This chapter contains an outline of some of the principles of pit layout and operation. The term pit is intended to cover any open excavation made to obtain material of value, whether it be coal, mineral ore, quarry rock, gravel, or fill.
Because of the complexity of the subject, treatment will have to be brief, and many operations omitted entirely.
Techniques of drainage, road building, and blasting have been discussed in previous chapters.
Most pit operations are started with the removal of soil or rock lying over the deposit to be mined. The problems involved in stripping will therefore be considered first.
Overburden may include topsoil, subsoil, sand, gravel, clay, shale, limestone, sandstone, and other sedimentary deposits.
The depth of overburden that may be removed depends on its character and accessibility, the value of the underlying formation, the comparative cost of underground mining, and the extent to which the spoil can be sold or utilized.
Need for Stripping. Stripping overburden may be a very large part of the cost of mining, and a number of factors should be considered before undertaking it.
First, is it necessary to strip it? It may be possible to mix it with the product, or to separate it at lower cost during processing.
If the pit has gravel or stone screening, separating, or washing equipment of adequate capacity, soil may be dug or blasted down with the pay dirt, and separated as part of the regular processing. In this case, thorough clearing is necessary as sod and brush clog screens and crushers.
Utilization of Spoil. If the land must be stripped, the next question concerns possible profit or use to be obtained from the material. Near large cities, topsoil can often be sold at a higher price than the regular pit product. This may also be true of peat deposits.
Any substantial layer of clay, or fine earth, should be sampled and analyzed. Clays which are superficially similar in appearance are used in widely different products, such as fire, paving, and common brick; tile, pottery, portland cement, flux, mud for rotary drills, and specialized functions in chemical processing.
Limestone is often found in overburden, and it is extensively quarried for crusher rock, building stone, and cement manufacture.
If it is not possible to get good prices for any part of the spoil, it may still be possible to get enough for it as fill to repay some of the stripping cost. The stripper is in a good competitive position because he or she has to pay for digging, and often for dumping as well, and is better off selling for a fraction of the excavation cost than not selling at all.
The limiting factor is the additional cost in making the spoil salable or delivering it. If selective digging that will slow operation, or finer fragmentation, is required, or if the stripper must truck material which otherwise could be cast, the salvage may cost more than it is worth.
When the spoil is being removed in trucks or scrapers, and no market exists for it, and it is not practical to set up any plant to process it, it might be used for real estate improvement or for goodwill.
Swampland along railroads or highways can often be bought very cheaply, and can be converted into valuable industrial sites by filling and grading, if the laws about preserving wetlands are not being violated. Filled land near towns may be sold for residential purposes.
Pits are frequently unpopular with the neighborhood because of blasting, heavy trucking, or dust. Local authorities may impose restrictions which would make operation more expensive or impossible. Under any circumstances, cultivation of local goodwill is sound business practice.
Fill which is being hauled and wasted can be utilized to reclaim land for parks or parking areas, building road or airport fills, and blocking gullies. Topsoil can be used to enrich farms, gardens, or lawns.
However, there is danger in such work of getting into much greater expense for extended hauling and grading than anticipated, so a careful study should be made in advance. The people or community benefited will often be willing to pay at least these extra costs.
As a general proposition, however, the pit operator must figure that the spoil from stripping will not be salable or of any use, and when it must be hauled away, it may be a problem to find a disposal area. This is particularly likely in large operations.
Regrading. A number of states now require grading of spoil piles and breaking down of walls of worked-out pits. This work should be included in cost calculations.
Replacing of topsoil and/or replanting may be required by law, or by private contract between the pit operator and the land owner.
Pit floors, slopes, and spoil piles are usually quite barren, and are often very rocky as well. They may be entirely lacking in nitrogen, and their potash and phosphorus supplies are usually in insoluble forms that become available to plants only after long weathering.
Reclamation involves grading to smooth contours; planting with grass, hardy trees, or other vegetation; and protecting the graded land against erosion until the plants have taken hold.
A reclamation plan must usually be filed and bonded before work starts. If the pay seam is thin, restoration of original contours will probably be required. If it is thick, or underlies rough land, new topography may be permitted.
Removal of coal often makes groundwater highly acid. Treatment of this condition within a project, and prevention of contamination of streams flowing from it, may be a major reclamation problem.
When the spoil cannot be utilized, the cheapest way to move it is by sidecasting—that is, to pile it alongside the cut within dumping range of the excavators. This disposal is possible when the pay formation has only a narrow exposure, or can be worked in narrow strips, so that the overburden can be economically moved across the pit, from the unopened to the worked-out area.
Sidecasting is generally not practical in working thick layers of quarry rock or mineral ore, because of need for wide working space below the face. Its best-known application is in the strip mining of bituminous coal.
One-Spot Strip Mining. The diagrams in Fig. 10.1 show the basic process of stripping, mining, and backgrading a narrow lens. It consists of three operations aside from the drilling and blasting frequently necessary. Top material is cast out of the way, pay material is dug and trucked away, and the top pushed or cast back in.
Progressive stripping of a wide deposit, as in Fig. 10.2, resembles the action of a gigantic moldboard plow, taking slices off the high wall and laying them against the spoil heap. Mining is done between trips.
FIGURE 10.1 Steps in strip-mining a lens.
If the spoil piles are smoothed over where they fall, without substantial removing, the original grade will be raised in a ridge parallel with the beginning of the work, and lowered at the last cut, these changes corresponding to the land and the dead furrow of the ploughed field.
Bulldozers. Where the deposit is shallow and the pit narrow, bulldozers make good stripping tools, particularly when the land slopes across the pit. Figure 10.3 shows a sequence of operations. This can be repeated in successive strips across the deposit if conditions remain the same.
Such shallow overburdens seldom require blasting, but use of a rooter may speed the digging.
It will be noted that this machine backfills the face of the pay layer so that work must be done from the surface of the seam.
Bulldozers are also essential auxiliary tools in the heavier stripping work.
Scraper. Scrapers, or pans, are used in sidecasting, haul-away, and combined stripping. Their best application is to soils that can be dug easily with the power that is available, or that can be broken into fine fragments by ripping or blasting; and that are not deep enough to justify the use of shovels with enough reach to give the pit the width it requires.
Figure 10.4 illustrates a method of scraper use. These are considered sidecasting since the spoil pile is placed immediately behind the pit, but otherwise the work is identical with haul-away with the same machines. A particular problem that becomes more important as depth increases, is maintenance of haul roads across the pit and up the spoil bank.
FIGURE 10.2 Strip-mining a horizontal bed.
FIGURE 10.3 Bulldozer stripping.
FIGURE 10.4 Stripping shallow overburden with scrapers.
In Fig. 10.4 the high wall is smoothed off and the outcrop of the pay formation covered by bulldozers working down the slope. The scrapers then dig the high wall, working downhill, and build their fill on the full width of the empty pit.
Lengthwise digging is well adapted to combined operations. If only part of the overburden were to be removed by the scrapers, and the balance by shovels, a narrower fill would be made. The scraper work would thus serve to reduce the depth to be handled and the distance it would have to be thrown.
Scraper cuts are frequently made to dispose of loose or soft overburden on rock which requires blasting. This reduces the drilling footage, eliminates the need of casing large blast holes, and permits use of wagon drills on the exposed rock.
Front Shovel. For sidecasting work, front shovels are equipped with extralong booms and sticks to increase reach. These are compensated by extra counterweight and power or smaller buckets.
Crawler-mounted stripping shovels are made with buckets from 2½- (1.9-cu.m) up to 140-yard (107-cu.m) capacities. In strip work they are generally used for sidecasting, but except in the largest sizes they may also be used for loading trucks or trains that stand in the pit or on comparatively low walls.
Small- and medium-sized stripping shovels are generally more or less standard units that can be readily transported from job to job. Large ones are likely to be custom-made, shipped to the job in pieces, and erected at considerable expense. Their high cost is only justified when enough work is available in one area to repay it.
For casting, reach must be increased with the depth of overburden, as the slope of the pile progressively narrows the work area in proportion to top width of the cut. Increases in power and bucket capacity are required for digging harder and coarser material, and for greater output.
A shovel may make one, two, or more trips to clear a working area (Fig. 10.5). It may be worked from the coal or pay seam, or from the floor left after its removal. Maximum swing required is 180°.
The shovel is followed by one or more cleanup bulldozers. The toe of the high wall is scraped back, and the pay seam is cleaned of material left or dropped by the shovel, or which has slid or rolled from the pile. This debris is pushed into the spoil pile.
Coal may be injured by the grousers of heavy bulldozers. Rubber-tire dozers, or crawlers with the grousers trimmed back or covered by street shoes, can be used to avoid damage.
Dragline. Stripping draglines are being manufactured to carry buckets up to 220-yard (168 cu.m) capacity, and booms over 300 feet (91 m) long. Diesel and diesel electric power are standard up to 3½ yards (2.68 cu.m), and are sometimes used in machines as large as 8 yards (6.1 cu.m).
Crawler mountings are standard in small machines, and walking bases in large ones. The walker rests directly on the ground and moves by eccentric movement of shoes on each side. It is safer for use on high or loose banks because its ground pressure is low [5 to 12 pounds per square inch (0.35 to 0.84 kg/sq.cm) as against 50 or 60 (3.5 or 4.2) for comparable crawler machines], and it can change its travel direction without exerting any side thrust. The mechanism is described in Chap. 13.
Walkers are not adapted to use on pit floors because they can only walk away from the boom so that they may be trapped if unable to swing freely. In addition, they are usually wider than crawlers of the same weight.
The dragline strips by moving along the high wall, parallel with the pit, digging behind it and casting onto the spoil pile, as in Fig. 10.6. Maximum swing is about 90°. Standard practice is to dig within the radius of the boom point.
Large draglines can dig hard and coarse formations but are somewhat less efficient in them than power shovels.
Draglines can load part or all of the spoil into trucks or trains, on the wall or in the pit. They can dig selectively from the top down, handling different formations in succession from one stand. This ability makes possible a rough division of the bank into select material to be hauled away, and waste to be sidecast.
FIGURE 10.5 Stripping with a front shovel.
FIGURE 10.6 Stripping with a dragline.
Cleanup bulldozers follow immediately after the dragline and push their loads within reach of its bucket.
Stacker. Stackers are mobile elevating belts. The belt is carried in a boom which can usually be raised, lowered, and swung while operating, and may be adjustable in length as well.
The hopper is usually crawler-mounted and self-propelled. It is protected by a grizzly, so that pieces too large for the belt will be rejected onto the pit floor.
Figure 10.7 illustrates the use of one of these machines in conjunction with a standard-boom shovel. The shovel digs the bank and dumps in the hopper, from which the belt carries it to the spoil bank.
Dug soil can usually be moved more economically by a conveyor belt than by swinging a shovel heavy enough to carry a boom of the same range. However, the stacker is not as flexible and will not handle as coarse material as a stripping shovel or dragline.
The stacker is also used in the pit for loading trucks up on the bank.
Wheel Excavator. The wheel excavator is the newest big machine to appear in the coal fields. It is a self-propelled, crawler-mounted unit that carries a cutting wheel on the end of a long boom that can be raised, lowered, and swung; it also has a stackerlike conveyor belt for discharging the spoil, that may or may not be separately controlled. (Refer to Chap. 14.)
One of these machines was used by Peabody Coal Company in its River King mine in southern Illinois. It was teamed with a 70-yard (54-cu.m) front shovel to handle overburden from 65 to 135 feet (20 to 41 m) deep.
FIGURE 10.7 Front shovel and stacker handling overburden.
The wheel excavator took the top third of the overburden, which was mostly soil. Where the cut was 90 feet (27 m), the wheel took off 30 feet (9 m). The bottom 40 feet (12 m) contained hard rock that was drilled and blasted; this work was done from the shelf left by the wheel. The shovel dug this and piled it across the pit in the space from which coal was removed.
On the next pass, the wheel spoil was deposited above and behind the shovel piles, as in Fig. 10.8. This made an excellent division of work, as the wheel could not handle coarse rock, and its soft spoil was held from sliding back into the pit by the shovel’s rock pile. On the high wall side, the wheel shelf provided a better surface for the drill than the original ground, and drill holes were much shorter. The shelf also caught bank slides that would otherwise go down on the coal.
FIGURE 10.8 Wheel and shovel team (multiply feet by 0.305 to get meters).
The coal-loading shovel followed the wheel, making a 45-foot (14-m) cut. Overall pit width at the bottom, from high wall to the toe of the spoil bank, was 130 feet (40 m) to provide space for these big machines to pass each other.
Double Casting. If a space is required which is wider than that which can be stripped by available shovel and dragline equipment, and the spoil is too coarse for scrapers, a shovel may be used to swing the spoil out onto the pit floor, and a dragline to pile it on the bank. The illustration in Fig. 10.9 is of a quarry requiring a wide working area below the face.
For best results, the two machines must work together as, if the backhoe is allowed to build a substantial windrow, the dragline will need a longer reach to get to its far edge.
A dragline may also be operated on the previous spoil bank, to take off the top of the pile being built by a shovel and thereby to increase its disposal capacity.
The extra expense of double casting limits its use to special conditions.
Deep Stripping. Strip operations often run into areas where depth of overburden is too great for efficient sidecasting with available equipment. This may be handled by two trips of a shovel, by a shovel and wheel, or by a shovel and dragline.
Two trips with excavators may be made for drilling and blasting convenience rather than to make up for a lack of reach. Taking a high bank in two levels may result in worthwhile savings.
FIGURE 10.9 Overburden removed with dragline and shovel.
The strata in the upper and lower lifts may differ in quality so that different drilling and blasting techniques are used. In some cases one will require blasting whereas the other will not.
Very large draglines may be able to dig solid shale, but not sandstone. In such formations only the sandstone layers would need to be drilled and blasted.
When sidecasting does not provide a wide enough work area for removal of a pay formation, or there is no place for sidecast spoil to be piled, the overburden is loaded and hauled to a dump.
Depth of overburden may be a few inches to more than 600 feet (183 m), and the area may be only a few acres or several square miles (kilometers). In general, the method is adapted to thicker pay deposits and deeper overburdens than sidecast stripping.
Mines in which stripping is chiefly done by sidecasting are called strip mines, while those whose overburden is hauled away are called open pits. Most sand and gravel are obtained from open pits. Open-pit overburden may be any material, from soft, lake-bottom mud to the hardest varieties of rock.
Stripping Area. The size of the area to be stripped for full recovery of a deposit is determined by the area of the pay material, its bottom depth, the thickness of the layer of waste, the slope of the natural ground surface, and the steepness of the safe slope of cuts.
As a practical matter, this ideal area may be limited by property lines, the presence of valuable buildings, access or drainage requirements, the money available for stripping, or combinations of these factors.
Property lines may not be absolute barriers. Permission may often be obtained, at a price, to extend excavation across them. In the Mesabi range where open-pit mines adjoin each other, different companies often exchange both overburden and ore, balancing such accounts over a period of years.
If the mineral is valuable enough, whole towns and mills worth millions of dollars may be moved or demolished to permit pit expansion.
Cut Slopes. It is seldom possible to limit stripping to the area immediately above the pay deposit, unless the overburden is very shallow or very firm. Cutting must be kept at a slope that will stand during the period of the work.
Even the firmest rock formations should not be left vertical for over 200 feet (61 m). The majority of rocks are not safe until slopes are reduced to 38° to 45°. The only guide that is reasonably accurate is behavior of similar rock in other pits, as laboratory tests for shear and other characteristics are unlikely to include all the characteristics of large masses.
Unconsolidated soil has the same general slope requirements, except that a combination of fine-textured soil and groundwater may reduce possible slopes to 10 percent or less. Water is the most important single factor in the stability of soil slopes.
While natural cliffs may be vertical for thousands of feet, they almost always have rock heaps at their feet that show falls that would have been disastrous in a busy pit.
In benched cuts there are three slopes: that of the individual faces, the average working slope, and the final or residual slope that is left when the work is completed. The average and final slopes are taken from crest to crest. See Fig. 10.10. The final slope is usually steeper than the working slope.
Rate of Removal. It is usually necessary to remove some overburden before the pay material can be dug. When money is short, extraction work may start before there is really room for it, with resulting inefficiency. A well-financed project may remove much or all of the waste, perhaps at an expense of millions of dollars, before digging any ore.
It is usual for both stripping and extracting operations to be carried on through most of the life of a pit, either at the same time or alternately. In the Mesabi iron mines the same equipment that digs ore in the summer loads overburden in the winter. Gravel pits do their stripping in slack seasons, whenever they occur.
Some mines blast and load their own ore but let contracts for stripping off waste, so that the two jobs are active side by side through the whole working year.
The relative rates of stripping and mining are an important factor in mine efficiency. When stripping is well ahead, the mine has plenty of working space, with wide bench floors. If mining is faster than stripping, benches narrow and space becomes crowded, reducing efficiency.
The size of the pay formation may change during mining. Additional reserves may be located, or the cutoff point between usable material and waste may be changed by variations in price, processing methods, or market. Any extension of mining will probably make it necessary to extend the stripping. However, abandonment of part of the project cannot unstrip the affected area, or recoup any part of its substantial cost.
A lag in stripping is often due to lack of funds, or to reluctance to invest them any sooner than is absolutely necessary.
Stripping Costs. Stripping costs are affected by the type and amount of material to be removed, the distance it must be hauled, grades on the haul route, and efficiency of the operation. These costs would include all loading and hauling costs as well as supervision, but not drilling and blasting.
Scrapers can move suitable material at 50 to 60 percent of the cost of shovels and trucks, if conditions favor them. But in coarse and rough material their production goes down and costs go up rapidly.
Hydraulicking and dredging are limited to soft overburden, abundant water supply, and dumps suitable for ponding hydraulic fill. However, if conditions are right, they can remove overburden as cheaply as any method.
Rail haulage is likely to be cheaper than truck haulage if large volumes are to be moved more than 2 miles.
Small pits may be able to sell or use their overburden, medium sizes may have little difficulty finding places to dump it, and large operations may find considerable difficulty in finding adequate disposal areas. For this reason, large-volume stripping may show a higher cost per yard than small-volume, because of the longer hauls needed.
Working at the Edge. A pit in a deep sand or gravel deposit usually expands rather slowly, and stripping work is done at intervals, often by the pit machinery in slack periods. Stripping may be postponed too long, until the face is pushed back against the overburden, as in Fig. 10.11(A).
Stripping may then be done by either pushing into the pit and loading from the bottom, or throwing back with a dragline, backhoe, or clamshell. Consideration should be given to the question of caving of high banks.
Once the edge is cleared, as in (B), the burden can be moved back farther by recasting with the dragline, pushing with a dozer, or carrying in trucks or scrapers.
FIGURE 10.11 Stripping edge of expanding pit.
It is vital to pit efficiency to keep overburden stripped far enough ahead to be out of the way in rush periods. Failure to do this often results in spoiling or losing valuable material, and in inability to fill important orders.
Dump Location. A dump for waste from stripping should be as near the pit as possible to reduce haul costs, but it should not be within an area that might be dug away because of any conceivable extension of the stripping. These two considerations may be opposed, and deciding between them may be difficult. It is a matter of regret that initial economies have often resulted in disproportionate later expense in redigging and moving a dump pile.
For example, loading overburden might cost 90¢ per yard, hauling it 1 mile about 25¢, and additional miles 15¢ each. A single move of 2 miles (3.2 km) would cost $1.30, while moving it 1 mile (1.61 km), redigging, and moving another mile would cost $2.55 per yard. Double handling is always expensive.
If it can be managed without substantial extra expense, different types of spoil should be placed in separate dumps in such a manner that they will be accessible for redigging. Changed conditions may make previously worthless material valuable. Examples are the reclaiming of mine tailings and slag heaps.
Haul Grades. Grade of haul roads is important to economical hauling. A level run from cut to dump is desirable for speed and economy. An adverse grade (upgrade in the direction of haul) will cut both the speed and the load-carrying capacity. The extent of this loss depends on many variables. A rule of thumb is that production will be reduced about 5 percent for each percent of adverse grade.
The adverse grade will increase fuel consumption, tire wear, and maintenance costs. Wear on the truck engine and drive train is increased disproportionately on grades over 6 percent.
A downgrade in the direction of haul (favorable grade) is helpful up to about 2 percent, but steeper grades may reduce production about as much as an adverse grade. Downhill speed must be limited for safety reasons, and even empty trucks are slowed by upgrades.
Favorable grades over 2 percent and adverse grades over 5 percent call for special retarding devices in torque-converter-equipped trucks.
Grades may change considerably during a stripping operation. The floor of the cut moves downward, but its edges move outward and often upward. Dumps may stay at the same level, but if space is restricted, they usually build upward.
Haul Routes. Two-way roads for heavy hauling should be from 4 to 4½ times as wide as the vehicles using them. That is, highway trucks should have 32 to 36 feet (9.7 to 11.0 m) between gutters or banks, and 11-foot (3.4-m), off-the-road haulers from 44 to 50 feet (13.4 to 15.2 m). Hauling can be done on much narrower roads when necessary, but liberal width pays whenever large volumes must be moved. Even wider roads are made for some mines.
A haul route that crosses a public road is subject to serious traffic delay. For example, an automatic traffic light that is set against pit traffic, but trips within 10 seconds when a truck reaches it, will delay the hauler as much as an extra 1,000 feet (305 m) at 20 miles per hour (32.2 kmph). A full stop sign will cause the same or greater delays, depending on the density and speed of highway traffic.
A signal worker at the intersection reduces delays to a minimum if the signaler is allowed to favor the pit traffic.
Hillside Dump. The easiest way to dispose of stripping waste in trucks is to dump it off a bank, that is high enough so that it grows outward quite slowly. Height may be anything from 10 feet to several hundred.
Such a dump may be started by flattening off a hilltop enough to give trucks space to turn, or by cutting a pioneer road along a slope and dumping from it.
Capacity can be figured in two ways. Annual or daily capacity depends chiefly on the length of the dumping face, and to a smaller extent on its height. Total capacity is the volume that can be dumped without spilling beyond the boundaries. It depends on the area that can be used, the height of the fill, and the slope of the dumped material.
Blasted rock dumped off a bank usually has an angle of repose of about 1 on 1, or 45°, and is likely to stay at its original slope indefinitely unless the native soil beneath it moves outward.
Dumped soil tends to assume a somewhat flatter angle, which will depend on the size and shape of its particles. Wet soil flows, and it is important that no drainage from other areas flow into the dump. Rain falling on the turning area on the top may gully the slope and spread mud over a large area below it. This can be prevented by sloping the surface up toward the edge, keeping a berm or ridge of dirt at the edge, and providing another escape for the water.
Both rock and soil slopes offer a hazard of rounded and oversize pieces rolling far beyond the toe of the fill. In empty country this may not matter, and brush and trees check such objects naturally. But dirt or log barriers may have to be built to prevent rolling onto paths, roads, buildings, or other property.
The slope of dumped fill is likely to be between 27° and 35°, with coarser material having the steeper slopes. For rough calculations, assume that it will be 1 on 2. If the waste is already being dumped or piled, the slope can be measured for more accurate figuring of areas and quantities.
Dump Operation. Trucks are backed square to the edge and dumped. Methods of keeping them from backing over the edge vary widely. Sometimes the driver is just supposed to stop in the right spot and at the right distance, with or without a spotter. Or a dump log may be placed to both indicate the dump spot and protect the truck against backing too far. The chief problem here is that a log heavy enough to stop 30 to 60 tons (27,200 to 54,400 kg) of loaded truck is difficult to move.
The simplest and best protection for trucks is an 18-inch to 2-foot (0.48- to 0.61-m) ridge of dirt left at the edge by the grading dozer. If the edge is very soft, a ridge may be built at a distance from the edge.
Off-the-road trucks with standard spill chutes instead of tailgates will dump their loads clear over a ridge at the edge, so that piling up does not occur at the top until it builds up from the bottom. Tailgate bodies may or may not spill part of their loads on the top.
It is good practice to maintain an upward slope from the truck entrance to the dumping edge. One-half percent is sufficient on porous fills, and 1½ percent is sufficient for any material that is kept well graded.
Fills tend to settle under weight of traffic, and with time and weather. This sinking is most rapid toward the edges that are being built out. It is necessary to correct the resulting downslope with wedge fills as downslopes are dangerous to trucks and may cause gullying.
A wedge fill is made by dumping on the surface at the back or thin edge of the settlement, and grading with a dozer to restore or increase the original upslope, as in Fig. 10.12. The dozer first pushes toward the edge to establish the slope, then parallel to it so as to smooth and compact it and to leave an even windrow along the edge.
FIGURE 10.12 Building up a dump edge.
Topsoil is frequently the only material sold from a temporary pit. At other times, it may be a highly profitable sideline, or a costly stripping and wasting problem.
In this discussion, topsoil is defined as any layer or layers of soil containing sufficient humus (organic matter) and plant food to support a good growth of grass or other desirable vegetation. It is ordinarily on the surface but is occasionally buried by flood deposits or slides.
In the eastern United States, topsoils are predominantly brown, with a humus content between 3 and 20 percent by weight. Depth varies from zero on ridges to many feet in river bottom lands, but is usually between 4 and 10 inches (10 to 25 cm). Division from lower soil layers is usually definite. These soils will be the basis of most of the following discussion.
In arid and semiarid sections of the west, topsoil tends to be deficient in humus and rich in minerals. It is often difficult to distinguish from subsoil. It may occur in deep layers or deposits and in general does not obtain as high a price as in the east.
The prairie topsoils range from 8 inches (20 cm) to several feet (meters) in depth, may be brown or black, are rich in both humus and minerals, and generally have an excellent texture.
Swamp topsoils, in any section, tend to be gray to black and may contain up to 85 percent organic matter. Depths vary from a few inches (cm) to hundreds of feet (m). The richer deposits are not topsoil as defined above, and will be discussed separately as peat.
In general, the salability of topsoil is determined more by appearance and texture than by the ability to grow crops. The average topsoil buyer will seldom have soil tested, and tests are often not as reliable as good judgment.
Topsoils with high percentages of clay or silt will be heavy, slow-draining, and inclined to pack into hard lumps if disturbed when wet. Increase of humus content will soften the lumps.
Sandy or gravelly topsoil is loose in texture, drains readily, tends to dry out, and can be worked when wet without caking. Most soils are of an intermediate structure, with variable draining and lumping.
Testing. One test of topsoil is the observation of the type and condition of vegetation it supports before stripping. The vigor of weed growth on piled topsoil is an excellent index to its quality.
Laboratory or field tests can be made for humus content, grain size, acidity, and available plant food. Humus is measured by the ignition test to be described for peat, and grain size on screens used for testing sand and gravel.
The test for acid-alkaline balance is commonly made by pressing litmus paper against the damp soil, and comparing its new color with a chart. If the soil is dry, distilled water should be used to dampen it, as tap or pond water may give a false reading.
Acidity is expressed in terms of pH (percentage of free hydrogen ions). A reading of 7.0 is neutral, lower readings increasingly acid, and higher ones alkaline. Most plants will grow under quite a wide range of conditions. A slightly acid condition is desirable for most of them.
Excessive acidity is readily corrected by the addition of lime, which can be spread on the field before plowing or disking. Soils are made more acid by mixing with humus, oak leaves, or aluminum sulfate.
Kits obtained at garden supply stores can be used to measure available or soluble nitrogen, potash (potassium oxide), and phosphorus or phosphorus oxide. It should be remembered, however, that these chemicals are often taken up by plants, or leached out by rain, as fast as they become soluble. The real measure of prolonged fertility is the insoluble reserves that are gradually made available by soil organisms, plants, and weathering. Except for humus, which is rich in nitrogen, and often in the other plant-supporting chemicals, such reserves are difficult to measure.
Preparation for Stripping. The cost of properly preparing a field from which topsoil is to be sold is usually a small part of the total expense, and should increase the value of the soil so that it will either command a higher price or be more readily salable at a standard price.
Aside from clearing, field preparation may not be necessary if the soil is to be left piled long enough to rot the vegetation—usually 4 to 6 weeks of warm weather for sod—or if digging is to be done by a chain bucket loader. Plowed land is more easily and cheaply piled by a bulldozer than solid fields.
The field should be plowed to the full depth of the topsoil, if possible, or at least deeply enough to turn up most of the roots of the grass or crop. However, turning up of subsoil should be kept to a minimum, particularly if it is of a conspicuously different color. Therefore, if the topsoil depth is variable, or plow depth hard to control, it may be necessary to plow very lightly.
After plowing, the field should be thoroughly disked so that the vegetation is chopped up and well mixed with the soil. It is then ready for stripping.
If soil is to be removed in a wet season, it may be necessary to leave some strips of sod intact to support trucks.
Noxious weeds can be reduced or eliminated by planting and turning under one or more vigorous, close-growing cover crops. Buckwheat is particularly effective at smothering out.
Nitrogen Deficiency. When vegetation is turned under and mixed with soil, there is an immediate and rapid increase in the number of the microorganisms which cause it to decay. For their growth they need nitrogen. If the crop is a legume, such as clover or vetch, they will be able to obtain it as they break down the plant material; otherwise they obtain it, or as much as they can get, from the soil.
This is likely to result in temporary total exhaustion of available nitrogen. When the vegetation has decayed so that it no longer provides sufficient food, most of the organisms die and their nitrogen is largely returned to the soil.
During the interval, which is 2 or 3 weeks for fresh green material in warm weather, and longer when the material is dry or coarse, when soil moisture is deficient, or when the weather is cold, any crop which is planted will be starved for nitrogen and make little or no growth. This effect is most severe when conditions favor rapid decay.
Packing Soil. The value of topsoil is reduced or lost if it is packed into lumps. Except for a few very light and friable soil types, varying degrees of damage will be done if it is worked wet, or trucked over except when thoroughly dry.
Wet working breaks down the soil particles into a structureless mud that dries into lumps and sheets. The probability of damage increases with the amount of contained water, and its severity decreases with increased proportions of sand or humus.
Packing under trucks or other heavy machinery may produce the same result by bringing water out of small spaces between particles so that it will make a mud. Trucking on rather dry soils will produce compression cakes, which are usually softer than the mud lumps.
A rough test of condition may be made by rubbing a sample of soil between thumb and finger. If it smears, it is too wet, and if the particles remain separate, it is probably ready to work. The dirt turned by the plow or dozer blade should be watched for smearing, which indicates that soil is too wet.
Topsoil should not be trucked over, but it is often impossible to avoid doing it. The stripped areas may be too soft, or they may not offer enough space for maneuvering. If the latter, trucks may be routed to drive empty across the topsoil and run on the subsoil with loads.
It is usually better to completely ruin a narrow strip of soil by using it as a haul road than to damage a large area by allowing trucks to wander around on it.
Soil lumps are completely broken down by freezing and thawing, and usually disintegrate slowly in wet seasons. If still on the field, the roots of a cover crop, or sometimes only a thorough rolling or disking, will reduce them. If absolutely necessary, a hammer mill shredder of the type used for humus can be used to pulverize them.
Piling. The standard tool for piling topsoil is the front-end loader. The old standby, the bulldozer, does the same work but rather less efficiently, as it cannot make high piles without walking on them. But it may separate topsoil more cleanly.
When piling is done in advance of loading, the standard practice is to heap the soil in windrows (long piles). These may be run up and down the slope so as not to interfere with drainage; or across it to keep in uniform soil types, or for convenience in trucking. It may be necessary to make occasional breaks in windrows to prevent ponding of water above them. Piling should be started at the entrance to avoid trucking over unstripped areas.
Windrow size will vary with loading requirements and soil depth, and the size of the bulldozer. Small, closely spaced ridges are most easily piled, but loading machines work best in large, high piles. Large dozers and deep soil favor building big piles. Building of the piles is described in Chapter 4.
Careful separation of the topsoil from subsoil is a requirement of most stripping. Inclusion of even considerable amounts of loose fill in topsoil does not usually damage its usefulness, but it is a type of adulteration that is unpopular with the buyer. The damage to appearance and value is especially severe if the subsoil is a conspicuously different color, or texture, or is in the form of lumps or sheets. It is generally better to leave a thin sheet of topsoil on the field than to mix topsoil and fill.
However, if the stripping is done in order to get clean dirt or gravel for roads or other purposes, it may be better to concentrate on cleaning the surface, even if some fill mixes with the topsoil.
Topsoil and subsoil are separated chiefly on the basis of color. If the difference is prominent, the distinction is easy to make, but the results of a mistake are painfully obvious.
Light conditions may obscure the color difference. When the sun is very low, in the morning or evening, or high in a clear sky, subsoil and topsoil may look the same. Cloudy days and intermediate sun elevations give the clearest distinction.
Any shovel rig can be used for piling topsoil. The backhoe and dragline are best at it, and the toothed clamshell slowest. Shovels work rather slowly in shallow soil. The hoe is adept at salvaging soil along a wall or fence.
Shovels loosen and aerate the soil, and cause minimum damage in handling it when wet. Fill dug with the topsoil can be concealed by mixing in. Teeth prevent absolutely accurate work, and it is good practice to clean up afterward with a dozer.
Scrapers are used for piling topsoil chiefly when it has to be completely removed from a large work area. They generally make wide, low piles that are more readily rehandled by scrapers than by other excavators. They pack the soil heavily even when it is in good working condition.
If topsoil is stripped from one part of the job at the same time that it is spread on another, the scraper can combine the two operations very efficiently.
Scrapers drawn by fast wheel tractors may be used to dig topsoil and deliver it by road to local customers, who will usually appreciate having it spread.
Loading from the Pile. Topsoil is comparatively light and normally has a low digging resistance. However, it tends to push ahead of narrow buckets, instead of entering them, and this factor, coupled with the small size of the usual pile, reduces production to below that of hard digging in a bank.
Difficulty in filling the bucket may be reduced by thorough chopping of sod and weeds before piling, building large piles, and building piles on undug areas so that the bottom of the bucket will work in firm soil.
The front-end loader is the best equipment for loading because of flexibility, ability to load from either pile or field, to clean up as it works, to pile when not loading, and high production in relation to purchase price. Trucks are backed into the end of the pile, in variable positions to keep them at an angle of about 45° to the digging, as shown in Fig. 10.13. These wide buckets get heaping loads until the end of the pile is reached. The remnants can be pushed to the next pile.
The excavator is also used. It may have trouble filling the bucket, but its principal difficulty is in cleaning up. If the pile is narrow, as in Fig. 10.14(A), it can walk down the center and scrape in the sides by swing dragging. If the pile is heavy, as in (B), it can dig the bulk from one side, easily cleaning as it goes, and come back through the small remnant. In either case, it is best to have a bulldozer work with it, at least part-time, cleaning up. A light rubber-tire dozer is generally adequate.
When digging a windrow ending in a wall or property line, as in (C), the shovel should be started at that end and worked in, to minimize working soil over the end and losing it.
Draglines and backhoe shovels can be worked from one side of the pile, as in Fig. 10.15(A), or preferably from the smoothed-off top, as in (B). The top of the pile is particularly suitable for backhoes, as in (C), as they load more rapidly and easily if higher than the trucks. These rigs clean up the edges without extra work but can strip closer if aided by a dozer.
Any shovel cutting to a grade will occasionally go below it so that fill will be dug. The mistake is immediately shown by the color of the bottom. The bucket should be dumped off to the side to be wasted, or on the pile, to break up and mix the slice of fill. Subsoil may be deliberately dug and mixed in this manner if the topsoil is rich and the price is low or the buyer indifferent.
Loading from the Field. Topsoil is often loaded directly from the field, with no systematic preliminary piling. The efficiency of the operation depends on the depth of the soil, the machinery used, and the output required.
Best results will be attained in deep soils, using the shovel excavator or backhoe, as increase in depth reduces the proportion of time spent in trimming the bottom and in moving into the digging.
Unless the soil is very hard or rocky, the front-end loader gives good results, particularly as vegetation does not have to be disked ahead of it. Front loaders dig easily, but may roll up big masses of sod. Almost any loading machine can be used unless the soil is shallow or rocky.
FIGURE 10.13 Loading topsoil with front-end loader.
FIGURE 10.14 Loading topsoil with shovel.
High production cannot be expected of any machine, except possibly the front-end loader, in shallow stripping. However, if the trucks are scheduled so that the front-end loader has free time between them, it can stockpile enough to enable it to fill the truck quickly.
Sometimes a bulldozer is used to boost output by feeding soil to the loader. This may be done on the whole job or just on thin spots. Dozer cleanup after digging is usually desirable.
Screening. Topsoil may be coarsely screened on a portable grizzly, which is placed on the truck body prior to loading, or may be up on legs so that the truck can back under it. The latter type requires large piles for convenient use.
These are used either to market topsoil which is otherwise unsalable because of presence of rocks, roots, and trash; or to obtain a premium price. It is usually necessary to have a worker clean stuck material off the grizzly. Best results are obtained with dry or sandy soils. Cohesive soils require a coarse mesh and must be put on a small quantity at a time, or too large a proportion may be rejected.
Square openings from ½ inch up to 4 inches (12.7 up to 102 mm) are used, or similar bar spacings. A much finer product, with less waste in rejections, can be obtained by using a revolving screen on the discharge of a front-end loader.
FIGURE 10.15 Loading topsoil with dragline or backhoe.
Storage. Topsoil is often left piled for long periods. Texture is generally improved by standing over the winter, and quality does not seem to be damaged much, even by years of storage. In hot, dry seasons, it may bake out to colorless dust, which is unpleasant to load and hard to sell, but a wet season will restore its original texture.
However, piled topsoil will support a luxuriant growth of grass and weeds, which quickly forms a sod that is very undesirable. The irregular shape of the piles makes cultivation with any ordinary tools almost impossible. Flame guns are tedious to use and ineffective, and chemical weed killers are selective and uncertain in action.
Such growth can be kept down, at a price, by cultivation with a bulldozer, as in Fig. 10.16. The machine is run along the crown of the windrow, cutting off the top and allowing the dirt to flow down the sides. This uproots or buries a large part of the weeds. Later the bulldozer pushes the sides back up to the top, as in (C), destroying the rest of them. Most economical results are obtained by allowing a week or more to pass between the two operations. The job is done over as often as necessary.
This serves not only to keep the soil clean, but also to enrich it by the decay of the weeds.
Restoring Vegetation. When topsoil stripping is not followed by other work, areas are left denuded of vegetation and the ability to grow it. They tend to cause a dust nuisance and to erode badly, becoming a mass of unsightly gullies, and often silt up streams and block roads with the waste.
FIGURE 10.16 Weed control on piled topsoil.
As a result of such nuisances, many communities now forbid the stripping of topsoil, or impose restrictions on the work. The least of these is to require a guarantee to restore the vegetation on the stripped areas.
The process is similar to that required to restore worn-out and eroded farmland. The ground is loosened with rippers and plows, loose rocks are picked up or pushed off, a liberal application of manure or fertilizer and perhaps of lime made, and a crop planted. Fertilizing and seeding should be heaviest in drainageways. This crop should be one that will grow readily on poor land. The local Farm Bureau will be able to advise one or more suitable for the conditions and season.
In many localities, buckwheat or soybeans make a good summer crop, and a rye and vetch mixture is suitable for fall or early spring. When the plants are in flower or beginning to seed, they should be plowed or disked under, and additional fertilizer supplied to spots that did not grow properly. After an interval of 2 or more weeks, depending on local custom, plant another crop. A self-seeding legume such as sweet clover is good. This seed should be inoculated with a culture of the proper nitrogen-fixing bacteria. Some patch fertilization and replanting may be necessary later, and any tendency to gully can be checked with topsoil patches, heavy fertilizing and planting, or brush mats.
With luck and good farming, the second crop may hold the land so that no further plantings are necessary. Decay of plants and nitrogen absorbed from the air will enrich the soil, so that the native vegetation of the area will soon be able to reestablish itself.
Reclamation probably gives best results on glacial till soils, which will develop a new topsoil cover in a surprisingly short time.
Floors of deep pits will respond to the same methods but much more slowly. Deep subsoils rarely have proper tilth, or plant food in quickly available form for crops, so more “green manuring,” or turning under of vegetation, is required.
Animal manure will give better results than chemical fertilizers, particularly on floors of deep excavations, largely because it contains many organisms essential to a healthy soil. However, in many localities it is difficult and expensive to obtain.
It is advantageous to plant as soon after stripping as possible. If an interval is to elapse between piling and loading, it is good practice to plant a cover crop between the piles to prevent the development of a dust nuisance and loss of topsoil remnants.
Bank Gravel. Bank gravel is a useful and highly varied material. It consists chiefly of sand, pebbles, and cobbles, but may also contain clay, silt, and boulders, mixed in or in accompanying layers or pockets. The gravel proper is the pebbles and cobbles in sizes from ¼ to 2 inches (0.64 to 5.1 cm).
The specifications which gravel must meet to do certain jobs, and the proportions found in deposits, vary widely.
Bank gravels consist mainly of deposits laid down by fast-running streams, often of glacial origin, but they are also formed by waves on the seashore. The quality depends on the original stone, the proportion of sizes, and the angularity of the particles. Wave-formed gravels are predominantly rounded, glacial ones subangular, and product of other streams variable.
Talus gravels, formed at the foot of cliffs by falling and sliding, may be coarse and angular, but are often weak stone.
If gravels are not sufficiently angular for their job, and contain oversize stones, they may be run through a crusher which will produce angular fragments.
Fines in bank gravel act as a cement or binder, holding it together when dry. Gravel without binder becomes too loose for road use in hot, dry weather.
Fines in excess of 8 or 10 percent may cause a gravel to become sloppy after repeated freezing and thawing when wet. Fines over 15 percent may cause it to soften under prolonged soaking. Softening is made more likely by a high proportion of fine sand in the mixture, and less likely if thorough compaction precedes the freezing or soaking.
Any gravel will become sloppy if soaked when freshly dug, but if of good quality, should drain and firm quite quickly.
Gravels derived from continental glaciers are largely of hard rock. River and mountain glacier gravels are derived from upstream formations, and occasionally include too much shale or other soft rock for some purposes.
There are a number of tests for gravel, for field and laboratory use. A sample, with stones over ¼ inch (6.35 mm) removed, can be shaken up with water in a glass jar, then allowed to stand. The pebbles will form a layer in the bottom, with coarse and then fine sand on top. Silt and clay will settle out more slowly, and may take an additional day to compact. The relative amounts of the different-size particles can then be determined by inspection.
In the laboratory, gravel is dried, weighed, and put through a vibrating screen with many different meshes. The particles caught on each tray are weighed. Any lumps have to be broken up. This operation gives a classification of the specimen for size gradation.
Gravel can be tested for abrasion resistance by rolling in a cylinder with steel balls or other hard weights. Resistance to breaking up by freezing can be tested with cold, or with chemicals which duplicate its effect.
Clean bank gravel of the proper sand-gravel proportions is frequently mixed directly with cement for concrete.
Sand. Most bank gravel deposits are more than half sand. In addition, sand deposits occur in many areas where no gravel is found.
Ocean beaches are typically sand, and river deposits usually contain high proportions of it. If the river flows slowly, the sand may be mixed with silt and clay, which usually must be separated before use.
Most sand is largely particles of silicon dioxide, best known in the form of quartz. It is very hard and withstands the abrasion of water working, which reduces other minerals occurring with it to fines. Calcium carbonate, mica, feldspar, gypsum, and many other minerals may also occur as sand.
Many sandbanks are clean enough for use without processing, but in most cases it is safer to screen and wash before using in concrete.
Occurrence. Sand and gravel deposits occur in all parts of the world, and with special frequency on or near past or present shores, glaciers, and mountains. They may be thin, irregular deposits, or in heavy masses. In general, gravel is more variable than sand in size and type of particles, and thickness and shape of beds.
Running water needs higher velocity to carry large pieces than small, and in general, gravel is deposited nearer the source than sand, or at times of heavier stream flow. However, a stream which is building up a deposit alternates, bringing in materials and cutting parts of it away. Channels wander over the whole area. Oversize material beyond the capacity of the water to carry may be rolled long distances along the bottom. Clay and silt may be deposited in temporary pools and cut-off and stagnant channels.
The result of these factors is that gravel, sand, and clay deposits are often extremely variable and uncertain. When this is the case, mining them requires constant good judgment in deciding which horizons should be combined and which separated; and what can be used and what must be wasted.
Processing. Sand and gravel may be processed to clean out dirt; to separate into different sizes; to combine different sizes and materials; to remove or crush oversize stones; and for combinations of these purposes.
In variable formations, the primary processing is selection at the bank as discussed under selective digging.
The processing plant proper may consist of a washer, a screen, a crusher, or multiples or combinations of these units, together with feed hopper, and transfer and discharge conveyors. These plants, available in both mobile and portable types, are described in Chap. 21.
By the use of units of proper size, any desired reduction, combination, or separation can be secured. It should be remembered, however, that no plant can produce a coarse product from fine particles. Deficiencies in gravel content must be made up by mixing in stone of proper size, or oversize up to the crusher capacity, in addition to the run-of-pit material.
Clay. Clay, like sand and gravel, may be found in massive deposits or in irregular layers and lenses. It is often interbedded or mixed with other materials in very complex ways.
Underwater clay may be soft enough to be dug with a small dragline, or quite hard. Dry clay grades from hard shovel digging to shales requiring heavy blasting.
Pit operators usually find it economical to loosen up dry clay with at least light blasting, to facilitate digging. Electric or gasoline-driven augers are extensively used for drilling, and slow to standard velocity explosives for blasting.
When valuable clay is in narrow and confused beds, it is often blasted, then separated by hand into piles which are loaded by machine.
Most primary pit excavation is in formations deep enough to be loaded directly from the bank. The material may be in its natural state or loosened by blasting.
Bank Height. In free-flowing material, such as loose dry sand, the only limit to bank height is that imposed by safety. This will be discussed below.
If a formation will stand in vertical or overhanging walls, and is dug from the bottom, the face should not be higher than the machine can reach, as it may be necessary to dislodge overhanging pieces with the bucket to avoid danger from falls. Half this height is usually more convenient and may allow greater production if the top of the bank does not keep falling as the lower face is cut.
For example, excavators of 2½-yard (1.9-cu.m) capacity may do their fastest loading in banks in which the dipper teeth do not have to be lifted above 12 to 15 feet (3.7 to 4.6 m). However, they can trim banks up to about 25 feet (7.6 m).
When working in rock, it is the height of the blasted rock heap that counts, not the height of the face. The amount of settlement depends largely on the proportion between the height of the face and the depth to which it is blasted.
A 200-foot (61-m) face blasted back 20 feet (6.1 m) may yield a muck pile only 15 or 20 feet (4.6 or 6.1 m) high. A 20-foot (6.1-m) face blasted back 200 feet (61 m) might produce a 30-foot (9.1-m) high rock pile.
The height of a rock face may be limited by the length of a drill feed. Changing steels or adding drill rod takes time. This time can be saved by fixing the face height somewhat lower than the length of the feed. Rotary drills with 50-foot (15.2-m) masts are used extensively on 40-foot (12.2-m) banks.
Low faces require frequent moves. A height of less than one-half of the shipper shaft height may make it difficult to fill the bucket.
Benching. Whenever a noncaving formation is too deep for convenient digging, it is removed in layers. In shovel work these are called benches. They may be anywhere from 6 to 200 feet (1.8 to 61 m) high. In highway work 12- to 20-foot (3.7- to 6.1-m) faces are common, being suited both to the mobile light drills favored by contractors, and to the ¾ to 3½-yard (0.57- to 2.7-cu.m) power shovels they use.
In very massive rock cuts in highways, and in large-scale quarry and mine operations, heights of 30 to 60 feet (9.1 to 18.3 m) are usual, with drilling by rotary or down-the-hole drills, and loading by 2½- to 8-yard (1.9- to 6.1-cu.m) shovels or the biggest front loaders.
The best place to start benching is at the top. This simple fact is often obscured by other considerations, so that hillside work may be started at the bottom or the middle, and the pattern straightened out later.
The width of a bench, from the edge to the toe of the unblasted rock, should be at least 50 feet (15.2 m). Greater widths are better.
Types of Machinery. Loading machinery used for pit excavation can be roughly divided into tractor loaders, which depend on traction on the pit floor for digging power; revolving shovels with dipper, clamshell, or skimmer front ends, which stand on the floor while working; revolving shovels, with dragline, backhoe, or clamshell rigs, which load from the top of the bank; and scrapers and bulldozers which work down the bank slope.
Selection of machinery will depend on the location and digging characteristics of the formations, the volume of output required, the type and importance of other work that must be done by the same machines, the type of haulage or conveyor units, and the costs involved.
Production Factors. Big machines are suited to hard and coarse formation and to high production requirements.
Practically all excavators are available in different sizes. Production usually does not increase in direct proportion to power and weight, as the more massive construction of heavier units may require lower speeds, and space may be lacking for convenient operation.
Manufacturers’ data on output should not be accepted without careful study. Some firms deliberately underestimate production to avoid arguments, while others exaggerate it to make sales. Others base it on time-motion calculations, with little reference to field conditions.
Production ratings based on loose yards, or on a 60-minute hour, will be higher than for bank yards, or a 50-minute hour.
Also, there is room for honest difference of opinion about whether a formation is hard or soft, and conditions average, poor or ideal.
A rough index to output can be obtained by timing a machine at work in various materials. A stopwatch should be used and the results written down. The cycle time is the elapsed time between a certain movement, such as entering the bank, and the repetition of that movement. Average number of cycles per minute, from a number of observations, multiplied by the average bucket load in yards, will give the production rate in yards per minute in simple work such as sidecasting. Extra passes made to trim the bottom, or to break out or avoid boulders, may be averaged in or considered separately.
If the machine is loading, the loss of time in spotting trucks and trimming up their loads should be observed.
Data for calculating production are included in Chap. 2.
Big machines can almost always dig hard material better than small ones of the same type, but this factor is even harder to calculate. A rough index to penetration in material of even texture can be obtained by dividing the force which can be applied to the bucket by the width of the edge, or the combined width of the teeth. The extra resistance to the thicker teeth of the heavy bucket may be negligible in brittle formations and important in resilient ones.
In poorly blasted rock, or boulder-filled banks, the gain in penetration is much greater, as nearly the full power is often applied in succession to points of greatest resistance.
A wide bucket may be at a disadvantage because of inability to get between obstacles to attack them separately, or benefit from its capacity for large chunks.
In any digging, sharp cutting edges are essential to best work. In hard formations, teeth of proper spacing will give better results than straightedges.
Mobility is an important factor for machines which may dig for short periods from a number of different bank sections, or are used for loading from storage piles as well. Ability to do several types of work is liable to be useful, particularly in small pits.
It is good practice, although not always essential, to match the size of loading and hauling units. If large shovels are used with small trucks, time is wasted centering the bucket and material will be spilled off the sides. Truck tailgates may be jammed by oversize pieces and trucks damaged by impact. If the trucks are too large for the shovel, they must spend too much time being loaded; the shovel may be unable to fill them from one stand, and high body walls may hamper it. Generally, the loading equipment should govern the production of the operation, but to be cost-effective the number of haul units should not lead to noticeable idle time for the haulers.
Revolving shovels are usually teamed with trucks which will carry between five and ten bucket loads. Capacity is not as important for tractor loaders, but body walls should be low enough to permit easy placement.
Tractor Loader. Tractor front-end loaders include crawler types, which can do quite hard digging and heavy bulldozing; four-wheel-drive, rubber-mounted units suited for medium-hard banks, and two-wheel-drive loaders for soft or loose material and firm ground operation.
Crawler-mounted loaders are easy enough to move around pits of moderate size, but the wheel mountings are superior in speed and cause less wear to themselves and to the roads while traveling.
Four-wheel-drive loaders may be obtained with standard buckets up to 24 yards (18.4 cu.m) in capacity. They can often replace an excavator-and-truck combination in filling hoppers, and in short to medium hauls.
Front-end loaders are described in Chap. 16. Almost all of them now have an open-front bucket that rotates forward to dump and backward into a cupped position for slicing upward in a bank and for carrying a load.
Multipurpose or four-in-one buckets are easier to dump into high trucks, and are invaluable in handling bulky objects. However, their greater weight is a disadvantage in ordinary digging and loading.
A good truck loading pattern for a front-end loader is shown in Fig. 10.17. While these machines are flexible and can dig under very awkward conditions, best production is obtained if both angle of turn and walking distance are kept to a minimum. Best height for noncaving banks is somewhere between the height of the push arm hinges, and the maximum upward reach of the bucket edge.
Loaders retain a high degree of efficiency in very low banks, but are seriously exposed to slides or cave-ins in very high banks.
In spare time or in emergencies, either crawler or wheel-mounted front-end loaders can do almost any bulldozer work except scraper pushing. They are used for tidying the pit, smoothing haul roads, shifting heavy machinery, carrying heavy or bulky objects, and rescuing or starting stuck trucks and other equipment.
FIGURE 10.17 Loading from bank with front-end loader.
Excavator. Hydraulic excavators or front shovels are excellent machines for bank excavation. Although fastest loading is in soft material that will heap on the bucket, they can maintain good output in very hard or rough material. They are more costly in proportion to capacity than the tractor front-end loaders, but have lower repair requirements as the tracks do not move during the digging cycle.
They will dig from any graded floor that will support trucks. Best production in noncaving material is usually obtained when the bank is about as high as the shipper shaft (dipper stick hinge).
A short arc of swing is important in getting maximum production. The bucket can usually be moved from break-out position at the bank, to correct height and distance for dumping in a truck, during 30° to 45° of swing. Any longer swing required by truck position slows the digging cycle.
When a shovel is walked straight into a wide bank, the initial swing required is about 60°. As the machine works in, the swing becomes longer, finally approaching 180°. See Fig. 10.18(A) to (C).
Except in the case of a through cut being made in just this width, it is obviously inefficient to penetrate so deeply into the bank on one path.
If the shovel is walked parallel to the bank, as in (D), trucks can be spotted ahead, at a slight angle to the bank, with a minimum swing of about 40° and a maximum of 140°. If trucks are also placed behind, the maximum swing can be reduced somewhat. As long as the shovel is kept slightly outside the toe line of the bank, as shown, it can do its own cleanup. However, production is increased if the shovel operator can dig roughly, depending on another machine to smooth out the floor.
If the shovel is kept deeper in the bank, as in (E), a ridge will be left near the toe line, reach to load across it will be longer, and a dozer will be needed. However, production from each stand will be greater, which in a low bank may be an important factor.
When the pit floor is narrow, sandy, or wet, so that trucks must keep to beaten paths, a drive-through pattern, as in (E), can be used. The shovel again works along the toe of the bank, and the trucks run parallel to it, at a convenient loading distance. Only one truck can be spotted at a time, but it can be moved into position much more rapidly than when backed in.
If the pit floor is too soft for trucks, and only a front shovel is available for loading, trucks may be put on top of a low bank. This works well only with a big shovel and low trucks.
In banks offering a danger of slides, the shovel should be worked straight in, so as to be able to back out directly if partly buried. Cuts should be kept shallow by frequent moves to different parts of the face.
FIGURE 10.18 Loading from bank with excavator.
Clamshell. The clamshell is so versatile that it is difficult to set up patterns for it. It can stand at the foot of a fairly high bank and dig from the top, or stand on the top and dig from the foot, or can work at any intermediate level. It digs straight down, gathering in its load, without pushing or pulling the surplus. These features make it very valuable in selective digging.
The clamshell is adapted to various types of digging by changing buckets or bucket plates. Heavy-duty buckets of great weight and reduced capacity will dig very hard dirt and even soft rock. Rehandling buckets are larger, light, and often lack teeth. Medium-duty or general-purpose models are intermediate in weight and have teeth.
The clamshell has a smaller output than other shovel rigs and is more often used in stockpile, rehandling, and hopper feeding than in primary digging.
Dragline. The dragline is the best machine for loading from the top of the bank if it can dig the material. Small draglines usually are quite helpless in tight or rocky soil, but very large ones will dig even tight, unblasted shale.
Difficulty of penetration increases with depth. For deep work, the boom should be long and digging done well out. This minimizes the upward pull of the drag cable, which decreases the effective weight of the bucket.
A dragline can dig harder material from a face than it can cut vertically. If a wide ditch is started by other machinery or by blasting, it can be continued back into the bank by a dragline. If hard, it will tend to narrow down and become shallow.
The most efficient arrangement for hard digging is with the machine digging from the top of the bank and the trucks in the pit.
A dragline’s reach enables it to stand well back from treacherous banks so that it can usually make deep cuts safely.
The dragline loads best if it digs inside the boom point, at a medium depth, with a swing which takes no longer than the raising of the bucket, and the haul units are on the pit floor beside the bucket.
“Throwing” the bucket, that is, casting it so that it digs beyond the boom point, adds substantially to the area a dragline can reach from one stand or one line of work. But it is a practice that is best reserved for special situations, such as small cleanup jobs where access is difficult.
Throwing the bucket may add from 10 to 50 percent to the cycle time, since it usually requires that the bucket be pulled in, swung out, then dragged in with a full load until it can be picked up. If digging is done close in, the bucket is simply dropped and raised as soon as it is filled.
The extra reach is rarely more important than the time consumed. Also, careless casting causes or increases damage to bucket and cables.
When the loading or piling area is on the top of the bank, digging becomes slower as the reach becomes deeper, because of the time required to reel in the additional hoist cable required. At usual hoist line speeds, an extra second is needed for each 2½ to 3 feet (0.76 to 0.91 m) of depth. However, if the swing is long and unobstructed, the time of raising the bucket may not affect the length of digging cycle. If trucks are in the pit, the bucket may be raised only a few feet (meters), regardless of depth.
If the dragline is not overloaded, it should have power enough to perform simultaneously the three functions of raising and braking the bucket, and swinging without lugging down the engine. If the bucket is lifted to dumping position before the swing is completed, it is the length of the swing which determines the loading speed. If the swing is delayed in order to raise the bucket, it is the hoist, and therefore digging depth, which regulates it.
Carrying Scraper. Scrapers are not ordinarily considered to be bank-digging tools, but they may give lowest cost on combined digging and hauling.
Self-loading models, whether elevator, crawler-drawn, or two-engine, are most suitable for pit use, as they can work alone.
The bank is first shaped to a slope that may be between 10 and 25 percent if the machines are to climb it, and steeper if they reach the top by a haul road. It is desirable that the top of the bank be flat or have only a gentle grade to reduce the danger of tipping while turning. The scrapers are loaded by driving straight down the slope, which should be long enough to give them plenty of space to load. Rippers and pushers should not be needed.
FIGURE 10.19 Loading from bank with scrapers.
The scraper then hauls its load away to dump into a hopper, build a storage pile, or deliver it to a job. On its return, it is driven up the bank, or a haul road, turned on the top and again loaded coming down. The cycle is illustrated in Fig. 10.19. Semitrailer machines may be backed up the slope if turn space is lacking.
Once the bank is properly sloped, a single scraper may perform all the functions of digging, hauling, and storing without help from other machines or personnel. Such scrapers can also be used for digging in the pit floor, building haul roads, and grading.
Bulldozer. Bulldozers can load trucks from banks high enough to permit the machine to push into or over the body. For occasional loads, this may be done directly from the bank, as in Fig. 10.20(A) to (D). Considerable material is usually lost in building the bank out to the truck, and repeated loading extends the bank out into the pit, requiring a longer push with each load. The truck may get stuck in the spill.
A retaining wall and platform, as in (E) and (F), will eliminate this difficulty. Many other constructions are used. The platform should have steel strips or rails to keep the blade from digging into the timbers. These should be spaced so that the tracks will not have to walk on them. If they are raised above the wood, they will cause a dirt cushion to be built up which will protect the wood from the grousers.
Dozer push loading is most effective in high banks with a slope steep enough to allow pushing of large loads and to still allow the tractor to back up easily. Much steeper banks can be used in clay and hardpan than in loose sand or gravel.
Spoil can be pushed in the same manner into a hopper and conveyor, which may be a light homemade arrangement such as that in Fig. 10.21(A), or a factory-built, high-capacity belt loader such as that in (B). In either case, when the material within efficient range is exhausted, the hopper should be moved.
It is common practice to postpone moving for much too long. Dozer loading on level or slight grades is inefficient and should be avoided. No part of the push should ever be uphill.
Bulldozers are also used to push bank material within reach of excavators which are stopped by rock outcrops in the toe, and to keep high banks sloped to prevent undermining and caving.
In “glory hole” excavation, which is usually in rock, a tunnel is driven in from the toe and a connecting shaft run to the surface. Rock blasted from the sides of the shaft feeds by gravity to a conveyor, drag scraper, or railcars, which haul it out of the tunnel. A bulldozer is not required until the pit has widened its slopes so that rock will no longer slide.
FIGURE 10.20 Loading from bank with a bulldozer.
Bank Slides. Most materials will rest temporarily at a steeper slope than their natural angle of repose. Some sand and gravel may stand in vertical or overhanging banks when freshly cut, but eventually fall or slide to slopes between 1 on 1 and 1 on 2.
The danger of undercutting high, noncaving banks is obvious. It is less apparent when a bank caves and slides steadily when dug at the bottom, preserving a fairly uniform slope. Such a formation may gradually become too steep to be stable, without giving any indication of its condition. A change in moisture content, a blast, thunder, or the dropping of a rock may start a slump, which reduces the slope of the face by lowering the crest and advancing the toe into the pit.
The danger from such slides increases rapidly with bank height and steepness. In many cases, workers have been killed and machinery buried in them.
Changes in moisture content affect both internal friction and weight, and either drying out or becoming soaked may create or intensify unbalanced conditions.
Aside from this danger, a high, sliding bank offers the best possible bottom-loading conditions. Because of the constant supply of fresh, loose material, digging is easy, and the excavator has to move forward only at long intervals.
Damp clay will usually stand vertically when cut, but will slump or fall eventually. Vibration from passing machinery or nearby drilling is liable to break down its structure so that it will flow. If the movement starts at the top, a dangerous collapse may be caused.
FIGURE 10.21 Loading a hopper with a bulldozer.
A face of clay or silt exposed to alternate freezing and thawing or internal water pressure may liquefy and flow out on the pit floor, eventually assuming a gradient as low as 10 percent. This action is usually too slow to be dangerous, but should be allowed for in figuring clearances for haul roads or in parking machinery.
High, steep rock walls should be checked for fissures running parallel to the face, which would allow sections to fall off. These are particularly dangerous if filled with dirt that might absorb water and exert a push.
No high face of any kind should be undercut widely without adequate bracing.
The safest way to dig high, steep banks in general is with a drag scraper. Sometimes a dragline with a very long light boom is used to pull the crest down to the excavators.
On lower slopes, and on firm material, a dozer can be used.
Benching. It is usually good practice to limit the height of shovel cuts by taking the materials in a series of layers or benches.
Two methods of benching a hill slope are described in Chap. 8. Pits are liable to take much larger areas and require many more benches.
FIGURE 10.22 Benching from the top and bottom.
Benching may be done from the top down, or from top and bottom, as in Fig. 10.22. A boundary cut is frequently carried down below the pit floor when the higher parts are exhausted.
A large number of benches may be worked at the same time, or in rotation. Each bench should be large enough to provide ample space for shovel and trucks. If it is accessible at each end, oneway traffic can be maintained and the need for turnaround space avoided. However, narrow roads are often blocked by stalled vehicles, slides, rockfalls, or overbreaks.
If the bench is accessible from only one end, the shovel should work from that end so that the width of the new cut will be available to traffic.
When layers are taken from the top down, starting at a hillcrest, or a back line which will stand steeply, the working area may be made about as wide as desired. The excavating done on the top widens the area available for the next cut.
When cuts are worked up from the bottom, width is largely determined by slope gradient and face height; if the slope is 45°, a 1-foot (0.3-m) height is required for each foot (meter) of bench width.
Top benching is preferred for steep slopes whenever immediate access can be had to the top.
Material is frequently obtained by sinking the pit floor, or a part of it, in thin layers without developing a bank except at the boundaries of the excavation. The material may be piled before loading, in the same manner as topsoil, but it is usually more convenient to dig directly, with carrying scrapers or cable excavators.
Rooters may be used to loosen the ground for scrapers and for cable excavators except in wet digging.
The wheeled scraper is more flexible in digging, can vary the dumping spot readily, and by change of the number and size of the machines can excavate at almost any rate desired. The machine may also be used in other pit work or outside jobs when the cut is idle.
Under favorable conditions, the cable excavator can dig at lower cost per yard. The fact that it is difficult to move is sometimes in its favor, as it will be there when needed.
Rain. Rain will usually stop excavating and hauling operations. In addition, it may soften stockpiles and turn pit floors and haul roads into swamps or ponds so that work may not be resumed for days or weeks.
Some pits are in such porous soil that any volume of water will soak away quickly, and neither mud nor standing water will delay work more than a few hours. Others are in such dry climates that it is better to run a small risk of water delay than to spend the necessary thought, time, and money on arranging for drainage. The majority, however, are so situated that at least routine precautions should be taken to keep them usable.
A first principle is to shape pit floors so that they will drain. In cutting into a hill, the floor should slope slightly upward toward the face so that water will flow away from this line of greatest activity. If this rear drainage is not practical, the slope may be made to the side or to channels or drains in the floor.
If the pit is sunken, drainage can sometimes be arranged into a deeper portion which will take it off the working floor and allow it to soak away gradually. This will often be a pond dug under the water table to supply the plant with water. See Fig. 10.22(C).
If pumping is necessary, it should be done from a sump that will hold a large volume of water, and in which the pump can be protected against being covered if heavy rain falls while the pit is shut down. Such a sump may serve as a storage reservoir for plant supply.
Runoff. A pit may be troubled by water running off surrounding areas, either during rains or in the form of permanent streams, which should be diverted around the working areas or channeled through them in such a way that it will cause minimum interference.
The best practice is to dig diversion ditches to lead the flow in other directions. However, if the pit is expanding, these ditches will require relocating and may cost more than they are worth. This is particularly likely if the ground is steep or rocky so that ditching is difficult.
Also, the water may be needed in the pit for plant or dredge supply.
If the water flows only occasionally, it can be led through the pit in wide shallow channels which can be crossed by machinery and trucks at any point. If it flows often or continuously, it should be in a ditch and taken through haul roads in pipes or on rock-paved fords.
Layout, machinery, and methods in a pit may be affected by the water table, or level of groundwater.
This unseen water surface may be practically level, evenly sloped, or irregular. The water generally appears to be stagnant, but it is almost always in slow motion and will slope down from the source to the outlet. The angle of this slope is the hydraulic gradient, which is determined by the resistance of the material to the passage of water, the pressure and volume of the supply, and the relative heights of inlet and outlet.
Porous materials, such as gravel and sand, have low gradients, and tight ones, such as silt or clay, steep ones.
The water table tends to follow the slope of the land, but at reduced grades so that water will be farther from the surface in hilltops than in valleys.
Above the true water table is the so-called capillary table, which is kept wet by water rising in the spaces between soil particles. The finer spaces cause greater rise in fine-grained soils than in porous ones. Capillary water gives comparatively little trouble in clean sand and gravel, but causes serious softening of other soils.
Capillary water may come up higher when ground is compressed, as under a haul road.
Underground water ranges in quality from fine spring water to solutions of salt or chemicals unfit for any use.
A shallow pit may be kept well above the water table, which is then of little interest except as a source of water for processing. A pit carried sideward into a hill may cut into the table and have drainage problems of varying severity. A downward cut will get into water eventually, except in arid climates, or very tight ground.
Surface water falling into a depressed pit as rain, or flowing into it from adjoining areas, must also be taken into account in coping with the groundwater.
Permanent plants and all-year haul roads should be above the highest water levels.
When it is necessary to use materials lying at or under the water table, they may be obtained by wet digging, digging in dry seasons only, draining, pumping, and combinations of these methods.
Digging Underwater. Any machine which can dig from the top of a bank can dig underwater to some extent. However, there are a number of special difficulties, including inability to see the work, weaker penetration because of decreased bucket weight and interference of water currents, loss of material carried out of the bucket by water, and sloppy condition of the spoil. Wet banks are also more liable to cave under a shovel.
Loose underwater material, such as sand, is efficiently dug with a clamshell, as it is securely held in the bowl while being lifted. Draglines, or long-boom hydraulic hoes, have highest production. For large areas, hydraulic dredges are preferred.
If a lake or river borders a pit, or a large enough pond is dug in it, a hydraulic dredge can be trucked in, assembled, and floated. The largest dredges may be able to cut 100 feet (30 m) underwater, but 40 feet (12.2 m) is the maximum depth usually recommended. The spoil is usually pumped through a pipeline directly into a processing plant, but storage piles can be built. It is also possible to put the plant on the same hull as the dredge and discharge processed material into barges or conveyors.
The dredge can also enlarge the pond by undermining banks so that they cave within reach of its suction. High banks that do not slide should be lowered by land machinery to avoid danger of damaging the dredge when they come down.
Unless the pond is very large, or has a large inflow of water, the dredge may depend on a prompt return of water pumped out with the spoil. If the product does not contain many fines, water may be returned directly to the pond through a pipe or sluice. Fines can be filtered by allowing the water to drain back through gravel or sand, or by holding it awhile in a settling basin. In either case a larger water supply may be required than for direct return.
When the dredge has cut the working area to the maximum depth allowed by its ladder, it may be possible to reach further supplies by lowering the pond level. This may be done by partial drainage, by diverting the wastewater from the plant into other drainageways, or by combining diversion with pumping.
If the material is soft enough not to require cutting, suction pipes can be extended below the ladder to the desired depth, but recovery will probably not be complete.
Care should be taken not to locate sunken and wet pits where they will interfere with the orderly development of higher layers. It should be remembered that drainage through a bank may cause it to settle to a very gradual slope, which might run it much farther back into the pit floor than was intended. This type of spreading is particularly apt to occur when the pond is used as a water source, and wastewater returned by being allowed to soak into the ground.
Drainage. The costs of wet digging are generally higher than those of doing the same work dry. If the ground surface slopes down far enough in the vicinity of the pit, it may be more economical to undertake even a large drainage project than to dig wet or to pump.
Draining is particularly feasible when spoil from an open cut ditch is of the same type as that which is being mined in the pit.
Where possible, provision in the original work should be made to drain to the full depth of the outlet or of the deposit. This may involve a very wide top cut, stable side slopes, and trucking out of spoil. If the spoil cannot be used, trenching, installing of drain pipe, and backfilling may be more economical.
Sometimes it is practical to run a tunnel, or one or more drilled holes, from the low spot into the edge of the pit area where a connecting surface shaft can be sunk. Each level can then be connected in turn to drain into the shaft. Precautions must be taken against the entry of dirt or trash that might block the tunnels.
Sand and gravel will generally drain into either a shaft or trench with little further attention, but other deposits may require trenching of various types. The problems are similar to those involved in digging and draining a large basement, and are discussed in Chaps. 4 and 5.
When a sidehill cut gets into water, a curtain drain may be required at the toe of the bank to avoid damage to the floor from seepage or flowing water.
A high water table may be supported by an impervious layer separating it from well-drained formations, as in Fig. 10.23. Such a perched water table can be drained by drilling or digging through the impervious clay layer below.
Surface ponds on sand or gravel deposits will often drain if the silt layer on the bottom is opened. A few sticks of dynamite exploded on the pond floor, or a shallow dragline cut, may do the work.
Pumping. When drainage is not practical, water may be removed by pumping. This may be the preferred method if the water can be used in the processing plant, then discharged outside the pit.
Pit pumping follows the practices described in Chaps. 5, 6, and 21. It usually consists of removing open water standing against or over the deposit being dug. A small sump is generally made by digging part of the hole more deeply and placing the suction hose in that.
Success in pumping depends on the relation between the volume of water in the hole and the rate of inflow, and the capacity of the pump or pumps. Costs per gallon are usually smaller, and work can be started or resumed more promptly if the pumps are oversize and can handle many times the volume of inflow, so that a large part of their capacity can be used to lower the open water.
Extensive gravel layers may contain billions of gallons of water over areas of many square miles, which will drain into the sump. If no rain falls, the rate of flow gradually declines as the continued drainage flattens the hydraulic gradient, but this situation requires handling so much water that expenses are usually too high.
FIGURE 10.23 Perched water table.
If the gravel is of limited depth, much of the inflow might be sealed by cement grout forced down into the gravel at pit boundaries, or where flow channels are suspected. Pumping should be stopped before and during the grouting.
In gravel formations of limited size, and in tight materials such as clay and peat, the original rate of inflow usually declines rapidly, and the underground reservoir may be exhausted so that inflow will stop until it rains.
Each time a wet pit is enlarged by working, the pumping job of reopening it becomes greater because of the increased pond volume.
Pumping should be done in dry seasons when the water is lowest and interruption from rain least likely. In general, it is better practice to pump out, dig a large volume as quickly as possible, and allow to refill than to maintain pumping and a slow digging rate over a long period. This is particularly true in the porous, quick-filling formations.
Both surface and underground water are sometimes removed by pumping out of deep wells drilled in the pit floor or near its boundaries. This method is particularly suited to plant supply.
Pits may be opened casually by digging in a roadside bank, or large sums may be spent on investigations, plans, road building, and site preparation, before work is started. Most of them start with small equipment and output and increase in scale if they prosper.
If a large-scale operation is planned involving plant and other equipment bought specially, the building of haul roads, clearing and stripping, or if the deposit is of such value that every yard must be removed, then the formation should be carefully investigated for extent, quality, accessibility, and water conditions. An option to buy or to develop should be secured before this investigation.
Zoning. In many areas, zoning regulations absolutely prohibit opening a pit as such. Frequently, however, if it can be shown that the land will be improved by the work, other types of permits can be obtained which will allow limited or complete operations.
Favorable conditions include deep road cuts through hills and the removal of underwater deposits to make a lake. Other projects include taking away ridges that block view or drainage, or leveling of land for residential or industrial development.
It is probable that restriction of mining has been carried too far in many areas. Where acute shortages of sand and gravel exist, there are sometimes millions of yards of these materials made permanently inaccessible by reserving the land for houses.
On the other hand, there is no question but that a pit in a residential area is a dust, noise, and traffic nuisance, and is often an eyesore as well. A pit operator who takes care that no machinery is operated without mufflers, or on Sunday; that calcium chloride or oil is used freely to keep down dust; that the floors and banks are kept trimmed and reasonably neat; and that finished areas are promptly topsoiled and planted, will encounter minimum resistance to expansion or to opening other pits in the vicinity.
Wherever possible, operations should be screened from the public view by leaving or planting trees and shrubs, or by natural or artificial ridges covered with vegetation.
Permits. In areas which are not zoned against digging, it still may be necessary to get both state and local permits to operate, and to put up bonds to guarantee that the area will be smoothed and planted afterward.
Heavy fines may be imposed for any failure to abide by such regulations, and the operation may be closed down, perhaps permanently.
Investigation. The only certain way of finding exactly what is underground is to dig it out. However, inspection and mapping of surface indications, study of other pits or holes in the area, and talks with geologists and local old-timers can provide at least an idea of where to look.
Next may come some test digging and/or boring. A tractor-mounted hoe can dig inspection pits 10 or more feet in depth. If the soil does not cave, quite deep holes can be sunk by a clamshell of sufficient bucket weight. But do not go down in them, unless protected by very heavy bracing. Get your information from what came out, and possibly by inspecting the walls with a light and a mirror.
Rotaries and downhole drills grind or pound soil and rock to chips, sand, and dust. Spudders (churns) and some rotaries complicate the problem further by mixing the spoil with water. The resulting mud may contain material eroded from the walls of the hole.
Interpretation of such samples requires knowledge and skill. Augers give a fairly accurate picture of formations which they can penetrate, except that the material is loosened and mixed.
Core drills give excellent and reliable samples of rock, but may not pick up soft or loose material satisfactorily.
Any hole or shaft should eventually indicate the water level. In porous soils this takes a few minutes, in tight ones as much as a month. If a record of the seasonal rise and fall of the water is to be kept, the hole should be lined with perforated casing, unless it is in rock.
Market Analysis. The next step is to figure the extent and durability of the market for the products to be mined, and the likelihood of competition from similar projects or from small workings with less overhead.
If the total consumption of the products within shipping range is small, investment in a big plant would be inadvisable unless new outlets could be opened by lower prices, superior quality, or building of consumer plants. If the demand is large but is already adequately supplied, the question will be whether the new pit can supply better material or service, or cut prices, or create additional demand. The efficiency of existing pits and the extent of their reserves should be studied.
If the potential market is large, and the supply limited, the question of future competition will depend on the availability of similar material to others, and whether the intended plant will retain its relative efficiency long enough to repay its cost.
The amount of processing required to fit sand or gravel to specifications of highway departments and other wholesale users may be an important factor in costs.
Capital. A contractor already operating in other lines may not need to make any extra investment in a pit until business is brisk enough to justify it. In general, however, the minimum capital required consists of down payments on machinery and land or digging rights, and money to carry payroll, operating expenses, and installments until there is income to take care of them.
If the pit must build up its market gradually, or if the demand is seasonal so that stockpiles are accumulated throughout the year, to be sold during a short period, substantial capital will be needed to carry through the slack periods.
The necessity of selling on credit, so that short to long periods elapse between loading the material and getting the money, will sometimes tie up more capital than all the other investments combined. This problem is discussed in the next chapter.
The risks of pit operation, as of any business venture, are greatly increased by a lack of surplus funds or borrowing capacity reserved for emergencies.
Selling without Loading. A pit may be opened or operated without capital by the owner of the land or digging rights, by selling material “in the bank” to customers who will dig it themselves. Such arrangements are usually based on a price per yard, measured either in the bank or in the trucks. Occasionally, a certain portion of the deposit is marked off and sold, or the buyer may be allowed to dig all he or she needs for a certain job, for a lump sum.
A customer taking a substantial amount is usually expected to do her or his own stripping of overburden, keeping this section of the pit orderly and doing any required pushing back of topsoil after completion.
Sales of bank yards involve measuring the ground surface before and after their removal. This is usually done by surveyors, who work out a grid or a series of profiles. The original measurement may be fairly expensive. Later ones are much cheaper if the bench and location marks have not been disturbed. This method is best adapted to large yardages.
It is often necessary to limit a buyer to a small area so that he or she will not wander around picking out pockets of especially good material, making unsightly holes, and leaving substandard remainders.
Working Space. A pit which sells directly from bank to customer may require only enough working space to back in a truck. However, it is usually desirable to have a flat area in which to park idle machinery, to pile topsoil or other good material not immediately salable, and to place boulders and stumps until they can be disposed of.
If a crushing, screening, or other processing plant is used, space requirements are greatly increased. It is unusual to be able to sell products in the same proportions in which they are produced. There is usually a surplus of one or more grades that must be stored for future sale.
Since it is often easier to ship directly from the plants than to reclaim from storage, piles may grow when demand for their particular item is weak, and remain untouched when demand is good. This may result in steadily increasing storage requirements as the pit is enlarged.
If a pit is started in a small way and preserves a more or less level floor while being dug into a hillside, storage area may increase automatically with requirements. But if a big operation is to be started full scale, level land outside the pit area must be obtained or built up with waste overburden.
The plant is almost always located at or near the original ground level, and hauling products down to any of the cut benches for storage, and back out to sell, would be uneconomical. This type of pit will require storage space outside the digging area, which is best provided at the beginning of the work.
Excavating Patterns. Hill pits may be opened by a straight cut-in or by benching. After reduction to the level of the surrounding land, they are dug as sunken pits.
Subsurface workings, called sunken or dig-down pits, may be opened with front shovels and ramps, or by dragline or backhoe work from the top, in much the same manner as a haul-away basement excavation. The circular pattern shown in Fig. 10.24(A) and (B) is also widely used, for both subsurface and slopes with gentle gradients.
Backhoes can take gentle slopes in a series of benches, as in (C). It is necessary to level a strip for walking, as accurate loading is difficult on a slant.
This heading includes screens, crushers, and washers with their feeding and discharge mechanisms. These units will be described in Chap. 21, and are discussed here only in relation to pit layout and other operations.
Portable Plants. The simplest screening equipment is that described earlier in connection with topsoil. These pickup or skid grizzlies can be used wherever a shovel can work, but require ramps if they are to be used with tractor loaders. Their product is not well graded, as narrow oversize pieces pass through readily.
Mobile plants having screening, usually crushing, and occasionally washing, equipment mounted on one or more wheeled trailers, require from a few minutes to several hours to move up to a bank and start work. Short moves in the pit require less downtime than highway transportation as conveyors and other projecting parts need not be removed or folded in.
One of these units is usually able to eliminate primary hauling or to reduce it to a single truck shuttle, or a short conveyor. For direct loading from a low bank, particularly by a short-range excavator, it may be desirable to keep a tractor constantly on hand so as to move the unit up.
The use of portable units allows almost as much flexibility as in direct-from-the-bank selling. However, highway requirements keep their maximum size and weight below those required for many jobs; and the necessity of packing everything neatly in minimum space causes them to be harder to service than fixed plants of the same capacity.
They can often be used profitably for handling the more distant banks, or for filling special orders, in a pit that has a fixed plant. They may also be used for outside jobs or subsidiary pits. Hauling is a major cost factor in gravel and crushed rock, and the ability to open and process banks near the job may be valuable.
A mobile screener and crusher will reduce the risk involved in opening a new pit. Although not very readily disposed of, they are more or less standardized, and if found to be of the wrong size and type, can be sold or turned in, with a far smaller loss than a fixed plant.
Also, the use of one or more of these units may permit the pit to be developed until adequate space is dug for a fixed plant. If the pit is in a hillside, with steadily increasing bank height, getting the permanent plant well back in it will result in cheaper primary hauling over the whole job.
Fixed Plants. A fixed plant should be the right type for the material to be processed; it must be large enough for the job and within the capital budget. The first consideration is generally the most important, for if business is good, the plant can be expanded although at relatively higher cost; and exceeding a budget may be less damaging than getting the wrong equipment.
Plant manufacturers are ready to supply good engineering advice on every aspect of plant layout. It is a sound plan to get at least general recommendations from two or more companies, and to compare their findings with local practice. Even with these precautions, no person without a good working knowledge of the business should make a heavy investment in machinery from catalogs.
Plant Location. A permanent or semipermanent plant should be as close to the digging as it can be without being in danger from blasting and slides. If the pit is wide, or includes many sections, the plant should be near the center, or the side which produces the greatest yardage.
Dig-down pits may supply the plant by means of ramps and trucks, trucks with cable assists, vertical bucket conveyors, clamshells, or elevators. If supply is vertical, the plant should be located on the pit edge, or as near to it as firm footing can be found. This method is ordinarily used only in rock pits, but not all rock will support a factory on the edge of a cliff.
If the spoil is trucked up, it is best to locate the plant well back from the pit to allow for ramps, storage, and room for expansion.
Wherever a fixed plant is placed, the cost of hauling to it will increase as the digging progresses, whether laterally, downward, or both.
Taking down, moving, and setting up a permanent plant are usually tedious and expensive operations, particularly if it is of a large size. Even if it is of prefabricated, knock-down construction, rust, wear, and patching may make it hard to handle and foundations are generally left behind. Many millwrights consider it best to salvage only the operating units, and to order or build new frames to carry them.
It is usually sound policy to charge the entire cost of such a plant against the material that can be handled at its original location. If the pit area is definitely limited by property boundaries, zoning restrictions, or change of ground, and the depth of the deposit is known, the yardage can be calculated. It is best to make a liberal allowance for occurrence of unexpected masses of unusable material.
Pit hauling includes the movement of material from the bank to the plant or to storage, and between the plant and storage in both directions. It also involves delivery from these three locations to the job, although a variable amount of the product may be hauled from the pit in the customers’ trucks.
The principal hauling units for pit use are trucks, including semitrailers and full trailers, and conveyor belts. Railroad freight trains, of either narrow or full gauge, are used in big pits, the latter particularly in taking raw material from banks to a distant market. Digging units such as scrapers and cable excavators may also do a substantial amount of hauling.
Conveyor belts and cable excavators and, to a smaller extent, scrapers are largely confined to work inside pits. Trucks are equally adapted to inside hauling and outside delivery. On very long hauls, heavy materials are more economically moved by standard gauge rail.
Conveyor belts may be considered either hauling units or part of the plant itself. They move and elevate material with minimum effort, but are usually difficult to set up and locate. They may be used instead of haul roads and trucks for delivery of a heavy volume of material to a single point many miles away.
Trucks are excellent flexible, general-purpose units. They are available in a wide range of standard sizes and can be adapted to different-size loaders or production schedules by varying the number on the run.
Scrapers, to operate as such, need ground they can dig and hoppers which they can drive across, or storage areas giving them room to maneuver. Banks which they cannot dig can be loaded into them. However, scrapers are more costly and are usually slower than dump trucks of the same size, so it is not good practice to use them steadily under shovels.
A scraper can dump beside a sunken hopper which is kept filled by a dozer.
Truck hauls may be kept short by adding conveyor belts to the plant. The new belt will dump on the receiving end of the previous belt. Such installations may be quite long and are justified whenever considerable yardage will be handled.
Hoppers which are built so that the truck can drive straight across, instead of backing to dump, are more expensive to construct but will allow a faster truck cycle. Such hoppers can also be used for scrapers.
Selective digging may be done to separate, at the face, two or more materials of value and to remove them; to remove one or more formations, leaving unwanted material; or to dig two or more materials so as to combine them.
Any or all of the spoil from these operations may be hauled away or sidecast.
Layers. If the different formations are in vertical sheets, as in Fig. 10.25(A), any machine which is accurate enough to work the narrowest vein can be used. If they lie horizontally, as in (B), any excavator can move them if they can be cut as separate banks. If they are horizontal, and two or more must be removed at once, the excavator should be able to work from the top down. If divisions run in several directions, and separation must be exact, a Gradall excavator, with assistance from hand labor, could be used.
FIGURE 10.25 Selective digging.
When horizontal layers are separated by a dragline, as in (C), it should have a boom at least twice as long as the bank is high. The boom angle should be low and the dump cable short to make possible picking up the bucket at a distance.
A clamshell can do the same work with a shorter boom as no allowance need be made for space to drag the load.
Inclined strata fall into any of the above classes. In general, it is bad practice to remove enough of any layer to leave the one above it overhanging.
Selective digging is quite commonly required in stripping overburden, and in gravel and clay pits. The operator may have the responsibility of choosing the section of bank most suitable to plant or customer requirements, and supplying deficiencies by mixing different sections or layers.
Mixing. A good way to mix at the bank is to build a stockpile by dumping several materials on one spot. A conical pile will be built, with each bucket load separating and sliding down the sides. A succession of very thin layers will be made which, upon redigging to load, should mix together quite smoothly.
Such a pile will tend to concentrate round or coarse pieces at the bottom, but these will be remixed in handling by a machine working from the bottom.
Mixing is also done directly at the bank by digging from one formation, dumping on another, and then loading the two together.
If the layers are horizontal, digging in slices from the bottom up will mix them. Scrapers may bring to the top of the bank material to be mixed into it by loading.
Pockets. When an irregular deposit, with sloped or vertical edges and numerous interruptions, is dug from the bottom, a cluttered pit is left. All parts of it are accessible to digging equipment and trucks, but there may be little room to maneuver and none for storage. If the digging is done from the top, the area may become a badland, with little or no access and probably deficient drainage.
Such areas should be dug out, or smoothed down, at the first opportunity, particularly if any further work is to be done behind or under them. Many pits have strangled themselves out of business while they had ample reserves, because leaving obstacles to orderly digging has forced haphazard development with increasing excavation and hauling costs.
Cutting a pit floor by pursuing a good vein across it is a bad practice that is sometimes difficult to avoid. In general, it should not be done unless its value is sufficient to cover the cost of backfilling. If floor and walls of the cut are smoothed off, it may be filled with material to be stored which can be recovered when the whole floor is lowered. If no further digging is to be done in the floor, the slot can be filled with waste of a type which will not become too soft to carry pit traffic.
Cuts down from a floor, whether pockets or the start of new levels, should be near the outer edges of the pit.
Boulders. A common problem in pits which are dug without blasting is the occurrence of boulders too large for the loading or processing machinery. These are found in glacial and stream deposits, in disintegrated rock, and near steep slopes.
In pits selling only directly from the bank, there is no convenient way of utilizing boulders or of disposing of them. Blasting will reduce them to a size which can be loaded, but the market for coarse rock is so limited that they may have to be sold as second-grade fill, or wasted. The pit operator will generally prefer to allow them to accumulate along the bank, or will have holes dug to bury them. Occasionally, an abandoned pit is close and deep enough to permit disposal by pushing them over the edge.
If allowed to remain where they fell out of the bank, or pushed into occasional piles, they will present obstacles to orderly development similar to those left by pocket digging. In general, the nuisance value increases with the size of the pieces, relative to the power of the dozer which must handle them.
It is occasionally possible to sell boulders for use in jetties or breakwaters, at a price high enough to justify hiring a machine big enough to load them.
Stockpiling is most efficiently done on hard, flat, clear areas. Dumping may be done on the flat, off piles, or from side banks. The location should be convenient to the face, the plant, and the market, the relative importance varying with the use of the material.
Trucks. If available space is very large compared with the bulk to be stored, trucks may dump piles against each other, as closely as possible, without further grading or heaping. Large trucks make high piles and place maximum yardage in an area.
This method takes a lot of space, forms a bank too low for efficient loading by many machines, and causes maximum danger of mixing the stored material with the floor.
If packing by trucks will not cause damage, such a piled area may be smoothed off by a dozer, and one or more additional layers added. Factors limiting the maximum height are the slope in from the edges and the more gradual grade for the truck ramp, which steadily cut in on the area available at the bottom.
Figure 10.26 illustrates the building of a stockpile by backing trucks up on the dump and building it up in layers. Ramp grade should not be so steep as to strain the trucks or prevent them from dumping cleanly.
At any time the building of the top can be discontinued, and loads dumped off the end. The trucks are then usually driven up forward and turned on the pile.
These two methods can be alternated so that both height and area can be increased to the limits of the space available.
FIGURE 10.26 Truck-and-dozer stockpile.
If the material is somewhat too soft or loose to support trucks, the road may be strengthened by the use of wire mats, or small quantities of screenings, soil, or other binders, if their use will not spoil the value of the stockpile. However, it is usually easier and safer to use other piling methods with such material.
Trucks can build a stockpile by dumping off a high bank. This involves a minimum of dozer work. The special problems are loss and contamination because of mixing with the bank.
Reclaiming may be done by a loader at the foot of the pile, or a dragline or backhoe at the top.
Trucks may be kept off a pile, either for safety or to avoid packing, by dumping on a level and piling by machine.
Heaping up may be done by dozer or by loader, on either tracks or wheels. The front loader is more efficient, as it can combine lift with push for higher, steeper piles with shorter moves and less power consumption. In discussions of jobs where either machine can do the work, “dozer” may refer to either of them.
If the stockpile can be laid out as a windrow, and the dumping kept close to it, the operation is almost as efficient as immediate heaping, allows the use of the dozer on other jobs, and makes hauling and piling work largely independent of each other.
The dozer heaps up stockpiles rapidly, is entirely flexible in placing them, or varying their size or shape, and can be used for a variety of other work. However, it must move its entire weight up the pile with each load, has constantly working tracks which may be subject to severe wear in sand or other abrasive piles, and may pack or crush soft materials so as to reduce their value seriously.
Choice of tracks or wheels depends on availability of equipment and the type of material. Wheels provide more compaction and cause less breakage into fines. They wear less in sand or gravel, but become ineffective under slippery conditions.
Loaders of either type can be used to reclaim the pile, loading it into trucks or carrying it to hoppers or to the area of use. If the carry becomes longer than 50 feet (15 m), the tires have an increasing advantage in speed and economy.
Clamshell. The clamshell is commonly used for building stockpiles from shallow dumps, barges, and railcars. A light rehandling bucket, with toothless lips, is used for loose material on a floor.
It can be used at the same time for loading trucks from either the dump or the stockpile. It often alternates hopper loading from the stockpile with the heaping work.
It is flexible in regard to placement and shape of piles, although not as much so as the dozer. It does not crush or pack.
Drag Scraper. If the storage location is fairly permanent, it may be more economical to heap up the truck dump with a drag scraper than with the trucks and bulldozer. The mast should be high enough to allow for piling the maximum amount to be stored, and the tail tower should be readily moved so that a wide area can be utilized.
The drag scraper may also be fixed to reclaim the material with a reversed bucket, and dump it into a hopper from which it is conveyed into the plant, dump bins, or directly into trucks.
If several classes of material are to be handled, both head and tail towers may be mobile.
A scraper installation does not have the flexibility of dozers and is not available for other work. However, it is very efficient in that only the weight of the bucket needs to be pulled up with the load, it does not pack or crush the pile, and only the bucket is subject to abrasive wear.
Carrying Scraper. The rear-dump scraper builds a pile in about the same manner as a truck. It can operate on a steeper ramp and in much tighter places, particularly if towed by a crawler.
The bottom-dump scraper is most efficient at building a long pile, in the same manner as a highway fill. The sides are built up, starting at the outer edges, as steeply as the material will permit. The entrance ramp at one end and the exit at the other are started at gentle slopes and steepened as necessary. The pile may be started full length, or made short, and extended when additional capacity, or an easier ramp grade, is needed.
A dozer or a motor grader should be available for at least occasional trimming of the surface. If the material tends to get soggy when water-soaked, or is unstable at the edges, compaction with sheepsfoot or rubber-tire rollers might be advisable. Ordinarily, however, the scrapers themselves provide sufficient compaction for stockpiling purposes.
Scrapers may also make shallow piles for rehandling by drag scrapers.
Conveyors. Stackers and other conveyors of the boom type will build high piles with material dumped into a hopper. If these machines are wheel- or track-mounted, they can be towed away from the pile so that it will build into a windrow. Width of pile can be increased or separate piles made by pivoting the boom or the whole machine.
These are most easily fed by a drag scraper or revolving shovel, but a skid-mounted ramp that will allow trucks to dump into the hopper can be made out of heavy timber.
Long conveyors, of either elevating or horizontal types, may be made so that a dumping device can be inserted at any spot desired. This makes possible the building of a windrow the full length of the conveyor, or making a series of separate piles of different grades. Lengthening the dump conveyor will increase the area which can be used.
Different-size pieces in a material being piled or distributed have a tendency to separate from each other, so that a disproportionately large amount of coarse pieces will be found in one part of the pile, and finer ones in another. This process is called segregation or separation.
Whenever size gradation is important to the use of the material, the system of handling must be checked to make sure that it either will not cause segregation or will include adequate remixing.
Even if aggregate is to be rescreened before use, as in a mix plant, the screens must be supplied with a proper assortment of sizes.
Dry. In dry materials, a principal cause of separation is sliding and rolling down slopes. When a pile is built from the top, material falls or settles onto the pile, picking up greater or less momentum from the downward movement.
Small particles develop little energy, and tend to come to rest almost immediately. Larger ones have enough momentum to slide or roll down the surface, the biggest tending to reach the ground, while intermediate sizes stop on the slope.
The extent of the separation increases as the range in sizes becomes greater, and when the material has low internal friction. Fine or sticky material tends to pile up to unstable slopes and then fall or slide in masses, minimizing separation. For example, damp sand does not usually separate, dry sand or fine crushed rock separates a little, while coarse-and-fine rock and rounded river gravel may segregate almost completely.
When sliding in chutes, or subjected to random movements or strong vibration, large pieces tend to be wedged upward by small ones working under them, with a layering effect opposite to a top-built pile.
The segregation caused by building a pile of assorted-size pieces from the top, with a clamshell or a fixed-discharge conveyor, is shown in Fig. 10.27. The pile is always surrounded at ground level by a ring of the coarsest pieces. As the pile expands, its bottom layer is made up of these. Above it is a zone of somewhat smaller particles, with a fairly even gradation to mostly fines at the top.
Such separation is almost never complete, as big pieces will get trapped in the top and small ones get to the bottom, but it is often sufficient to make the pile unusable until recombined.
The clamshell operator can largely avoid layering by estimating pile area in advance, and building it in layers. Each successive layer must be enough smaller than the one below it to prevent sliding over the edges.
Remixing. A dragline working from the top of a pile, or a long-boom front shovel at the bottom, can make long shallow cuts from bottom to top, to get some of each layer in each bucket load. A clamshell should dig at various layers in turn, trying to provide a good average mix in the hopper or hauler.
FIGURE 10.27 Segregation in a dry gravel pile.
FIGURE 10.28 Layers in dredge pipe.
Layers are more or less mixed when loading is done from the bottom by a loader. Undermining causes the finer upper part to fall and slide, mixing with the bottom.
A hopper gate or other fixed-in-place reclaiming unit under a pile must take what it gets, so it is essential to keep material properly unsorted above it.
Thin Liquids. Some processes, such as hydraulic dredging, move mixtures of water with heavier solids. These behave more or less as true fluids as long as they are in sufficiently rapid motion, but separate into liquid and solids when slowed or stopped.
If speed of flow is marginal, several zones may develop in a pipe or stream. Coarse pieces slide or roll along the bottom, somewhat finer pieces are partly pushed and partly carried, and still finer pieces remain in suspension and function as part of the liquid (Fig. 10.28). The margins of the zones are moved up and down by turbulence.
With decreasing velocity, coarse pieces no longer move and medium sizes slide or roll sluggishly, until only fines are moving with the water. Buildup of the coarser parts is likely to plug the pipe.
On the other hand, if velocity is increased sufficiently, all pieces will be swept along as part of the fluid, although there is usually a higher percentage both of solids and of their coarse portion toward the bottom.
If such a high-velocity fluid is released from confinement, as at the discharge end of a pipe, it will lose speed and force abruptly. Coarse pieces will be dropped immediately, finer ones carried somewhat farther, and only very fine particles kept in suspension. This behavior is used to advantage in building hydraulic fill dams. See Chap. 14.
In a washing plant, it means that an open-pipe discharge to a stockpile will build a coarse center and top, and finer edges. As it grows, it may tend to have a fine layer at the bottom, and coarser ones toward the top.
However, if the material builds in steep slopes, this effect may be balanced or reversed by the gravity separation described for dry material.
A pit near a city may yield a much larger profit from being refilled than from the original digging. Disposal of raw garbage, incinerator ash, scrap, and industrial waste is a severe and increasing problem in many areas; and a worked-out pit or pit section may provide an ideal dumping area.
The pit owner may charge the city or the garbage contractor on the basis of each load dumped, by the cubic yard of dumped and settled or compacted waste, on a monthly or yearly service basis, or at a flat price for use of a certain area. Leveling, compacting, covering, and/or burning may be done by either the refuse hauler or the pit owner.
Arrangements for refuse disposal in or near settled areas should include detailed coverage of the methods of operating the dump, as it can easily become a nuisance, and in any case it would be subject to regulation, interference, and possible closing by state or local authorities.
There are a great many ways to dispose of refuse, ranging from open dumping of mixed garbage and scrap in fields, to filling of prepared holes with thorough compaction and immediate coverage with clean fill. If the dump is kept free of litter, and all garbage is promptly compacted and covered by dirt, it is a sanitary fill.
Segregation. A dump may be expected to handle a wide variety of material, from old automobiles and heavy steel scrap to semiliquids. Both of these extremes must be avoided on the main area of a commercial dump, as they make it difficult or impossible to handle rubbish or garbage efficiently when they are mixed with it.
Burning. Some or all of the material on a dump is usually burned. This reduces its bulk substantially, and often makes it more manageable by destroying long or awkwardly shaped pieces. Such burning may be done in a series of separate fires on the top, by keeping the forward part of the dump itself continuously burning, or by both methods.
Surface fires of wood, dry paper, tires, and other flammable scrap seldom cause disagreeable smoke or odors. However, fires in mixed materials including garbage may be offensive to workers and drivers on the dump, and to nearby communities. Such fires may be started by a surface fire working down, by hot ashes or spontaneous combustion in the trash, or by deliberate setting. They may go out by themselves, or persist in spite of determined efforts to extinguish them.
Spreading. Refuse is dumped from trucks and spread by a bulldozer or front-end loader, usually a crawler-mounted model. Rubber tires are too apt to be damaged by sharp pieces of metal. Tracks may get badly snarled with wire, cable, or bedsprings, but can be freed by wire or bolt cutters, hacksaws or other tools with little or no damage. A dozer operator on a dump should develop a special knack to avoid track tangling.
The dump may be built out in a single high face, in a series of thin layers, or in compartments. A high face is the least work, makes the worst mess, gets the least compaction, and is likely to cause maximum difficulty with rats and insects.
A dozer crushes and compacts the surface of the trash as it moves over it. A bull clam or a rolled-back loader bucket may be used to flatten it ahead of the machine. Compacting effect usually goes down 1 to 3 feet (0.3 to 0.9 m).
Compaction is desirable to increase the amount of refuse that can be put in the available space, to make a stable fill that will be able to support graded surfaces and buildings, and to reduce the number of rats.
Rats. Garbage dumps usually provide abundant food and shelter for large numbers of rats. They are a major nuisance. They spread disease. They kill or drive away many species of desirable birds and animals in adjoining wild areas. When a dump is closed, thousands of starving rats are likely to desert it and pillage the neighborhood.
Rats may be controlled to a limited extent by poison, inoculation with disease, and encouragement to use them as targets for .22 rifles. But the most effective way to suppress them is to compact the garbage so thoroughly that they cannot tunnel through it, and to cover it with dirt before they can feed on it. The same work eliminates breeding places for flies and mosquitoes.
Litter. Papers and other light trash articles blow around a dump and its neighborhood, producing an untidy appearance that may be one of the chief factors in local hostility to the operation.
Litter is controlled by enclosing the area with a high fence, putting wire or weights over papers as soon as they are dumped, and prompt burning or burial of all trash that might blow. It is often necessary to spend considerable time retrieving pieces by hand that have escaped in spite of one or more of these control methods.
Smell. Objectionable odors around a dump may be caused by decay, burning, or chemicals, or various combinations of these sources.
Decay odors can be almost entirely prevented by prompt burial in sanitary fills. Separate burning of clean trash, and avoiding fires in the dump itself will eliminate burning odors. Dumping of chemicals may have to be controlled or prohibited.
A pit that will contain any sort of contaminated material needs to have a lining to prevent that contamination from migrating to the surrounding earth and groundwater. This lining could be an impervious, compacted clay layer, a flexible membrane, or other geosynthetics. The latter are used in the United States to satisfy the Resource Conservation and Recovery Act regulations for liner systems.
Liner Quality. To ensure that no contaminants leak out of the pit, there needs to be well-executed construction quality assurance. That may call for a double geocomposite liner system. The system may have a primary liner and a secondary, high-density, polyethylene liner along with two compacted clay layers. The contractor would begin liner construction by preparing the earth base covered with a nonwoven geotextile. This provides confinement to sandy, base soil, allowing better compaction of the clay layer above it. Then the second liner of polyethylene membrane can be placed, and to ensure tightness, the seams are sewed together instead of merely overlapping the joints. The final compacted clay layer is laid to protect the membranes from cuts or tears that would destroy the containment quality of the pit lining. This liner system may be considered the ultimate lining, and one less complicated might be accepted as sufficient for site conditions.
A sanitary fill is usually a garbage dump in which the rubbish is thoroughly compacted in thin layers and is promptly covered with clean fill, and that is therefore free of persistent bad odors, large rat populations, and severe litter nuisance.
Dug-Cell Method. The cell method usually depends on obtaining clean fill from the dump area itself. Procedure is probably not exactly the same in any two dumps.
Figure 10.29 illustrates the parallel or double-trench method. First, a wide flat bottom trench is dug, usually by the dozer that spreads the garbage. The soil is piled to the side for use in final grading. Garbage is dumped at one end of the trench, where a ramp is provided to permit trucks to back in. Garbage is spread in one to six layers (two shown), depending on trench depth and thickness of layers. They are compacted by the back-and-forth and turning motions of the dozer.
FIGURE 10.29 Sanitary fill, dug-cell type.
Another trench is dug alongside the first one, in the waiting time between trucks. Fill obtained from it is pushed or carried across to cover the surface of the garbage, either the top layer only, as shown, or each layer as it is made. Thickness of dirt layers in between garbage lifts may be only 4 to 6 inches (10 to 15 cm), but the final cover should be 18 to 36 inches (46 to 91 cm).
A single trench may be used in progressive fashion, where fill for the day’s garbage is obtained from digging another section for the next day. Or the trench may be made by sidecasting, and backfilled from the side to cover rubbish.
Trench cell methods are efficient only in areas that have a good depth of firm dry soil, preferably with a sand or loam texture. Wet conditions would require using a dragline or backhoe for excavation and a pump to keep the trench dry. Mud difficulties would be expected until the bottom layer of garbage had been placed. Pump failure or heavy rains might stop work, and float rubbish out of the area. These conditions involve extra expense and nuisance.
Hard, coarse, or boulder-filled ground, such as makes up the floor of many old pits, will make cell digging impractical.
Ground level can be raised only a few feet by this method, because of the inefficiency of dozers in deep trenching.
Dug cells are therefore poorly suited to land improvement, as they can be made only in land that is fairly good to begin with. A community would use its rubbish to better effect by filling swamps or burying rough, rocky, or stump-filled areas.
Covered Fills. A sanitary dump for improvement of poor land may be made up of one or more layers of garbage covered by imported fill. The bottom layer should be deep enough to keep trucks and spreading equipment safely above water and obstructions. The dozer should make enough spreading passes to compact it thoroughly. The layer should then be sealed off with a 4- to 6-inch (10- to 15-cm)soil cover, and another garbage layer started. (See Fig. 10.30.)
FIGURE 10.30 Building up sanitary fill.
Maximum density is obtained if layers are limited to a compacted depth of about 2 feet (0.61 m). High density permits putting more rubbish in the same space, and prevents future settlement. Some types of trash compact satisfactorily in lifts as high as 5 feet (1.5 m), but this is unusual.
A garbage fill can be called sanitary only if clean cover is put on within a few hours of spreading the garbage. Fill trucks or scrapers may haul at the same time as garbage trucks, or stockpiles may be made from time to time within reach of the dozer.
At the end of the day the cover should be extended over the face of the layer, so that no trash is left exposed. Except in the first layer on wet ground, the dozer should walk down the slope of the face while spreading fill, to compact and seal it.
When a first layer must be built in standing water, the operator should try to work large, heavy pieces of trash out into it, to minimize the floating away of light pieces.
As with all fills, the surface should be given a slight grade preferably toward the face, to prevent ponding of water on it. The project should be arranged so as not to interfere with natural drainage either during filling or after completion.
Artificial Hills. If a community runs out of low places to be filled with garbage, it may decide to build it up into a hill. This idea usually does not create popular enthusiasm. In flat country, the dump may become the most conspicuous feature of the landscape, for example, the more than 50-foot (15-m) high sanitary landfill north of Glenview, Illinois.
But as long as it is built carefully by sanitary fill methods, its appearance need never be distressing. And upon completion, it can become a very pleasant park or a golf course, as is planned for the Glenview landfill.
Recreational Park. A landfill site in Cambridge, Massachusetts, was studied carefully to evaluate site conditions, development possibilities, and potential public health risks. The studies investigated settlement, combustible gas migration and generation, air and groundwater quality, radioactivity, storm water drainage, and revegetation.
After all was evaluated and methods were declared satisfactory, the landfill was made, creating a park including softball and soccer fields, children’s play areas, and others, along with 2.5 miles (4 km) of trails for walking, jogging, and biking. And there are about 20 acres (80,940 sq.m) of slopes planted with wildflowers and more than 800 trees. A 2-acre (8,094-sq.m) wetland meadow for storm water control and restrooms and parking facilities complete the landfill park.
