The discussion in this chapter will be limited to ponds varying from a hundred square yards to a few hundred acres in area, which may be built according to the judgment of the landowner or the contractor, rather than according to detailed specifications.
Such ponds may serve as small reservoirs for domestic and industrial use, or to provide water for fire fighting, for animals, and for fishing and other recreation. They may also be useful in swamp reclamation, groundwater replenishment, and flood control.
They may be made by damming streams, digging holes for streams to fill, digging below the water table, or combinations of these techniques. Dry hollows may sometimes be converted into ponds by diverting streams, tapping springs, or lifting underground water by means of windmills or other pumps.
During these modern times in the United States, before one can think of reforming the earth to build a pond and, perhaps, doing away with a naturally wet area, it is necessary to consider the subject of wetlands. Wetlands are those areas that are inundated or saturated by surface water or groundwater at a frequency and duration sufficient to support a growth of vegetation typically adapted for life in saturated soil conditions. Wetlands are valuable natural resources that provide important benefits to people and the environment. They help improve water quality, reduce flood and storm damages, provide important fish and wildlife habitat, and support hunting and fishing activities.
Clean Water Act. In 1986 the Congress of the United States included in the Clean Water Act provisions for the protection of wetlands. It laid on the Environmental Protection Agency (EPA), in partnership with state and local governments, the responsibility for restoring and maintaining the chemical, physical, and biological integrity of the nation’s waters. Consequently, the EPA is also charged with protecting wetland resources. The major federal regulatory tool for this is Section 404 of the Clean Water Act, which is jointly administered by the U.S. Army Corps of Engineers and EPA.
Types of Wetlands. There are two types of wetlands, coastal and inland wetlands. The coastal wetlands are linked to estuaries, where seawater mixes with fresh water to make a variety of salinities. The inland wetlands have fresh water commonly in flood plains along rivers and streams, in isolated depressions surrounded by dry land, and along the margins of lakes and ponds. In moving the earth, the wetlands of concern are the inland variety, except where dredging along the coast may have to be done.
Altering Wetlands. Physically wetlands might be altered by filling, draining, excavating, clearing, flooding, diverting water away, etc. They might be altered chemically by changing nutrient levels or introducing toxics and biologically by grazing or disrupting the natural population. Any of these forms of alteration needs to be planned with approval. Anyone filing with the Corps for a dredge-and-fill permit under Section 404 must first attempt to avoid or drastically reduce the impact on the wetland. Failing that, compensation through mitigation may be required for unavoidable wetland loss, by either restoring an existing wetland or building a new one.
Wetland Evaluation Technique. The Federal Highway Administration is teaching state departments of transportation (DOTs) and their contractors how to determine the value of a wetland with the wetland evaluation technique. The DOTs require the contractors to take a 4-day course before contractors are allowed to bid on a job involving wetlands, because created wetlands fail when the basics are not being done. If the site for a created wetland is too narrow and the slopes are too steep, the wetland will remain barren. Once built, a newly created wetland must be monitored by a state DOT or developer for 5 years to make sure it is self-sustaining or viable.
Soil Conditions. Swamps which are wet all year are logical places to dig ponds. The spoil taken out of the excavation can be used to build up the area around it so that the section worked is changed from a bog into open water and dry land.
Swamps commonly have a top layer of soft peat or muck soil, which may be of any thickness from a few inches (centimeters) to 100 feet (30.5 m) or more. This organic material is easy to dig but provides very treacherous support for machinery. Below the muck, any type of soil or rock may be found.
The reader is referred to Chap. 3 for techniques in handling the mud that is one of the usual problems of swamp work. Mud may be reduced or eliminated by working in a dry season; by diverting, draining, or pumping out the water before or during the work; or drying up the area by sump or well-point pumping. These techniques are discussed below and in Chap. 5.
It is very advantageous to get rid of as much water as is reasonably possible. Water prevents the operator from seeing the bottom he or she is cutting, with resultant wasted passes and gouging. It reduces the digging effectiveness of the bucket so that some soils which can easily be dug when dry cannot be penetrated when under 1 or 2 feet (0.3 or 0.61 m) of water. Even with skillful operation, water will mix with soil in the bucket, making sloppy spoil piles that reduce the amount of digging at a stand, and which sometimes will flow back into the excavation or cut off the shovel’s exit.
Excavators. For decades the dragline excavator has been the choice of equipment to excavate ponds. It has good reach and can excavate the slopes for the pond very well. More recently, a full-revolving hydraulic backhoe, also known as a hydraulic excavator, has been used to excavate ponds. It does not have the reach of a dragline, but the hydraulic excavator has more power to dig hard material. These forms of power shovels are described in detail in Chap. 13.
For this discussion of pond construction, the use of draglines will predominate so that there is not much concern for the footing of the excavator. Footing is a problem for the hydraulic backhoe, because it has to get closer to the area to be excavated than the dragline does.
Digging Plan. Figure 6.1 shows a general plan for digging a pond in a swamp and using the spoil to build up the unexcavated parts. A drainage hole is dug at the downstream end, and the water level lowered by a ditch drain or by pumping. A sill may be placed at the entrance to the ditch or the sump hole to hold water back a few inches above the floor of the proposed pond.
If the swamp is fairly dry, and the digging is fast and continuous. the removal of water may be unnecessary as the expanding excavation may keep the water at a low enough level that it will not cause trouble. Surface water may be diverted around the excavation by shallow ditches or dikes as shown, or allowed to flow into the hole.
If no obstructions prevent, the pond is dug from the center toward both sides, with the dragline walking along the longest dimension, which is usually parallel to the direction of water flow, as in Figs. 6.1 and 6.2. The machine keeps back far enough from the centerline that it can reach it with an easy cast. It usually works on platforms or other artificial supports, but if the swamp has been drained enough in advance to be firm, or has gravel soil, or a heavy mat of bushes, these may not be necessary.
FIGURE 6.1 Digging plan for swamp pond.
The bottom is kept on grade by digging just enough to let the water back over it. If there is not enough water to cover the enlarging bottom, the grade may be checked in the same manner as in a basement excavation.
The length of a pond of this type can be increased indefinitely without change of method. The width, however, is limited by the reach of the dragline and the depth of the hole. The reach determines the width of the strip in which it can dig and pile, and the height of the piles; the depth governs the part of that width which must be reserved for piling spoil.
If a wider pond is required than the machine can dig in one round trip, as illustrated, it must go behind the piles, drag or swing them away from the excavation, and then widen the hole.
Size and Depth. Calculation of the size and depth of the pond should involve a number of factors. A large shallow pond gives the most for the investment, at first appearance. A deep pond is desirable in that it can be fed by seepage from lower levels, loses a smaller percentage of its water by evaporation, does not lose area by silting as readily, discourages growth of bottom weeds, and is more suitable for fishing and swimming. Against these advantages are increased cost and a possible drowning hazard.
Deep ponds may often be obtained from shallow excavations, or without excavation, by building of dams and dikes; but for the present we will consider results obtained by excavation only.
The pond should be dug to a clean bottom, if possible, and should yield enough spoil to build banks 1 foot (0.3 m) or more above the water at the edge, and sloping up away from the pond for drainage. In a limited area that is to be reclaimed, an increase in water surface reduces the area of the banks and the amount of fill needed for them. Fewer yards need be moved for a large shallow pond than a small deep one of the same capacity; although a larger proportion of the yardage may have to be moved more than once.
FIGURE 6.2 Making the excavation.
Cut and Pile Relationships. Figure 6.3 shows some of the relationships between the cut and the spoil piles. The diagrams show a machine with a 40-foot boom, digging beside and behind itself, and have been simplified by assuming a constant dumping height of 12 feet (3.66 m); an increase in volume of the spoil of 20 percent; nearly vertical slopes in the cuts; a 1-on-1½ slope on the piles, and soft soil, permitting deep digging. Figures on the piles show cross-sectional areas. Muddy conditions cause piles to be lower and wider, reducing the width of cuts.
The dumping height increases with higher boom angles, an advantage partly offset by a shorter dumping reach. A low boom is preferable for deep digging and to obtain a good digging reach without casting the bucket. Extra pile height may be obtained by raising the boom to put a top on it, but keeping it low for the bulk of the digging. A dragline with a live boom can lower it for digging, and raise it for dumping during the swing; but the extra power needed limits this to occasional use.
The swell and the slope of the soil piles indicated in the diagrams might or might not be applicable to a particular job. The regular cross section shown will be found only in sand or other free-flowing, nonsaturated material.
The volume of the piles may be increased by moving the dragline behind them, and pulling them or swinging them back periodically. This extends the pile toward the rear and leaves room for more on top. This is practical if ground conditions permit work without platforms, but otherwise takes too much walking time. A second dragline working behind the windrow and swinging it back makes a very effective combination.
If the excavation is narrow and shallow, and the banks are narrow, a long boom dragline may excavate without building a pile by placing each bucket load in its final position.
Double Cuts. If the width of the pond is to be greater than can be dug in the two cuts described, the dragline may make additional cuts, first removing the soil windrow, then the ground under it.
The windrow is usually moved by standing behind it and digging with a short dump cable, so that the loaded bucket can be picked up without pulling in. It is swung to dump as far back as possible, and a level runway made to permit the dragline to work along the rim of the original cut, as in Fig. 6.4.
The runway is normally made at the original grade, as vegetation or drying makes it firmer than the soil underneath. However, if the soil is hard to cut, the runway may be lowered to improve digging efficiency. If digging is easy, and the dug soil dry and firm enough to support the shovel, the ramp may be made higher, as in Fig. 6.5, to increase the dumping height. The cross section of a pile increases about in proportion to the square of the height, so the advantage gained is important. The freshly moved dirt is left higher than the undisturbed part of the pile, as it may settle seriously under the dragline. If possible, the machine should be kept on the consolidated part.
If the pile has a wide top, it may simply be leveled by a bulldozer, or by the dragline raking and patting it as it travels along it. However, it should be remembered that the tops of piles are treacherous at best, the machine should be kept back from the edges, and the pile watched carefully for evidence of caving, particularly toward the digging.
FIGURE 6.5 Working from top of pile.
As shown in Fig. 6.5, a dragline working on top of a pile is able to not only move the whole pile back but to dig the ground under it in one operation.
For even wider ponds, additional cuts may be made. Each additional slice involves moving all the material which has been dug, with increasingly complicated patterns, and expense mounts rapidly. Usually more than two cuts are made in one direction only when the operation involves cleaning off vegetation and shallow digging. In deep work, trucking the spoil or removing it with conveyors is more economical when several rehandlings with a dragline are necessary.
Trucking. A flattened windrow, such as is shown in Fig. 6.5, may be used as a truck road if it is dry and substantial enough, and is connected with dry land at one end. The dragline, or backhoe, may be started at the far end and worked toward the exit. It may dig to final grade, leave a shelf for further dragline work, or it may load the dry material in the trucks and cast the wet stuff from the bottom to build up a new windrow.
Abnormal delays may be experienced in the trucking. It is usually not practical to build turnarounds or two-lane roads, so the trucks must back in the full distance from shore, and no other truck can enter until the previous one has left, thus leaving the shovel idle each time. There is also danger of trucks going too close to the edge, or encountering soft spots, so that they get stuck or overturn.
If the windrow can be connected with dry land at both ends, scrapers may be used to remove the dry upper part.
Material may be trucked away economically if the pond is to be large in proportion to the size of the shovel, so that several shovel handlings would be required; if there is insufficient space in the digging area to pile the spoil; if fill is needed elsewhere on the project; or if the spoil can be sold. Under some circumstances, the entire cost of making the pond may be repaid by the sale of the spoil.
Selling Spoil. Materials that might be sold out of a swamp include the organic earths, such as topsoil, muck, and peat (humus); and inorganic subsoils such as sand, gravel, clay, and loam. Frequently, the values of these materials are destroyed if they are mixed together, as the presence of organic matter makes subsoil undesirable or useless as a structural fill; topsoil or other organic earths are not salable if coarsely mixed with subsoil.
Swamp topsoil and mucks are generally too heavy in texture for use in lawns or gardens, but may improve greatly if left in piles for a year or two, or if mixed with sand or light loam. Nearly pure peats, however, are widely used to enrich soil, and command a fairly good market. These have a very high water content, and may shrink 50 percent or more in the pile.
If the spoil is to be used for the pond banks, or similar unloaded fills, organic and inorganic dirt is generally dug and handled together. If they are to be sold, the topsoil should be stripped and piled to the side, and the subsoil dug afterward in a separate series of operations. As the surface of the subsoil is often below water level, adequate drainage or a dependable pumping system should be provided before putting the shovel on it.
Separation of materials in several layers may be very complex. Examples are digging gravel interrupted by seams of unwanted clay and digging clay seams lying between sandy layers, the decision as to what is desirable being a matter of local demand.
If the dragline is large, or has a long boom in proportion to the area to be dug, these layers may be removed and piled separately, or they may be stripped back separately in the same manner as topsoil. Often, however, a better solution is to truck away the least bulky or sometimes the most valuable of the materials during the digging. Trucking may be possible only during a short period late in the dry season, after a protracted period of artificial drying by drainage or pumping, or only after construction of fills or timber roadways.
Selective digging should be done in the dry, so that the operator can see what he or she is doing. It will be discussed further in connection with borrow pits in Chap. 10.
Predraining. Whenever the soil is to be trucked out, or when very sloppy conditions are found in a swamp, it is advisable to investigate the possibility of drying up the area before work starts.
Almost any spot can be dewatered, at a price, by diversion of any streams or other surface inflow, and well-point pumping.
Sump pumping may be more economical when surface water can be diverted, and when underground reservoirs are small, or when relatively impervious soils cause groundwater movement to be slow. The soil must be fairly stable.
The sump is a deep hole with a bottom below the proposed digging. One side should be sloped gradually or terraced so that a pump can be set up wherever desired. Outlet hose, or a flume or ditch, must be provided to lead the water out of the area being dried.
For best results, pumping should be started around the beginning of the dry season. If the swamp is underlain by porous gravel or sand, most of the water can be removed from small basins in a few days, and additional pumping will be required only occasionally. In large basins, continuous pumping might be needed during the job. If the soil is tight, less water will be removed at first, and seepage into the hole will be reduced very gradually.
This operation should cause the swamp to dry up, so that difficulties with mud will be reduced or eliminated. However, the effect may be largely lost if heavy rains saturate it again before work is started.
Both the speed and effectiveness of sump pumping can be greatly increased by drainage ditches leading into the sump. A horseshoe-shaped trench enclosing the working area, but leaving an undisturbed space for entrance of machinery, is a convenient layout. Such a trench may involve piling hundreds or thousands of yards of spoil, but should be a good investment if it allows dry digging for the bulk of the project.
Peat will burn except when saturated with water, and a peat deposit drained as suggested might be entirely lost by fire. If this material had no value, this might be the quickest way of removing it. The fire would also loosen any stumps, and if deep, might consume them.
A peat fire might have black or bad-smelling smoke (or be almost smokeless), burn for a long time, and be difficult to prevent from spreading. Such a fire should be started only after consultation with local fire and police officials.
Choosing the right size dragline for pond dredging in swamps involves consideration of a number of factors. Small machines are more easily supported on soft ground, are easier to salvage if they get in trouble, can work in restricted quarters, and usually have a faster digging cycle. However, they must handle material more often to move it the same distance; cannot dig as deep or pile as high or penetrate hard material or take out stumps as effectively as larger machines; and cost more for each yard of digging.
Small draglines may be equipped with long booms to match the reach of large machines, but this reduces their speed and may interfere with stability. Additional counterweight is advisable, and the inertia of this and the reduced leverage on the bucket load cause it to take more time to start and stop each swing, increasing the cycle time by 1 to 5 percent for each foot (meter) of boom added. If the swing clutches or engine power is barely adequate to manage a standard boom, loss of time through excessive slipping or lugging down will be much greater than with a high-powered machine of the same rated capacity.
Undersized buckets are often used with extralong booms. These reduce the tendency to tip, and speed up the cycle somewhat, but reduce the payload and the ability to penetrate hard soil.
Digging Cycles. A three-quarter-yard (0.57-cu.m) dragline with a 40-foot (12.2-m) boom digging at the end of its reach, and swinging 180° to dump, can move part of the material 70 or more feet (21.3 or more meters) at each handling and complete two to three digging cycles a minute. The same machine, in digging up its own track, may move the dirt only 10 to 20 feet (3 to 6.1 m) at the rate of three to four buckets a minute. The average distance the soil is moved may be found by measuring from the middle of the cut to the middle of the pile.
If the spoil is to be moved back through several handlings by the dragline, the long move is desirable; but if it is being stockpiled, the short, quick cycle is best. Most digging patterns include both long and short swings, to get maximum work out of minimum walking.
Building Up Ends. If fill is needed at the upper and lower ends of a pond, the dragline may dig and pile in a curve as it rounds the ends, as in Fig. 6.2. Another method is to make straight cuts across the ends, as in Fig. 6.6. Such work at the outlet will block the drainage ditch, or interfere with pumping, so that it should be the last spot to be dug.
Drainage may be maintained by laying pipe in the ditch before covering it. If the spoil is to be spread after the first cut, such pipe may be left permanently, being fitted with a cap or valve so that it may be blocked to fill the pond, or opened to drain it. If two or more handlings of the spoil are necessary, temporary pipe must be placed and then lifted as the digging progresses, for which purpose corrugated steel is most satisfactory because of ease of handling and resistance to rough treatment.
Islands. Both the quantity and difficulty of digging can often be greatly reduced by building islands. In cases where a shallow cut is made, and the balance of the depth obtained by building a dam, most of the spoil can be disposed of in islands. If cutting is deep, some soil may still be piled on them and their bases do not have to be excavated.
Islands may be built at spots most convenient for disposal of spoil, or according to other requirements. A finished height of 2 or 3 feet (0.61 or 0.91 m) above water is desirable. In humus, piles 6 feet (1.83 m) or more above water level may be needed because of loss through shrinkage, smoothing the top, and slumping under water.
Grading should be done by the dragline immediately after piling, or at least before the pond is filled.
Trees. Pond sites are usually not as open and regular as the examples discussed above. One of the most common obstacles in the way of systematic work is a tree, or a group of trees. One tree can cause an increase in dragline time of 300 or 400 percent for the digging in its immediate neighborhood, and two trees may make the digging impossible without the use of trucks.
This is one reason why most pond diggers recommend making a clean sweep of all trees within boom reach of the excavation area. Another factor is that if the water level is raised by construction of a dam, or if fill is placed around a tree, even with protection for the trunk, it is liable to die, and large and old trees are particularly sensitive to any such changes around their roots.
Pond construction may cause injury or death to trees at some distance from the work. If a dam is built, the water level may be raised not only at the pond edge but to a decreasing amount for hundreds of feet (meters) back. Water level may be raised even when a dam is not built by ground-water backing up behind impermeable fill from the pond, or as a result of raising the grade of the banks. In addition, seepage may drown out trees below a dam. See Fig. 6.7.
Moderate lowering of the water table will only occasionally damage trees. Such lowering may be caused by digging into and draining a water-bearing layer formerly having an outlet on higher ground, or, in the case of a pond without a dam, by digging back into a bank.
Methods of protecting trees during construction, and from the effects of changes in ground and water level, will be discussed in the chapter on landscaping.
Removal of small or worthless trees near the pond, and of trees which would shade a beach or float, is desirable from landscaping and recreational standpoints. A strip of grass on the pond edge that can be trimmed will make it easier to keep the shoreline free of bushes and water weeds.
FIGURE 6.7 Changes in level of groundwater.
However, none of these factors justify an indiscriminate destruction of trees in the pond area. A tree may be a landscape feature of as much value as the pond, and the cost of digging around rather than through it may be only a small fraction of the entire cost of the project. Also, the cost of cutting and removing a large tree and disposing of the stump may be greater than the cost of operating the shovel for an extra day.
It is a good plan to consult a local tree expert, informing her or him of the scope of the excavation, the changes in water level, and the height of any fill to be placed near the tree, to get an expert opinion as to its chances of survival. This information may be given to the owner with an estimate for the job both with and without removing the tree.
Rock. Ledge rock and boulders cause less difficulty as they interfere only with the digging, not with the swinging. They may often be advantageously used for shoreline or islands, as a rock slope is usually more attractive than a mud or grass bank.
Rock to be blasted must be cleaned off, and should be above water during the work if possible. Standard blasting techniques are used. Refer to Chap. 9.
Bank Preservation. If the pond site includes a dry bank of suitable height, it may be advisable to leave it as one edge of the pond, digging away from it as if it were the centerline in Fig. 6.1 for the first cut, and disposing of all spoil on other edges. This technique can also be used to avoid tangling with trees or landscaped areas. If the pond is wide and without truck access, saving the bank may be too costly.
Digging Pattern. If a pond is to be excavated in the valley of a flowing stream, special precautions should be taken.
The diagrams in Fig. 6.8 show procedures in excavating a pond similar to that in Figs. 6.1 and 6.2, except that a brook runs through the center of the swamp.
A diversion ditch deep and wide enough to contain the brook is dug, starting downstream at a point lower than the proposed pond bottom, where it meets a ditch from a drainage hole, somewhat deeper than that described earlier. The diversion ditch is continued back to a meeting with the brook on the upstream side of the job. Spoil is piled on the pond side of the ditch to form a dike and to dam the original channel.
The dragline will have to walk across the brook. The banks may be dug away and platforms laid on them or in the water, or the stream may be partly filled with saplings or long logs to permit a crossing.
After diverting the water, one side of the pond is dug. A channel is cut below grade, midway between the centerline and side lines. At the upper end, the shovel cuts a gradual ramp up from the channel bottom, usually after crossing the dry streambed. This ramp should be covered with a brush mat weighted with rocks, or logs or planks should be placed crosswise in the bottom to avoid excessive erosion when the dike is cut and water permitted to flow down it. Sometimes a hole is dug below grade at the foot of the incline to trap most of the dirt washed down, and the protections are omitted. The water should now follow the channel in the pond bottom, and flow over the sill into the drainage ditch, keeping away from the centerline where further digging is to be done.
Another method is to straighten out the stream channel so that it lies on one side of the centerline. The other side is dug and the pile extended well across the stream at the upper end, forcing it to find its way behind the pile and back to the stream below the pond.
Temporary Stream Diversion. It is sometimes possible to put a temporary dam across the stream well above the work area, and to divert it across a low ridge into another valley, or into a trench, flume, or pipe running along a hill slope in the same valley. A large pump may be used to raise the water into such diversions.
Before arranging to pump out a stream, or building a flume or pipeline to carry it, its volume of flow should be measured. This may be roughly done by placing a rectangular wood trough or flume 15 to 20 feet (4.6 to 6.1 m) long in the streambed, and packing around the upper end with mud so that all the water will enter it. Chips of wood may be dropped at the upper end, and the time they take to drift through checked with a stopwatch. Mud can be dropped into the water to find if the bottom or sides have a perceptibly slower current than the top.
The flow in cubic feet (meters) per second may be found by multiplying the depth of the water in feet (meters) or fractions of feet (meters), by the inside width of the flume, and multiplying the product by the distance the wood chip traveled in 1 second.
If the stream has a fairly regular channel, the cross-sectional area of the water in it may be calculated, and the speed of flow measured in the same manner.
Most streams are subject to considerable and sudden changes in volume, and pumps used should have extra capacity, unless it is possible to abandon the job during high water and return to it when the flow is reduced to a volume that the pumps can handle. Diversion channels, pipes, dikes, and dams should also be built to withstand high water.
Digging in Streams. If the stream cannot be diverted, the digging of each strip should start at the upstream end and move downstream. The dirt loosened but not picked up by the bucket will be washed downstream in considerable quantities which might entirely silt up any downstream excavation.
Riparian Rights. Laws relative to stream use and pond construction vary in different states and localities. Where riparian rights law holds, owners of land on a stream below the job must give their permission before the stream can be diverted, even temporarily. They also can collect damages if mud from the excavation work chokes the stream, or is washed onto their property. Excavation permits are often required. It is good to have the law and the neighbors consulted before starting any important pond project.
FIGURE 6.8 Digging in streambed—gravity drainage.
Permanent Stream Diversion. A pond usually is kept in the best condition and appearance if a strong flow of water goes through it. However, if the pond is to be managed for the production of fish, or if the stream is likely to fill the pond with silt, it may be advisable to make a channel for the stream around the pond, keeping only a controlled flow from it into the pond through a pipe or ditch.
It is difficult to overestimate the power and destructiveness of even a small stream in flood, and it is at flood time that the greatest damage can be done to a pond. It is therefore important to take every precaution to prevent the stream from breaking out of its prepared channel.
The diversion should start, if possible, in a stream section headed in the right direction; and often should be reinforced with heavy rocks or posts driven into the base of the bank on the outside of the curve. If the turn must be in the artificial channel, it should be a gradual one, protected with rocks, posts, or a well-anchored timber bulkhead. A high earth dike should be built between the stream and the pond, planted with sod, bushes, or trees.
Figure 6.9 shows a safe arrangement. A low dam of concrete, masonry, or fitted rocks is placed across the stream to raise its level a foot (meter) or so. A pipe leading to the pond is placed below this water level, and a dike of earth or concrete placed over the pipe.
Water in excess of that which will pass through the pipe will flow over the dam into the channel. The pipe may open into a ditch, or continue into the pond. Flow may be reduced by means of a gate valve, or by partially obstructing it with a board, or by placing stones at its mouth.
Spreading Piles. After a pond is dug, it is usually surrounded by spoil piles whose size and arrangement depend on the reach of the machine, the digging plan, the shape of the pond, and obstacles to digging or walking. These piles may be left to dry for ultimate sale or removal, but more often are knocked down and graded into banks and slopes. This can be done immediately, but if time is available and considerable yardage is involved, they may be left to dry and shrink. Granular soils and some fine-grained ones should become firm enough that the shovel can work without platforms, saving time and work. Peats and mucks are less likely to become firm, but lose substantially in bulk and weight.
The dragline or a backhoe are the preferred tools for spreading such piles. A bulldozer can be used if the piles are dry, and is frequently used for finishing after the dragline has knocked them down.
A dragline spreads piles by a combination of dragging down with lifting and swinging. First the machine approaches the pile closely and digs off the top. Each time the bucket is filled, it is pulled closer to the shovel than necessary, sliding several times its capacity ahead of it. It is then lifted, swung, and dumped in a low spot, and the process repeated until the dirt piles against the tracks.
The shovel is then backed a few feet (meters), and the digging and dragging are continued, cutting to somewhere near final grade. The shovel continues to back and dig until the pile is exhausted, when it pulls down the lip in front of it, and walks up on the freshly graded area to work on the portion of the pile that was originally beyond its reach.
In Fig. 6.10 the pile is shown to be on the edge of the pond excavation. The dragline digs this shore to its final slope, widening the pond in the process. It is good procedure to cut banks back to a slope which will be stable under water, as it reduces the accumulation of soft mud at the edge of the bottom from parts of the bank sliding and falling in.
If the spoil is in windrows, the shovel may be walked parallel to the pile, digging and pulling it down until it starts to fall against the tracks, then moving on to wreck another section, continuing until the end is reached. It then comes back, parallel with the windrow but farther back, digging and dragging in the same manner. The ridge pulled against the tracks can be dug and spread behind the shovel. If the windrow is small, one trip may be sufficient.
Grading. It is sometimes possible to do the final grading while spreading the piles, but more often it is necessary to distribute them roughly to get an idea of the amount of material available, after which the area, or parts of it, is gone over for a light regrading. The finishing is done by raking and patting with the bucket.
There is probably no type of dragline work in which a skillful operator is as valuable as for spreading piles and grading banks. A long boom helps considerably.
Unless the operator is an expert, a better result will be obtained by finishing with a bulldozer or a grader, if the soil will support it. Also, it may be economical to make a quick rough grade with the dragline, and release it for other work while another machine finishes up.
When grading is complete, the area is usually disked and planted. Most peat soils need some time to drain, with the addition of lime, before they can support ordinary field vegetation. Testing outfits may be purchased at garden supply stores, and these may show the cause of trouble if things do not grow well.
Shore Drainage. The grades of the above-water parts of pond banks are affected by disposal of material, the securing of proper drainage, the nature of the surrounding area, and the personal preference of the person in charge.
A difficult drainage and landscaping problem is presented by a swamp sloping gently up away from the center, changing gradually from wet swamp to dry meadow. Even when tapered to a thin edge, the fill is liable to create a wet spot where it meets the meadow, either because a dike is formed holding surface water on the lowest part of the meadow, or because the whole fill is relatively impervious to water, stopping underground seepage and causing it to overflow at the top edge of the fill.
FIGURE 6.10 Leveling piles with dragline.
If possible, the excavation should yield a sufficient volume of spoil to carry it far enough into the meadow to be well above pond level. When sufficiently dried, the lower part of the meadow and the upper part of the fill should be deeply plowed and disked, then blended together with a bulldozer or grader.
If wet spots still appear, they can usually be relieved by mole drainage starting at the pond shore. If this is ineffective, tile or rubble drains may be required.
Shore Erosion. Freshly built banks will wash and gully badly in heavy rains unless protected. Drainage coming from undisturbed slopes across the fill is particularly destructive. This can often be diverted by shallow ditches made with a plow or by hand. These may be leveled after the banks are anchored by a firm sod.
Disking hay or straw into the surface of the ground increases its resistance to erosion and may supply ample grass and weed seeds. Unless applied with a nitrogen fertilizer, it may delay the growth of vegetation by temporarily absorbing this element from the soil. Such mixing in or scattering hay on the surface is helpful in holding soil that has been graded too late in the year for planting.
Beaches. If a pond is to be used for swimming, a beach is very desirable. It may also be of use for wading, picnicking, and getting small boats in and out of the water.
A maximum exposure to sunlight is desirable for at least part of a beach. This is best obtained by locating it on the north or east bank, so that midday or afternoon sun, or both, comes over the water. Reflection intensifies its heat, and the slope of the beach is favorable to its reception. In most localities, more swimming is done in the afternoon than in the morning.
If the beach must be located so that its sunlight comes over the land, it may be necessary to cut a number of trees to obtain exposure. If the beach is large, enough trees should be spared to give shade over some part of it, or over a lawn area adjoining it. Sometimes a tall tree that is removed may be profitably replaced with one or more smaller ones to shade a smaller area.
If the pond is being dug, or can be emptied, the beach site can be graded. A gradual underwater slope is desirable for small children and nonswimming adults. Vigorous swimmers are likely to prefer a steep underwater slope, particularly if the water is usually cold. The dry section is usually gently sloped or flat.
A beach must be protected against runoff of water from surrounding land, as this will wash away the sand, spread dirt on it, or do both. A grass-covered ridge immediately behind the sanded area will serve to divert water, and may also function as a very welcome windbreak and heat conserver.
It is desirable for the beach subgrade to be a cut rather than a fill, and be of firm material. If this is the case, 3 in (7.6 cm) of sand might suffice for a cover for swimming purposes, but not for building of sand castles. Six inches to a foot (15.2 to 30.5 cm) is a safer but more expensive depth.
If part of the pond bottom is sand or fine gravel, some of it may be pushed or carried to the proper location during the excavation work.
If the subgrade is soft mud, an attempt may be made to stabilize it with clean bank gravel, pea gravel, or fine crushed rock. A layer of lawn clippings or hay, placed immediately before the sand, may prevent mixing with the mud. This, of course, is not practical underwater.
An attempt should be made to extend the sand blanket to a depth of 4 or more feet below pond level, so that swimmers who are sensitive about walking on mud will be able to take off before they reach it.
Any clean sand that is suitable for concrete or plaster can be used for beach sand. Coarse grades are more attractive than fine, and light colors are better than dark. Where obtainable, white sand from ocean beaches or bars is most satisfactory, but it is apt to be much more expensive than pit or mason’s grades. Sometimes the bulk of the beach is made with sand of a cheap quality, and the surface dressed up periodically with a better grade.
A beach will usually require an additional 2 or 3 inches (5 or 7.5 cm) of sand after the first year or two, and occasional freshening up with smaller quantities afterward.
Fire Control. A pond is a valuable asset for fire fighting. Country fire apparatus and many city units have suction pumps so that they can get their water supply from ponds as easily as from hydrants. Even a small pond provides enough water to supply hoses for a considerable time. Many fire crews carry enough hose to utilize water 0.5 mile (0.8 km) or more from the blaze. An accessible pond may reduce fire insurance rates substantially.
Suction lines are short, so a rock fill or other firm surface should be provided to allow equipment to get close to the water in any weather. A deep hole should be dug near the shore. A wood or masonry wall to allow the suction hose to enter the water vertically instead of sloping down a bank adds to pumping efficiency. Any shallow bars separating the pumping hole from the bulk of the pond should be ditched.
Such a deep spot also serves well as a location for a diving board and an entrance ladder.
Clearing. Land clearing adds materially to the cost of reclaiming swamps, sometimes being more expensive than the earthmoving. Trees, and usually brush, should be removed or burned in advance of digging. Because of soft footing the cutting is usually done by hand, but tree trunks may be dragged out by tractors or winches.
Stumps. The stumps should be cut high in the area to be excavated, and very low where the spoil is to be piled. Height gives leverage which helps in digging them out, and affords a grip for chains for handling them, but makes disposal more difficult.
High stumps in the area to be filled will cause major difficulties during grading by hanging up machines, or tipping or breaking platforms, and by requiring excessive depth of fill.
When a large stump is dug, a slow process of cutting roots, overturning, and dragging out may be required. They are sometimes too heavy to pick up and must be dragged out of the way, cleaned and reduced by hand, or split by blasting.
Lighter stumps, which can be hoisted, usually cannot be held in a bucket and must be gripped with tongs, or other front end attachment, or chained.
Stumps should not be allowed to drag along the ground while being swung, as they will put a serious twisting strain on the boom.
Swamp stumps seldom have taproots, and the lateral roots are very close to the surface, so that they tend to come out as rather thin sheets. These can be most conveniently picked up by a pair of tongs inserted somewhere in the root mat, and chained to the bucket. If the ground is soft, the butt of the stump may be turned down, and driven into the mud in the fill area by patting the roots with the bucket. It is sometimes possible to entirely dispose of large numbers of stumps in this manner.
If this cannot be done, and the dragline or backhoe is working alone, it may be necessary to put the stumps in the spoil pile, where they cause trouble in rehandling. When the spoil is being spread, an effort should be made to rake out the stumps first and put them in the lowest spots, driving them in if possible in order to bury them. If they are too big to bury, they must be piled up to dry for eventual burning, or, more rarely, loaded in trucks and removed.
Digging in stumpy swamps is greatly simplified by the help of a crawler tractor, preferably equipped with a winch, which can stay on firm ground and pull the stumps away as the dragline or backhoe digs them out. These stumps may be winched or bulldozed into low spots for burial under the spoil; scattered around to dry before piling for burning; or piled immediately by passing the winch line through a pulley held high by a tripod, or a stout tree.
The high pulley arrangement decreases the power needed to drag the stumps, but unchaining them on the pile is a messy and somewhat dangerous job. The tripod or tree is usually destroyed when the stumps are burned.
Logging tongs are the preferred tool for gripping muddy stumps for winching. When a chain is used, notching the butt reduces the inclination to slide off.
Boulders. Large boulders also interfere with digging and are very likely to cut the drag cable if lodged in front of the tracks. It is sometimes possible to dig deep holes in which they can be buried, or to line them up along the edge of the pond where they should improve the appearance of the bank. However, they are more difficult to winch out than stumps, because of difficulty in getting a grip on them, and are an even greater nuisance in rehandling spoil. Often it is best to break them with dynamite, or air or hand tools, into pieces small enough to bury or mix with the spoil.
Hard Digging. A small dragline has great difficulty digging hard or rocky soil. It will do its best if the soil is not covered by water; if the bucket teeth are sharp; the boom held at a low angle; and the shovel footing kept as low as possible.
Occasionally a swamp floor may be of cemented gravel or decomposed rock which can be broken up with a tractor-drawn ripper. Such floors, and most hardpans, can be effectively dynamited if charges can be sunk deep enough in drilled or hand-dug holes. It is sometimes sufficient to blast a small area in which the dragline will be able to cut to depth, as it may be able to maintain this depth through the undisturbed material around it. If the whole bottom needs to be blasted, it will probably be cheaper to use other machines. Sometimes a single heavy blast will soften clay throughout the whole area.
Backhoe. A backhoe has quite effective penetration, but is hampered in pond digging by the inability to pile spoil at a distance. This limits the amount it can dig and exposes it to the danger of getting caught in slumping piles. A very good working team is a backhoe digging the hard soil, as in Fig. 6.11, and a dragline taking it away as fast as it is dumped. Best results are obtained if they work together, but because of the exact timing required to avoid accident, it is safer for the hoe to cut as much as it can pile and move on, with the dragline following at a discrete distance behind. At the end of the strip the hoe may turn and work back, building a new pile to be removed by the dragline.
Clamshell. A clamshell with a heavy bucket has good penetration but works quite slowly, and is at a disadvantage in sticky soils because of suction holding the bucket down. If used, it may do the digging and the casting back and spreading; or the rehandling of loosened material may be left to a dragline.
Front Shovel. A front shovel can break up the bottom, provided that drainage is adequate and dependable. The machine can operate on pontoons and ramp itself down to the required depth, and dig as wide a cut as it can reach, or is required, dumping the spoil in a ridge behind it. It may come out of the pond site elsewhere, or turn and emerge near the entrance point, as in Fig. 6.11. Material broken up in this way can be easily dug by a dragline, but may be so soft as to be unsafe even on platforms, until it is well drained.
It rarely happens that a layer under a swamp is sufficiently firm to carry trucks, but in this case regular basement-digging techniques can be used.
Bulldozer digging in softer mud is done by the methods described below for pond cleaning.
Water Level. The highest water level in a pond depends on the height of the overflow point, whether it is a streambed or an artificial spillway.
The best way to decide upon the new water level is to lay out a grid and take elevations throughout the area. The boundaries of a pond at any level can readily be sketched in, and the amount of cut required for desired depth, and the spoil to be disposed of in the dam and the banks can be roughly calculated.
A less laborious method which is usually satisfactory is to select some spot that would make a good shore and use a transit or hand level to find the corresponding shoreline at other points. This is done by reading a rod set on the selected point and moving the rod up and down any slope in question until the reading is the same, at which place it will be on the same level as the original point.
FIGURE 6.11 Loosening hard bottom with a backhoe or front shovel.
Readings can be taken on the dam site and on high, low, and normal points in the pond basis, and distances measured with a tape or by stadia.
Digging may be done according to patterns outlined previously, with one or more cuts made across the bottom of the pond and piled for the dam. Or the digging may be done parallel with the dam and all the spoil used in its construction.
A dam should fulfill three requirements. It must be high enough in relation to the spillway such that water will never flow over unprotected parts; it should be stable enough not to break, slump, or move under any conditions; and it should not leak.
Usually earth dams for small ponds are given a freeboard, or height above water, of 2 feet (0.61 m). If the spillway is wide, wave action very weak, and the material thoroughly consolidated, 1 foot (0.3 m) may be enough; but under reverse conditions, 3 or more feet (0.91 meters) may be required. For further protection, an earth dam should be covered with rocks, a strong sod, or bushes and trees.
No dam should be built to hold a depth of over 6 feet (1.83 m) of water, or any considerable volume of water which would flood an inhabited area if released, without competent engineering advice. In many localities, plans must be filed and permits obtained before building any dam.
Earth Dams. For stability, an earth dam should rest on a base of firm soil or rock without stratification, dipping away from the pond. It should be well bonded to its base by removing vegetation and plowing or ditching parallel to the axis of the dam.
The dam should be at least 6 feet thick at water level, and slopes should not exceed 1 on 2 on the downstream face, nor 1 on 3 upstream. If the top is to be used as a roadway, it should be at least 10 feet (3.05 m) wide.
The soil used should be stable enough to hold itself up, to resist both the push and the softening effect of the water, and to carry any traffic or other loads on the top of the dam. It should also be fine-grained and compact enough to give maximum resistance to movement of water through it.
Stability in the presence of water is best obtained by the use of broken rock or clean gravel, but these materials allow easy passage of water. Clay, and soils rich in clay, is best for sealing off water, but may be inclined to slump and flow when saturated.
Large Earth Dams. In large dams, a clay core should be used to stop the water, with loam or gravel faces to support the clay, as in Fig. 6.12. In such dams, the type and amount of each material must be calculated. The dam may be built up in carefully compacted layers from material carried from pits in trucks or scrapers; or the fill may be mixed with water and carried to the dam in pipes laid along its sides. When carried mechanically, the clay core is usually built up a step or two ahead of the faces. The hydraulic method mixes all the soil types together, but as they come out of the pipe, the coarse material is dropped first, at the edge, and successively finer particles as the water flows inward. At the center a pond forms, and fine clay and silt particles are deposited to build the core.
FIGURE 6.12 Hydraulic fill dam.
Small Dams. These methods are not well adapted to small dams. The expense of setting up hydraulic equipment can be justified only by large-scale operations. Mechanical transportation and spreading are handicapped by lack of width. Even small trucks, scrapers, and dozers cannot work readily in strips less than 8 feet (2.44 m) wide, or in comfort on less than 12 feet (3.66 m). The three sections would accordingly produce a width greatly in excess of that needed. A narrower dam could be built by the use of undersized equipment or hand labor.
A reasonably satisfactory dam may be made of mixed soil dug out of the pond or obtained nearby. This may be piled wet by a dragline or built up in compacted layers in the same manner as a road fill. Dusty soil should be dampened. If much sand or gravel is included, it should be mixed with fine-grained soil, or placed on the downstream face as much as possible.
If the dam is built of dry, uncompacted material, it should be allowed several months and some soaking rains to settle it before it is used to impound water.
If the soil is porous, the dam will leak unless sealed on the upstream side. It is not safe to wait for sediment in the pond to accomplish this, as leakage may liquefy the soil and cause the dam to fail.
The upstream face may be covered with a blanket of clay, heavy soil, or a bentonite mixture.
Bentonite. Bentonite is a volcanic clay which absorbs large quantities of water, changing to a jelly that effectively seals soil against water seepage. It is the expensive variety of clay mentioned in Chap. 3. It is used in many industrial processes and is available in bags in most cities. The pellet size is more desirable for pond work than the powder forms, which tend to float on the water surface for long periods and may be lost over the spillway.
A recommended practice is to mix one part of bentonite with four parts of sandy soil, or six or eight parts of fine-grained loam, and place a 4-inch (10-cm) layer of the mixture over the areas to be waterproofed. When more convenient, the pure material is spread over the ground and raked in. Satisfactory results are often obtained from more economical amounts, applied either in leaner mixtures or in thinner layers.
Either bentonite or the mixture can be shoveled into a pond over leaks, and allowed to settle into them. This is best done when there is no overflow.
Usable Materials. Small stumps can be used in a dam if the fill is muddy, so that it will form a close bond and fill cavities. It is good practice to cut roots back close to the butt. Boulders may be used in either wet or dry fills if the soil is carefully puddled or tamped around them, and they are not close to each other.
If the fill is rich in organic matter, considerable shrinkage must be allowed for in both height and thickness. Even after years of use, the dam may shrink still further if the pond is dry for an extended period.
Cutoff Trench. If the soil on the dam site is porous, a trench should be dug down to better material, approximately under the centerline of the dam. This should be filled with clay, well tamped or puddled.
If a deep layer of peat is found at the dam site, it would be best to find another place for the dam. If this is impractical, the peat may be dug or blasted out, or compacted by sand hole vertical drainage. If the budget does not include funds for any of this work, the dam may be built on the peat, and access for machinery provided so that it can be built up later if it sinks. If bulges appear in the peat above or below the dam, they should be left, as they serve to partly counterbalance the weight of the dam.
Settling and Cracking. In a dam, troubles to be guarded against are settling, cracking, slumping, seepage, erosion, and damage by burrowing animals. Settling is prevented by building on a firm base, using fill low in organic matter, and tamping or rolling it in thin layers if built dry. Cracking may occur in a dam with a high clay content when the pond level is low, and may be avoided by mixing in sandy soils. Such cracking rarely causes dam failure.
Slumping. Slumping may occur while building a dam with wet fill, and usually necessitates stopping work on the affected section until it has partially dried. Much more serious slumping may occur when water is impounded behind the dam before it is thoroughly consolidated. Wet fills that have not dried, or uncompacted dry fills which have not stood long enough to settle together, are apt to have this trouble. Seepage of pond water into the dam, softening it, and water pressure giving it a push all act together.
Water in a pond exerts pressure against its shores, which tends to balance inward pressure from the water they contain. If the pond is drained, removal of the water support may cause extensive slumping, which may be disastrous if it occurs in a dam.
A dam or causeway separating two ponds is particularly vulnerable if the lower pond is drained. For this reason, it is important to face dams with coarse, self-sustaining material that will resist slumping.
Seepage. No earth dam is watertight, as there is a slow movement of water even through clay. Water working its way from the pond through the dam is usually called seepage only when it is sufficient in quantity to show on the downstream side, where it may make wet spots on the dam face or marshy patches below it. Aside from the loss of water, such seepage may damage the dam by liquefying it until it slumps; or by making channels of increasing size by washing out particles of earth. Once definite channels have been established, the volume of flow may enable it to tunnel and destroy the dam.
The seepage appearing below the dam may damage it by undermining, but more often merely produces soft wet areas that may detract seriously from the value of the pond area.
Seepage may be largely prevented by cleaning and scarifying the subgrade, careful construction of the dam, using sufficient impervious material, compacting it well, and allowing it to set before raising the water level. If suitable impervious, clay material is not available, a plastic membrane can be used to reduce seepage. A 20-mil-thick (0.5-mm-thick) PVC membrane placed on the upstream side of the embankment and covered with a protective layer of granular fill will practically eliminate the seepage.
The surest and most expensive cure for seepage in an existing dam is trenching along it, with a backhoe shovel or clamshell, to solid foundation, and building or pouring a concrete core. This may be quite thin if of dense concrete treated with waterproofing on the pond side. Since a leaking dam is liable to have extensive soft spots in its interior, such a ditch may be dug safely only if the pond has been drained for several months, or very heavy bracing is used.
Driving a single line of sheet piling, or tongue-and-groove sheeting down the centerline of the dam, with grouting on the upstream side, is often effective.
The leaks may be stopped by laying a clay blanket on the pond side. The pond should be drained, if possible, to allow inspection. The leakage may be through the upstream side of the dam or in the pond bottom nearby. If the spots cannot be found, clay or fine-grained soil should be laid 6 inches (0.15 m) to 2 feet (0.61 m) thick on the whole face of the dam, and on the bottom, back about twice the height of the dam, and should be thoroughly tamped. The slope should be gentle, 1 on 4 to 1 on 6, and the clay should be covered with gravel or cobbles where subjected to action of waves. If the leaks are found, dig them out about 2 feet (0.61 m) deep, then tamp in clay or heavy earth patches. Bentonite mixture may be used instead of clay.
Impervious patches should never be applied on the downstream side, where the water is leaking out, if the leaks are low on the face. The water will generally work into or around the patch, soften it, and force it out. If the patch holds, the water held in the dam may liquefy parts of it, causing slumping and possibly complete failure.
The best treatment for the downstream slope of a leaking dam is to face it with gravel, with an underdrain below the bottom of the dam opening into the outlet brook, as in Fig. 6.13. The first coating of gravel should be bank run to allow the passage of water, while holding back any soil particles carried with it. Over the bank gravel should be clean coarse gravel or crushed stone, to correct any tendency toward sliding when saturated. If the area is to be planted with grass, stone should be covered with straw, hay, or cut weeds before placing topsoil.
This gravel blanket does not reduce loss of water, but it does stop damage to the dam and eliminates surface wet spots.
Seepage at the foot of the dam may be kept underground by tile and gravel, or stone drains, of the same type used in draining farmland.
If the dam is of pervious material, the methods suggested later in this chapter for stopping seepage into porous soil may be of use.
Overtopping. If the water is allowed to flow over the top of an ordinary earth dam, it may cut a gully to the bottom of it, draining the pond, wrecking the dam, and perhaps causing flood damage below. Freshly built dams are much more subject to damage from overtopping than old established ones that have set and are covered with vegetation.
Overtopping is due to the dam’s settling or slumping below a safe height, or an inadequate or too high spillway allowing the pond level to rise too much.
If a dam starts to slump, the water should be drained if possible and the dam allowed to dry, then the dam should be rebuilt with more or better material. If it is not possible to drain the pond, and pumping or siphoning is not practical, the dam should be reinforced by putting first gravel, then a heavy fill of coarse rock on the downstream side. An attempt should be made to puddle or blanket the pond side, and the top should be filled to grade. If it settles badly without slumping, the top should be built up, preferably with compacted fill. Sandbags, if obtainable, make an excellent temporary stop.
Sometimes a dam can be saved by partly draining the pond through a trench dug in firm ground nearby. Undisturbed soils can often carry a heavy flow of clean water without severe gullying, particularly if reinforced with roots, boulders, or brush mats.
Repair. When a gullied dam is fixed, the sides of the break should be smoothed and sloped sufficiently that the fill can be tamped against all parts of them, but it should not be cut into a straight ditch. The bottom should be dried up if possible. Fill should be dumped on the edge and pushed or shoveled down gradually, while workers at the bottom spread it in thin layers, tamping or tramping it thoroughly. If the break is large enough to allow machinery to work in it, it can do most of the spreading and compacting, but the bond with the walls must be done by hand. Dusting bentonite against the sides while filling should prevent seepage along them.
FIGURE 6.13 Seepage apron, small earth dam.
If it is not practical to dry up the bottom, fill should be dumped and kneaded until the water is absorbed into a stiff mud on which a layered fill may be built.
Burrowing Animals. Earth dams may be damaged by animals burrowing part or all the way through them. Muskrats make holes which run underwater to well under the bank, where they rise above the water. Such tunnels will cause leaks only when they give water access to some line of weakness that did not go through to the pond, or which had been silted shut. Muskrat damage can be largely avoided by using a low dam not containing enough dry ground for home building, or a wide one without porous veins.
Crayfish will at times dig burrows all the way through a dam, creating a water channel large enough to enlarge by erosion, unless a fortunate cave-in should block it. This damage is most apt to occur in soft peat soil, and it may sometimes be cured by injections of cement grout.
Burrowing animals may be discouraged by including ¼-inch (0.64-cm) mesh wire in the underwater part of the upstream slope. This affords fairly good protection for a number of years. It is usually laid on the dam, and 6 inches to 1 foot (0.15 to 0.30 m) of fill is spread on it.
Masonry Dams. Masonry dams may be used instead of earth fills. They are most suited to comparatively narrow sites with firm bedrock near the surface of bottom and sides. Reinforced concrete is the strongest construction, but fieldstone masonry is more attractive and may be less expensive in inaccessible spots.
Earth and decayed rock should be cleaned off the dam site, and the bedrock shaped or gouged in such a way that the dam will not be able to slide on it in any direction. Holes 2 or more feet (0.61 or more meters) in depth should be drilled in the rock, and reinforcing steel grouted into them so that it will project into the concrete or other masonry.
If the dam is to be more than a few feet high, it is advisable to have an engineer or a geologist check the ground, as fractured rock can make a leaky and unstable foundation.
The dam should have a bottom thickness of at least 2 to 3 feet (0.61 to 0.91 m) for every 3 feet (0.91 m) of height.
Masonry Cores. A masonry core dam consists of a thinner wall, preferably reinforced concrete, with earth piled on both sides. The masonry does not extend much above the waterline, and is ordinarily buried under earth. The core seals off seepage, and the sides support and protect it. It must resist the difference in pressure between the wet and dry earth on its two sides. Thickness is about one-fourth of height.
The core should be founded on a firm, impermeable material, preferably rock. The original surface is ditched for footings. The sides are carried into the banks until they meet rock, or until they are far enough from the water to make seepage unlikely. Rock should be roughened to hold the masonry against shifting.
The core is built and allowed to cure before placing the earth fill. The upstream face should be painted with waterproofing. If its ends are not keyed into rock, they should be fitted with vertical metal baffles sealed to the concrete, and the fill near the baffles should be mixed with bentonite. Failure to take these precautions may lead to serious leakage around the core.
Fills should be placed on both sides of the wall at the same time to avoid unbalanced pressure. If the dam is high, the fill should be carefully compacted. If it is low, this is not necessary unless final grading is to be done immediately.
The masonry core dam is the safest and most satisfactory construction for ponds, but is too expensive for casual use.
Removable Wood Dams. If a small pond is built on a small but fast-flowing stream subject to flood, there is the danger that not only earth or weak masonry dams will be washed out, but that, if the dam holds, the pond may fill completely with mud and debris in a single season, because the slowing and widening of the stream cause it to drop a part of its burden.
A removable wood dam may be used to advantage under such conditions. If the stream is narrow, 10 feet (3.05 m) or less, a heavy, well-founded masonry wall is put on each bank, having slots to receive 2- to 3-inch (5- to 7.6-cm) plank, as in Fig. 6.14. A masonry sill, similarly slotted, connects the piers on the stream bottom. Planks cut to the correct size and length are slid down the pier slots, resting on the sill and on each other, until the desired height is achieved. This structure will leak but will impede a brisk stream enough that part of it will flow over the top board, and the desired water height may be maintained. If the stream shrinks, the leaks may be reduced by jamming a tarpaulin in the sill slot, upstream, and pulling it over the dam face and top, and tying weights on the downstream side. Or tongue-and-groove planks may be used to cut leakage, with the top plank fastened down and all the joints packed.
FIGURE 6.14 Removable wood dam.
When a flood is expected, or pond use stopped for the season, the planks may be taken out and stored, allowing the stream a clear passage.
Gate Valve. When possible, means should be provided to drain a pond for repair, cleaning, and other purposes. The best, but most expensive, means is to place a metal or concrete pipe under the dam, connected with a gate valve, which may be located in the dam or at either end. Figure 6.15 shows an installation in which the valve is in the downstream face below frost line. To prevent burial and clogging, a vertical 8-inch (20-cm) pipe placed over the valve wheel extends to the surface, where it is plugged or covered. The valve is opened or closed by removing this cover and turning the valve wheel by means of a jaw on the bottom of a rod which can be turned from the top.
If the cover should be left off, and the vertical pipe filled with dirt and trash, it may be jetted out by the use of an engine-driven water pump, delivering water at pressure through a small pipe which is pushed down inside the casing, where it can break up and wash out the debris.
FIGURE 6.15 Drainpipe and gate valve.
Elbow Drains. A much less expensive installation, which can be used in climates where freezing is not expected, is shown in Fig. 6.16(A). A metal drainpipe under the dam is fitted with an elbow on the downstream end into which a vertical pipe is threaded. Space is provided so that this pipe can be turned into a horizontal position.
If the open end of the pipe is higher than the water in the pond, no water will move through it. If it is lower, the water will flow through it until the pond level is lowered to the same elevation. The pond level can therefore be adjusted to any height desired by turning the pipe up or down.
In cold climates, the exposed pipe would be subject to breakage because of water freezing in it. This is not likely to occur if the movable pipe is placed in the pond as in (B), because of less severe freezing and inward pressure of pond ice. However, the water makes the pipe difficult to get at, so that it must usually be moved by a line stretched to shore, as in (C). This will not pull it into a horizontal position and may have difficulty raising it from down position also.
A more satisfactory arrangement for underwater use is shown in Fig. 6.17. The drainpipe is extended by means of a tee, a close nipple, another tee, a short pipe, and a cap. The tees are fitted with pipes long enough to reach the surface of the water. These are set at an angle of about 45° from each other, and the tees welded together. One of the pipes is capped and a ring fastened to it.
This apparatus rests on a small block of concrete, which is cast around the edge of the drainpipe and around tar paper wrapped around the end pipe, but is enough below the tees so that they can turn.
Control is by a rope or cable stretched from the ring, past the vertical drainpipe to the shore. A pull on this line, by hand or machine, should raise the ring pipe and turn the drainpipe down. The drainpipe can be raised by pulling the line from the opposite bank.
With some risk of twisting the end off instead of turning it, the masonry block may be omitted and the outer tee replaced by a street ell welded to the inner tee.
The threads should be treated with waterproof grease or plumber’s dope, and wrought iron fittings should be used if possible.
Metal pipe is expensive in large sizes, and 6-inch (15.2-cm) is about the minimum for a pond drain, except for use in dry seasons only. Considerable expense may be saved by using concrete or tile pipe under the dam, connecting it near the end with metal pipe to the valve or other drain arrangement.
FIGURE 6.17 Elbow drain with pull arm.
FIGURE 6.18 Spillway drainpipe.
Vertical Tile. An overflow or trickle drain can also be made entirely with tile. A pipe is laid under the dam, ending on the upstream side in a concrete junction box, as in Fig. 6.18. From this a tile pipe with joints sealed with soft mastic rises to the surface. One of the pipes may have to be clipped short to obtain the proper height. The pond height is limited by overflow into the pipe.
The pond is drained by pulling the top pipe out of its joint and removing the next section when the water has gone down sufficiently, repeating the process until the bottom is reached. Sometimes the whole pipe will pull out of the box and drain the pond all at once. At other times, a pipe may refuse to move and may have to be broken with a hammer or crowbar.
The overflow type of pond drain serves to some extent as a spillway, but a regular or emergency spillway also should be provided for flood conditions, and because of the possibility of the pipe’s becoming clogged.
Pipes reaching the surface of the water can be protected against external ice pressure by tying several sticks or boards to the outside.
Drainpipes are a source of weakness to dams and must be carefully installed. It is best to place them before the dam is built, as this eliminates the difficulty of making a proper bond between fill and the wall of a ditch. Pipe joints should be watertight.
One or two collars of metal or masonry should be built out from the pipe, as indicated in the illustrations, and sealed to it by cement or welding. These will discourage seepage from following the outside of the pipe and cutting a channel along it. Clay, or soil mixed with bentonite, should be tamped or puddled around the pipe and the collars.
The first layer of fill should be spread rather evenly along the masonry pipe, as a full load in one spot might push it down enough to open the joints. If the ditch bottom is not firm, the pipe may be set on a reinforced concrete slab the width of the pipe and up to 6 inches (15.2 cm) thick.
A wood box, 3 feet (0.91 m) square or larger, may be built of rot-resistant wood around the upper end of a drainpipe and topped with ¼- or ½-inch (6.4- or 12.7-mm) mesh screen, to keep fish in while lowering the pond level.
Construction. Ponds which are made by excavation only, and do not raise the original water level, usually overflow through stabilized streams or channels that do not require any artificial protection against erosion. If an earth or masonry core dam is used, however, an artificial overflow channel, called a spillway, must be prepared.
A spillway may have a surface of any material that will resist the destructive action of the water which might flow across it. A steady flow calls for a structure, usually of stone or concrete, but occasionally wood, metal, or asphalt. A spillway that carries water rarely, as one which is intended to care for floods in excess of the capacity of a masonry or pipe spillway, or to provide for occasional overflow of a normally static pond, may be planted with grass or other well-rooted vegetation.
Spillway size may be calculated on the basis of the area drained, type of land and vegetation, and rainfall records in the same manner as culverts. However, a greater margin for safety should be allowed.
It is good practice to keep the spillway and dam separate if possible, as each is a source of weakness to the other. A recently constructed dam ordinarily lacks the stability necessary to support heavy masonry, and it is difficult to get a leakproof bond between dirt and stone. Any leaking through or around a spillway will be much more destructive to an earth dam than to a long established subgrade. On the other hand, practical and aesthetic considerations frequently require placement of the spillway in the dam.
If the dam including a spillway has a masonry core, the two structures can be combined. However, the core must be widened or buttressed, or the spillway provided with additional foundations as firm as the core wall. If the spillway is supported by a thin core wall and dirt fill, and the fill settles, the spillway will be left supported only at the core, and may break, or may twist and break the core. A preferred method is to extend the core footings far enough to carry piers to support the spillway.
If the overflow is to be carried around the dam, standard practice for masonry structures may be followed. Two more or less parallel walls carrying the water race, which may be a curve or a series of steps, is a standard type of construction. The structure is strongest if built of reinforced concrete well tied together, but stone and mortar make a more attractive appearance. The fill under the water race should be clean sand or gravel with good bottom drainage if the ground freezes in winter.
Settlement. If the spillway is to be part of a newly made dam, it may be based on the fill material, or may have footings in the native soil underneath the dam. In the first case, any settlement is liable to tilt or break the spillway and to settle away from it, leaving channels for leakage. In the second case, the masonry will stand firm while the dam settles under it and away from it. If the structure includes a core wall long enough to tie into the earth on each side, such settlement may not be serious.
Grouting. Leakage under a masonry spillway surface, resulting from dirt settling away from it, may be stopped by drilling holes in the masonry and pouring or pumping a cement and water grout into them.
A grout injector may be an air pressure tank or a pump. The tank is provided with an agitator to prevent separation. It is partly filled with grout and tightly closed. Compressed air is piped into the top of the tank, forcing the grout out through a pipe or hose in the bottom. The tank is opened and a fresh batch of grout poured in as often as necessary. Air should not be allowed to enter the outlet hose.
Special pumps may be purchased, or a fluid grease dispenser or a tractor grease gun used. Pumping can be continuous, with extra grout added as necessary.
The holes are drilled or punched to a depth where the leaks are suspected. The grouting tube may be fitted with a rubber collar to fit the holes and held in place by hand, if low pressures are used. For high pressures, a threaded iron pipe is cemented into the hole some days before and the grout pipe coupled to it.
The grout forced underground may penetrate and seal the leaks, may be washed away by water, or may escape to the surface of the ground. If possible, the pond level should be lowered to stop the water flow during grouting. The whole area should be watched for the appearance of grout, particularly at the leakage points.
A very thin grout made with 45 gallons (170 liters) of water to a sack of cement is good for sealing fine porous soil, but will escape readily through small channels. The thickest grout used, 4½ gallons (17 liters) of water to a sack of cement, will escape only through large openings, but does not seal fine passages effectively. Sand mixtures are not recommended for amateur use because of the tendency to separate, but sawdust or fine shavings may be mixed with grout used from pressure containers if the grout is otherwise washed out by water which cannot be stopped.
If grout is applied at a pressure of more than a few pounds (kilograms), care should be taken that it does not lift or break the spillway, or even split bedrock beneath. A tractor grease gun can develop pressure of thousands of pounds per square inch, and will break up strong masonry with little effort.
All grouting equipment should be thoroughly cleaned immediately after finishing the job, or for any shutdowns of more than a few minutes.
Wood Spillway. Trouble from settling under a spillway may be avoided by putting in a temporary structure upon completion of the pond, and removing or destroying it after complete settlement, then building the permanent spillway. Tongue-and-groove plank made into a box is a satisfactory construction. The dam surface on which the wood rests should be coated with bentonite, clay, or other fine-grained soil, and puddled until semifluid. The spillway should be stirred around or vibrated when set, and mud packed in along the sides.
A wood spillway may give satisfactory service for a great many years under favorable conditions.
Horizontal Pipe. Concrete, tile, or corrugated steel pipe of large size may be used, either, as described, under drains, or laid horizontally through the dam at water level, with the same precautions against seepage.
Water Supply. The ability of a pond to remain nearly full of water through a dry season is to a large extent the measure of its usefulness, except in semiarid sections where it is considered a success if it retains any water at all.
A pond level is kept up by water entering it through rainfall, surface wash, springs and seepage, and streams. It is lowered by evaporation, outflow, leaks, and seepage through sides and floor.
Once a pond is built, little can be done to add water to it except by pumping water from a well, by windmills or engine-driven pumps, or more rarely, diverting water into it. It is therefore important to locate and build it in such a manner as to take full advantage of sources of water.
Ponds dug in swamps may depend primarily on the water table existing before work is started. If possible, fluctuations of this should be watched for a year or two.
A dug pond may cut into active springs or extensive seepage areas which had previously been draining below the site, so that the pond may keep a higher level than the groundwater did. On the other hand, the swamp water might overlie a layer of clay or hardpan, which, when cut, would allow all the water to drain down into unsaturated porous soil, in which case it might be difficult to keep water in the pond.
The best way to estimate the water supply is to measure the drainage area. Figure 6.19 indicates approximate requirements throughout the country.
Seepage into Porous Soil. Outgoing seepage can be greatly reduced and sometimes stopped altogether by keeping mud in suspension in the pond water for some time. The water in seeping out of the pond takes the suspended particles with it and lodges them in the fine passages through which it travels, thus clogging them up. This process operating naturally over a period of years makes possible the existence of rain-fed ponds and swamps on sand dunes and gravel banks, high above the water table.
Digging in a pond will keep it muddy, as will driving livestock around in it several times a day. Fine-grained silt, powdered clay, or pellet bentonite may be scattered on the water with hand shovels, preferably when there is no overflow.
If the water is leaking through channels too large to be plugged by sediment, a layer of clay or a soil-bentonite mixture several inches (centimeters) in thickness should be spread over any outcrops of porous veins. If this fails to hold, the pond should be pumped dry and any leakage holes appearing in the clay should be dug out and filled with the blanketing material to a depth of a foot or more.
FIGURE 6.19 Drainage area map. A general guide for use in estimating the approximate size of drainage area required for a desired storage capacity in either excavated or impounding reservoirs. The numbers on the chart show the number of acres (× 4050 sq.m) of drainage area required for 1 acre-foot (1233-cu.m) of water impounded. (Courtesy U. S. Department of Agriculture.)
If the porous vein is comparatively thin and close to the surface, it may be sealed by injections of cement grout in the same manner suggested for spillways.
If leakage is along sod or brush which was not removed before placing fill for the dam, it may be stopped by chopping and mixing. A tamper such as is used in breaking street pavements can drive a narrow tool several feet underground, and repeated blows struck close together will mix the vegetation into the dirt so thoroughly that it will no longer provide water channels. Additional fill can be added to the surface if necessary, and a wide face tamping tool used for compaction.
Seepage along the old ground surface may cease when the vegetation rots, but this cure cannot be depended upon.
Seepage cannot be stopped entirely but will fall to a very small amount in a well-sealed pond, particularly if the water level is not high enough to create a strong pressure toward a nearby low spot.
Movement of groundwater is often nearly horizontal, so that much of the loss from a pond is through the banks rather than the bottom. This is one factor in the excessive shrinking of some small ponds during dry spells.
Evaporation. Evaporation acts constantly to remove surface water. It varies with heat, humidity, and exposure to sunlight and wind, and may lower the level of a stagnant pond from 5 to 15 feet (1.52 to 4.57 m) during a summer. This loss is most pronounced in desert regions.
The rate of evaporation is higher on small ponds than on large, and on shallow ponds as compared with deep ones. A number of factors are involved: The banks heat more readily than the water surface; capillary attraction draws water several feet up on the banks, thus increasing the surface exposed to evaporation; and a large body of water warms more slowly than a small one.
This loss from the water surface may be reduced by shading it with trees, but it is a question whether the trees use as much water as they save. If they are set well back from the edge, they may find a large part of their water supply elsewhere.
Dry Land Ponds. Losses of water through seepage and evaporation assume their greatest importance in ponds designed to fill with surface runoff in the winter or spring and to hold this water through a dry summer, even though the water table drops many feet below their bottoms.
Such a pond should be so located that the drainage from a large area will flow into it; not only so that it will fill even in years of subnormal rainfall, but also so that it will get the fullest advantage from any freak rains that might fall in the summer. But it should not be placed in the channel of a stream having enough force to fill the pond with sediment during flood time, or to require an unreasonably expensive spillway.
This pond may generally be dug in the dry season without any interference from groundwater. Any dry land excavator or set of excavators, down to small tractor-drawn scrapers and scoops may be used. Techniques are similar to those used in borrow pits and basements, except that banks must be sloped, not more steeply than 1 on 1, and it is usual to place a large part of the spoil so as to build up a dam.
Silting. Silting is a problem common to most ponds and reservoirs. Lakes of all sizes are shortlived geologically, because incoming water deposits sediments that fill them, and water flowing out tends to deepen its channel.
The amount of silting will depend largely on the local conditions. Steep slopes, cultivated or bare land, and fast stream flow bring heavy loads of sediment into ponds and cause them to fill rapidly.
Wastage of soil from farmland can be greatly reduced by contour plowing, terracing, and planting steep slopes to permanent grass or trees, with beneficial results to the land, the stream, and the ponds.
If it is not possible to alter watershed conditions, silt traps may be constructed. See Fig. 6.20. These may consist of small ponds built above the main one, or a very deep hole on the upstream end of the pond. Such traps should be so located that a dragline shovel and trucks can reach them for periodic cleaning.
Mud deposits found in ponds and lakes are made up of soil brought in by water or slumping from the banks; dust, leaves, pollen, and other debris falling from the air; and remains of plants and animals living in the pond. A combination of these sources usually produces a soft black mud which dirties and shallows the water. Near inlets and steep banks it may be chiefly silt or sand, and away from shores it is largely organic.
Removal. A hydraulic dredge removes such a deposit without draining the pond, but its use is often not practical. There must be enough work to justify transporting and launching it, enough water inflow or return flow for its needs, and adequate disposal areas.
Removal by machinery usually requires draining or pumping out of the water to avoid distributing disturbed mud throughout the pond.
After draining, the mud deposit will often be found to be so soft that it will not support machinery safely even on platforms. Given time, it will drain and compact so as to be fairly firm, in which condition it will not only support platforms but will stay in a dragline bucket. This hardening process, which may reduce its bulk as much as 80 percent, can be greatly accelerated by hand ditching into the subsoil for more thorough drainage. The ditching, however, is a sloppy job, and will be very discouraging at first because of mud flowing or slumping into the ditch and blocking it. The first digging should be very shallow and can be gradually deepened as the banks drain.
If the pond is narrow and accessible enough that all parts can be reached by a dragline on the banks, or if the mud overlies firm material that will support a dozer which can push the mud to a dragline, it may not be absolutely necessary to let the mud dry. If it is too thin to be picked up in the bucket, digging the ground under it several times may suffice to get enough of it out. In any event, such undercutting will eventually lower the mud so much that it will no longer be a nuisance.
It is seldom practical to just skim even dry mud off the old bottom. At least several inches of native soil are ordinarily dug with it, and this opportunity is often taken to deepen the pond substantially. In some cleaning methods, it is necessary to take enough subsoil to build firm piles.
The cleaning process differs from the original digging in the usually shallower cuts, the peculiar nature of the mud, the fact that trees and landscaping on the banks often must not be disturbed, and the undesirability of reducing the pond area by piling spoil inside it.
Bottom mud is generally useless for agriculture when freshly dug, but makes excellent topsoil after curing in piles for a year or two. Mixing with sandy subsoil speeds curing and improves its quality. It is often necessary to add lime to correct acidity.
Dragline. If a dragline can do the necessary cleaning from the banks, the problems are chiefly avoiding or cutting trees, and providing either places to pile the spoil or means of access for trucks to haul it away.
If the width is too great for the boom length, an unassisted dragline must work from the pond bottom, usually on platforms. From there it may pile spoil on the banks to be leveled off later; against the banks, to make a new shore for a smaller pond; load it in trucks on the bank, or build one or more windrows in the pond to be trucked out later.
Trucking windrows must contain enough inorganic soil that they will become firm as they dry, and must be high enough that capillary water will not keep them soft. This height will vary from about 3 feet (0.91 m) for a sandy mixture to 7 to 10 (2.1 to 3.0) for silt or clay. Lower piles, or any piles containing a lot of humus, may require a surfacing of better soil or gravel before they will support trucks. The height of the roadway will be substantially lower than that of the top of the original windrow.
The dragline may roughly level the piles as it builds them, or this work may be left to a bulldozer. It may be advisable to use the lightest dozer that can do the work, as unexpected soft spots may be found, due either to slower drying of sections of fine-grained soil or excessive amounts of humus in spots.
Since cuts are usually shallow near shore, and trees may interfere with maneuverability, it may not be practical to build the piles large enough to make a good land connection, in which case extra fill might be trucked in to bridge the gap.
When the windrows have dried and have been leveled off for a roadway, the dragline or hydraulic backhoe can walk out to the end of one, possibly with the precaution of using platforms or poles, and dig it back from the end, loading trucks backed to it from the shore. It may just dig the piled material, or go down into the pond, either to deepen it or to obtain fill. Sections of roadway may be left to form islands. (See Fig. 6.21.)
Use of this method involves deepening the pond 6 inches to 2 feet (0.15 to 0.61 m) or more. The double handling, the trucking, and the volume of material to be removed may make it prohibitively expensive for large areas, although it results in a pond which is better than new.
Dozer. If the bottom is firm enough to support a dozer, and if the mud is thick enough that a good load will stay in front of the blade, a dozer may provide the fastest and cheapest cleaning job.
The mud can be skimmed off gravel subsoils with little mixing. On softer footings, or wet soils which churn to mud readily, several inches may have to be taken with the mud. In any case, the digging down need not be as deep as with dragline work.
FIGURE 6.21 Trucking out piled mud.
FIGURE 6.22 Pushing mud up slot ramp.
Disposal of the spoil may be a critical problem. It is liable to be too sloppy to pile up high enough for a bank, and to contain too much organic matter to make a satisfactory shore.
It can often be pushed out. The average pond edge is too steep for a dozer to climb with a load, so a ramp or ramps must be cut in it (Fig. 6.22). If the shore is a dam, with low ground beyond, very watery muds can be trapped in the ramp entrance and pushed through. Because of light friction, a dozer may push five to ten times its normal yardage on each trip through the slot, but a part of the volume will be water.
The ramp is apt to soften and break down, particularly at the bottom. Also, disposal areas at its head may fill up rapidly. For this reason, a number of ramps are liable to be required, and backfilling these later may be a major project.
The front-end loader, with grousers bolted on every fourth or fifth shoe on each track, is the preferred tool for this work. The widely spaced cleats do not clog with mud. In cutting through heavy deposits, a front-end loader is more adept at side casting than a standard dozer, and can backdrag material out of bad spots. When this is done by filling the bucket, it may be necessary to float it while backing to better ground, as lifting it tends to make the front of the machine sink in. This machine can often unstick itself by using the bucket dump as a pushing or pulling device.
The second choice is a wide-gauge bulldozer. It usually has wide shoes that reduce its tendency to sink, and the width gives extra leverage for turning with loads on slippery footing. If grousers are worn down, a few of them can be built up to provide nonclogging traction.
When the bottom is reliably hard, a large dozer may be used. It is desirable because of greater production in both the volume of mud moved and area left cleaned by a single pass. It can also back into a deeper layer of soft mud without getting hung up than smaller machines with less clearance.
On soft or doubtful bottoms, lighter machines are much less apt to get stuck and are easier to rescue if they do.
Saturated clay, silt, or very fine sand may look and act firm when work starts, but soften under the weight and vibration of machinery. This change will be caused much more quickly by heavy than by light units. However, such soil will often continue to give adequate support to a dozer as long as it keeps moving, even after becoming too soft for comfortable walking. No machinery should be left standing for any length of time, particularly if unattended.
When a dozer is used for swamp digging, means should be provided for prompt rescue in case the bottom proves too soft, or careless operation gets it stuck. If the dozer does not have a winch, a hand or machine winch, or equipment capable of exerting a heavy pull, should be on the bank with sufficient cable or chain to reach any part of the area. Cut green saplings and hand shovels should also be available.
Drain holes in the flywheel and steering clutch housings should be plugged to prevent the entrance of water and mud. Plugs should be taken out periodically to drain any oil that might leak into them.
Fully sealed rollers which are greased on a twice-a-year schedule require no special attention. Other types may require greasing every 2 to 4 hours to prevent mud from working past the seals. Sand in the mud may make it very abrasive so that track wear may be several times as rapid as normal.
Under average conditions, dozer work in a pond bottom offers considerable danger of getting bogged down, and conditions are often found to be so sloppy that little effective work is done. But when it works, it’s fine.
Ramps. The ramp should be at the easiest possible gradient to facilitate pushing large loads and to minimize churning under the tracks.
When the ramp is roughed out, the dozer is backed into the pond mud until a good load is ahead of the blade or bucket. This is then pushed through the ramp to the disposal point, or parked in the ramp to be moved along with an additional load or loads.
The floor of the ramp will usually soften from absorbing water out of the mud being moved over it, and it will be worn down continuously by the push of the tracks and cuts by the blade. These effects are liable to be most severe in the pond at the foot where the dozer turns upward for its climb. A deep hole may be gouged here which will usually fill with a very thin mud. This ordinarily does not bother the machine any more than the same quantity of water, but will eventually reach the fan or other nonsubmersible parts, and the ramp will have to be abandoned or its foot relocated.
Such a hole may be convenient in freezing weather as the tractor may be placed in it overnight, so that the tracks will be underwater and the mud on them will not freeze. This will save a long and messy job of putting it up on blocks and of cleaning and hosing it at the end of the day. It is of course not practical unless the bottom is entirely safe.
The cleared space may be widened by other cuts fanning out from the ramp. This uniform expansion of area is not particularly efficient from the pushing standpoint, as mud tends to spread on each push over ground cleaned by previous passes, and both mud and subsoil become increasingly sloppy from reworking. However, it keeps the dozer close to dry land while bottom conditions are observed. This pattern is shown in Fig. 6.23.
FIGURE 6.23 Gathering mud near ramp.
FIGURE 6.24 Dozer-dragline team.
Dozer and Dragline or Backhoe. Ramp difficulties, or lack of nearby disposal areas, may make dozer cleaning impractical even when the bottom conditions are favorable. In such cases, the dozer may push the mud so that it can be reached by a dragline or backhoe standing on the shore or the dry pond bottom, which can pile it on the bank or load it into trucks, as in Fig. 6.24.
This method can be rather widely applied and is usually more economical than doing the whole job with the dragline.
Pump. Cleaning by machinery usually mixes some of the mud with so much water that it becomes too thin to be picked up or pushed, but can often be pumped. A diaphragm pump will handle heavier mud than a centrifugal, but the volume moved is much smaller.
If a water source is nearby, clean water can be pumped into a hose line and the mud stirred up, thinned, and driven to the mud pump or gravity outlet by a stream directed from a nozzle. If patience and workforce are sufficient, whole ponds can be cleaned in this manner.
After removal from the pond, the mud may be allowed to flow away from the work area, or to accumulate in natural depressions; it may be held in a settling basin from which it can be dug after it has dried; or it may be placed directly in tight-bodied trucks. The very thin muds which pump most easily are usually the hardest to dispose of. The contractor is liable for mud damage downstream or on adjoining property.
Water Plants. Vegetation in ponds is of many kinds, few of which are desirable.
The blue-green filamentous (stringy) algae, which grow in surface masses that may have considerable depth, are often a major nuisance. They may be dense enough to make swimming and even boating impossible. These algae grow vigorously (bloom) in the spring, and at irregular intervals throughout the warm months.
In large lakes, these plants can flourish only if the water is overenriched by drainage carrying sewage, detergent, fertilizer, or other nutrients. In small and shallow ponds, however, they often grow very well without such assistance.
Fortunately, such algae are very susceptible to poisoning by tiny amounts of copper sulfate, and die within a few days of contact with it. This chemical may be obtained in the form of coarse blue crystals from large hardware stores, or from dealers in commercial or agricultural chemicals.
The easiest way to apply it is to put in a burlap or other loose-weave bag, then tow it behind a boat, or pull it by hand while wading or swimming.
Two to three pounds (0.91 to 1.36 kg) to 1 million gallons (3,785,000 liters) of water, applied two or three times a year, should keep a pond clean. If it is allowed to become heavily overgrown, applications of two to three times that amount, at 2-week intervals, may be required.
Pond area may be roughly calculated from dimensions found by pacing, or by stadia as in Fig. 2.16. Soundings will indicate the depth. Area in square feet (sq.m) times average depth will give cubic feet (cu.m) of water. One cubic foot equals 7½ gallons (1004 liters/cu.m).
Copper sulfate cuts down the food supply of fish by reducing the vegetable food that is the basis of all animal life in the pond. Usually a balance can be preserved in which the plants are reduced enough to be unobjectionable but sufficient food is left for a large number of fish.
In the proportions recommended, or even in much heavier doses, this chemical is harmless to swimmers and to fish. However, the abrupt killing of very heavy plant growth may suffocate fish by absorbing the water’s oxygen into decaying material.
This danger can be avoided by using light, repeated doses, or by treating only part of the pond at a time.
There are a number of chemicals and compounds used to control algae in swimming pools, which can also be effective in ponds. However, their price is usually higher, and application may be more difficult without special equipment.
Many water weeds that resist copper sulfate may be killed by sodium arsenite. However, it has been found that this very poisonous chemical tends to accumulate in mud bottoms. Its use is prohibited in many areas, and even where permitted, it should be applied by an expert.
Pellets of clay containing 2,4-D or similar materials may be broadcast over the water surface. The pellets break up and release the poison gradually at the plant roots.
Emergent weeds such as cattails and water lilies can be killed by a 1.0 percent spray solution of 2,4-D, or kept from spreading by about one-tenth that strength.
Duckweed (lemna minor), a very simple plant consisting of small, flat, floating ovals with roots dangling several inches below, is a problem in many shallow ponds. It often covers the surface completely, giving the appearance of a pale green lawn.
Most of the growth can be killed by chemicals, including sprayed fuel oil spiked with 2,4-D, but it may grow back almost immediately. It may be necessary to remove organic mud deposits and deepen the pond.
State fish and game authorities can usually supply the names of contractors qualified to eradicate or control pond weeds. Permits may be required from state and/or local authorities for chemical treatments.
In small areas the most effective way to get rid of pond weeds is to pull them out by hand and with rakes, as often as they appear. This is a muddy job, but can be enjoyable as a group activity.
Hydraulic dredges are very efficient excavators in wetland. However, their use in digging small ponds is limited by the expense of bringing them in and setting them up (see Fig. 6.25), interference of brush and stumps with cutters and suction lines, lack of sufficient water, and problems in spoil disposal.
Pond-cleaning work does not usually involve handling stumps and brush, and most water weeds can be removed by hand or simple equipment. The pond provides some water. The competitive position of the dredge is improved by the great difficulties that land machines have in working pond bottoms too wide to reach from the shore.
This discussion will be limited to pond cleaning. However, the same problems arise in digging a new pond where water is available and vegetation can be handled by other equipment. Dredge construction, work characteristics, and operation are described in Chap. 14.
Water Supply. A 6-inch (15.2-cm) hydraulic dredge will pump water out of a pond at a rate between 800 and 1,200 gallons (3030 and 4540 liters) a minute, and will move 1,000 to 2,000 gallons for each yard (134,060 to 260,000 liters for each cu.m) of soil. An acre of water contains about 325,000 gallons for each foot (4040 kiloliters for each meter) of depth, or enough water for 4½ to 6½ hours of operation. Because of the slope of banks, each successive foot of water will contain fewer gallons than the one above it.
FIGURE 6.25 Loading a small dredge for transportation.
An 8-inch (20.3-cm) dredge needs about 60 percent more water and moves about 60 percent more soil than a 6-inch (15.2-cm) model.
If there is no natural inflow into a pond, it is necessary to return most of the water taken by the dredge to be used again. The most economical way to do this is to put the disposal area upstream, so that water will drain back. If this is not practical, a pump is placed so that it can return the water through a hose or pipe. A heavy-duty 4-inch (10.2-cm) pump can usually handle enough water to keep a 6-inch (15.2-cm) dredge busy.
Return water is usually dirty so that part of the dredge capacity is wasted rehandling the fines that did not settle out. The amount of circulating soil particles is greatest when the soil is fine-textured, and when settling ponds or areas are small.
The fine organic mud that forms a large part of the deposits on pond bottoms may stay suspended in water for long periods. The experimental uses of cyclones for separating soil particles from dredge water, and of polymers for clotting organic slime, which may reduce this problem, are discussed in Chap. 14.
Dirt in return water increases when the suction strainer of a return pump is too low, or the water is allowed to run on the ground and erode it. It is reduced by having a large settlement pond or area, and/or very sluggish open flow of return water. It tends to increase as the settlement pond fills up with use.
Reusing dredge water has the important advantage of reducing or eliminating the problem of fouling streams or other property below the fill with muddy water.
Any natural flow into a pond reduces the water problem. A pond with no inflow in the summer or dry season may have an ample supply pouring into it in the wet season. If the dredge is operated on a single shift, it could operate on a steady inflow of one-half or one-third of its output.
Disposal Area. Desirable features for a fill disposal area include location close to the pond, drainage back to the pond, good conditions for separation of soil and water, need for fill, and absence of trees and brush.
A 6-inch (15.2-cm) dredge may be expected to pump spoil from 800 to 1,500 feet (about 240 to 460 m), depending on its coarseness and weight, the percentage carried in the water, and the alignment and gradient of the pipe. Maximum distance is obtained with light load, straight pipe, and low lift. A downhill line could be much longer.
Production is reduced by long lines and uphill flow, as these conditions reduce both the volume of liquid and the percentage of solids that can be carried. For maximum output the spoil should be discharged close to the dredge, or downhill from it.
If it is necessary to reuse the water, and two or more dump areas are available, the loss of production from uphill pumping to get gravity flow back to the pond must be balanced against the expense of a return pump at the same or a lower level.
A close location reduces the cost of providing and handling pipe.
Soil settles out most rapidly and completely when water is still. A discharge or settlement pond is usually kept more or less agitated by flow from the pipe. Increasing its area and depth reduces rate of water movement and allows more particles to settle out. The overflow or the sump for pumping should be as far from the flowing water as possible. The size and depth of the pond diminish as it fills during work.
The cost of pond cleaning may be best justified when good use can be made of the material removed. The value of swampland may be greatly increased by building it up to a higher level. Rocky or stumpy fields may be filled over to smooth surfaces.
Fill obtained from cleaning a pond should include both a layer of the original bottom formation and rich black mud built up by the pond water. It is usually of rich fill or topsoil quality, and may be salable after drying.
Mud is held in the area to be filled by putting a dike around it. This may be built by a dozer on firm ground or a dragline where it is soft. A wood dam with movable boards, as in Fig. 6.16, will provide for overflow and height regulation.
Both the owner and the contractor may be liable for any damages caused on neighboring property or downstream by mud or too much water.
However, dredged material has been successfully used throughout the United States for the development of wetlands and aquaculture. It has been used for beach nourishment, shoreline stabilization, and erosion control projects. Also, its use has made for betterment of agriculture, forest, and horticulture. And the U.S. Army Corps of Engineers lists the use of dredged material for open-cast-mine reclamation, solid waste management, construction and industrial projects, and material transfer for fills.
Filling around Trees. Many swampy areas that should be filled are covered with vegetation ranging from brush to big trees. Undesirable brush usually survives partial burial, but trees are very likely to die. The biggest and most valuable specimens have a smaller chance of survival than younger ones.
Trees that will die anyhow should be cut before the fill is brought in. This will allow burial of stumps and unwanted logs. Such logs should be cut in short pieces to avoid later interference with trenching or basement digging.
If a tree is cut after ground is filled, there will be an unsightly stump. This will be extremely difficult to remove because of the depth of the roots under the new surface.
Brush should be cut and burned, or at least knocked down flat, before filling. Afterward it will be difficult to take out and will make grading very difficult.
Wood may decay very slowly under a hydraulic fill.
