-inch (7.94-mm) stainless steel rod, with a sharp point at one end and a handle at the other. This can be pushed down into any but the hardest soils.Landscaping may include the processes of cutting, filling, or grading to change ground contours; retaining or placing adequate topsoil; preserving, moving, or adding vegetation; and planning and installing walls, drives, and game courts.
An important purpose is to produce a pleasing appearance. This may be an end in itself but is usually secondary to the use of the land.
Landscaping is often the final step in jobs which involve earthmoving. It is required in connection with highways, particularly of the parkway or thruway type; to improve the appearance of home or business buildings not surrounded closely by other buildings and paved areas; to beautify parks; and to provide them with suitable recreation areas.
Plans should take into account proper drainage, which may include subdrainage.
Landscaping is often done under the personal direction of the landowner or a representative, but may be finished to grade stakes or left largely to the contractor’s judgment.
A large part of the annual landscaping bill is for work around homes and other buildings. Much of this is done during building construction or immediately after its completion, in connection with backfilling around the foundation, disposing of dirt dug for the basement or footings, and restoring surface drainage.
Such landscaping may include construction of terraces, retaining walls, and driveways; moving or planting of trees and shrubs; and making lawns.
The excavating contractor may perform the entire job or only the heavier parts.
Building Elevation. The type of grading close to the building is determined by its elevation relative to the land. The door sills or trim should be at least 4 inches (10.2 cm) above the finished grade of the topsoil. In general, exposure of more than a foot or two of foundation causes a building to look too high for current styles. The ground should slope down away from the building enough to prevent surface water from standing against the wall.
A building may be set high enough that dirt from the basement excavation can be used entirely in backfilling and grading up to it. If the floor level is determined in reference to the original grade, the bulk of the piles must be “lost” on the grounds, or trucked away.
Grading is also affected by the extent and type of basement excavation. A deep, full basement produces large quantities of fill, while digging for footings and a floor slab may yield little or none. When the building is to have a basement, is to sit low, and is to be built on a plot having a good grade, it will probably be economical to haul away all dirt not required for backfill around the foundation.
Desired depth of the foundation below ground line may be obtained by digging full depth and removing spoil; by putting the basement floor at the original surface and filling; or by an intermediate method. In general, the most economical way is to cut just enough to provide the necessary amount of fill to build the ground up to the building.
Rock and Water. The presence of rock or water near the surface may make a plot a poor investment, and in any case is important in deciding whether to have a basement, and the depth to place its floor.
Shallow rock can be found with a probe made of 4 or 5 feet (1.2 or 1.5 m) of -inch (7.94-mm) stainless steel rod, with a sharp point at one end and a handle at the other. This can be pushed down into any but the hardest soils.
However, it will not tell whether resistance is a cobble or ledge. A long sharp crowbar or prybar can be sunk by repeated dropping and turning. If it is stopped by an obstruction, lack of vibration as it strikes indicates a small stone; vibration only near the hole, a boulder; and a general jarring, a formation of bedrock.
Vegetation will tell a lot about water conditions. Bush willows and bog or bunch grass must have it wet in spring at least. Such water-loving plants on a flat indicate swampy conditions. On a slope they show a spring or seepage, and may warn of ledge rock as well.
If rock or a high water table is found on the site or surface drainage is poor, it is often good practice to reduce the depth of excavation and truck in fill.
No fill should interfere with drainage from adjoining property. If the land must be raised, drains must be placed under or around any dam that is formed.
A septic field on low or impervious ground may have to be placed in a filter bed (pervious fill) which may be quite costly.
Hill or Valley. A hilltop is almost always well drained, so that the wet basement difficulties discussed in Chap. 5 will not arise. On the other hand, it is much more likely to have rock close to the surface, so that the expense of basement digging may be three to six times greater than for dirt excavation.
Ground drainage can be too good. A person wanting to enjoy lawns and gardens will have difficulty with them in dry weather if they are on a heap of sand or gravel. Topsoil is likely to be poor, thin, and stony.
Building on low ground risks water trouble in the basement, if any, and the possibility of serious flooding from streams or drains. It limits view to the immediate surroundings, provides a higher average temperature but increases danger of frost damage (cold air flows downhill), reduces effect of cooling breezes in the summer and even more cooling gales in the winter, and usually provides rich and moist soil for lawn and garden.
If at all damp, a low site is dangerous to the health of arthritis and asthma victims.
Slopes may offer any combination of features of high and low land. Special factors to consider are that if the land slopes down to the south, it will be warm (or hot), and, down to the north, it will be cold in the northern hemisphere.
View. A building on high ground may be largely deprived of the enjoyment of a fine view by being set too low or too far back from a slope, or by careless grading or planting.
A common error is to build or fail to remove a high spot which, although lower than the house, blocks the view of nearby down slopes and hollows. See Fig. 7.1.
There is often much conflict between trees and view, which must be decided on a basis of individual preference. In general, ordinary young trees may be quite readily sacrificed while old trees or fine specimens of younger ones should be preserved if possible. Drastic pruning will often serve the same purpose as removal.
Shade. Shading a building and grounds from full sunlight is desirable, but too heavy shade will cause excessive trouble with rot and mildew and create unhealthy conditions, particularly for asthma and arthritis sufferers. Such trouble may be reduced by building in the open, by high trimming of branches of existing trees to permit full air circulation, and by use of discretion in planting.
Noise. If noise from a highway or railroad is of critical importance in determining building location, it should be remembered that noise travels chiefly upward, partly because of reflection from the pavement or roadbed. Even hundreds of feet up a hillside will not reduce it substantially if the source remains within sight.
If the river in Fig. 7.1 were a noisy highway, the construction which is wrong from a scenic standpoint would become right when noise only is considered. An earth berm or bank is a more effective sound deflector than a hedge or other planting.
Water Well Drilling. A substantial portion of both home and industrial building is in areas not reached by water mains. Most farms depend on groundwater for domestic use, and many use it for irrigation also. Factories, theaters, and other large users of water may find that they need a supply in addition to city water. Under such circumstances, the only method of getting a dependable supply of safe water may be to drill for it.
In sandy or gravelly soils, surface water outcrops, such as ponds and springs, give a rather good indication of the level and abundance of subsurface water. However, a well should go substantially deeper than this level, both for purity and for protection against unusual dry spells.
Where possible, it is best to get water from rock, or deep down in sandy soil. Danger of contamination is then negligible. Casing is driven down at least far enough to keep surface water and loose soil out of the hole.
Wells are usually located for convenience, on the first try at least, as prediction of underground water may be highly uncertain. This is particularly so when the soil is too shallow to provide safe supplies and water must be obtained from a rock formation.
Divining rods of various kinds are used in many sections to locate water. In tests these “dipsticks” have shown a somewhat better record than random drilling, but the difference can usually be accounted for by the good judgment of the experienced person who carries it.
The best place for a well for a residence is just outside the foundation line, so that it can be included in a small extension of the basement or connected by a short pipe, but can still be reached vertically from outside for pulling underground equipment and servicing the underground part of the pump. It is usually drilled and lined (cased) before the basement is dug.
Placing the well away from the building involves constructing a rather costly separate pump house which may pose a landscaping problem and will have to be connected to the building by water and electric lines. It does have the advantage of freeing the building from the noise of the pump and automatic switch, and the possible nuisance of water from leaks.
A well under the building is very convenient, and has become permissible because of improvements in pump design. The flexible plastic pipe and jet pumps, now most commonly used in drilled wells, can be serviced in spite of limited headroom.
Distance between sewage septic fields and wells may be subject to local regulations. Under ordinary circumstances, there is no conflict between having them in the same place if the well is deep, but there is a slight chance that the casing might crack or become disjointed and allow leakage into the water. For this reason, prudence dictates that the well top should be higher than the field, and at least 50 feet (15.2 m) away from it.
The truck-mounted spudding or well drill or rotaries and down-hole units have done the drilling. Flow in the well is measured by pumping or bailing.
A flow of 4 gallons per minute is considered adequate for a small residence, but double this is desirable to ensure a generous supply. A small water flow can be partly compensated for by a large storage tank.
Backfilling. In general, it is most satisfactory to backfill around a foundation after the interior horizontal supports for the basement walls are in place but before the upper framing of the building is started. This removes the piles of fill that form an obstacle and a hazard during construction, and provides space for entrance and piling of materials.
Backfill against fresh masonry must be done carefully. A heavy dozer should keep farther away from the wall than the diameter of the largest stone found in the fill, to avoid accidental punching of holes. It should not walk on fresh backfill parallel to the wall, because if it sinks on the side toward the building, it will exert a heavy thrust and be almost impossible to get out without causing damage.
Foundation backfill is seldom tamped when it is placed, but failure to compact it offers the danger of the loose dirt’s soaking up enough water during a heavy rain to crush the wall by hydraulic pressure. Good underdrainage around the footings, a proper surface slope away from the building, and compaction of the surface make such a disaster unlikely. Placing floor beams strengthens the foundation.
A foundation of concrete block is subject to damage even after curing. Unless the fill is wet, the weight of the dozer is unlikely to cause damage, but a stone may still be punched through the wall.
A front-end loader is the preferred tool for backfilling and grading around a building. Its ability to back and turn with loads, to cross graded ground with a load without excessive damage, and to place dirt exactly where it is needed enable it to accomplish much more work than a bulldozer of the same size. However, it cannot grade quite as closely to a wall because of the overhang of the back of the bucket in dumped position, and the fact that the bucket is little, if any, wider than the tracks.
Grading. Grading may be mostly or entirely a problem of disposing of surplus fill to the best advantage. At other times it will consist of arranging for proper drainage, removing objectionable humps or filling gullies, disposing of stone walls or boulders, reshaping to obtain a desirable view or to avoid an undesirable one, or rearranging contours for better appearance. These operations may produce a surplus of soil, or may require bringing in hundreds or even thousands of yards.
Soil in trenches and fills should be thoroughly compacted before the fine grading is done. Unfortunately, it is not common practice to attend to this on small jobs, with the result that an originally pleasing appearance degenerates badly in a year or two. Effects are bad when a level or evenly sloping lawn settles into humps and hollows, and are worse when game courts, stone walls, or paved drives are involved. See Fig. 7.2.
Trench backfill can be compacted by hand; with air, gasoline, or mechanical hammers; or with electric vibrators. If ample time will elapse before grading, ditches can be loosely filled then puddled by flooding with water. Full shrinkage will not occur until they have dried out, a process which takes a few days with porous soils and weeks with heavy ones. While wet, a puddled ditch is a dangerous trap for machinery.
Fills should be compacted by rollers or trucks. If trucks are used, each fill layer [preferably not higher than 10 inches (25.4 cm)] should be thoroughly rolled, first empty and then loaded. Running a loaded truck on loose fill puts a severe strain on its power train.
A medium-textured fill is more satisfactory for most purposes than either very porous or very clayey soils.
Lawns should not be perfectly flat for any appreciable distance. The maximum slope which it is convenient to mow is about 1 on 6 for long grades, and 1 on 3 for short terraces that are handcut. Steeper grades may be left in long grass; planted with vines, shrubs; or fixed as rock gardens.
Old Walls. In New England and many other sections of the United States, utilizing or disposing of old stone walls is a common problem in landscaping. They often contain huge stones which are so buried and bound that they are a problem to any but the largest machinery. For this reason, and because of the beauty of many of them, it is advisable to leave them in place when possible.
If the wall is to be removed, an attempt should be made to sell it. Weathered field-stone in small sizes is often in demand. Boulders can occasionally be used in deep fills, stream bank riprap, or breakwater construction. Prices obtained for large stone seldom more than repay the expense of handling.
If there is no market for stone, an attempt should be made to bury it. The bulk can be roughly calculated by measuring the length and the average height and width of the wall, including the underground part. If no gully or other natural disposal point is available, a hole or holes should be dug to contain somewhat more than the calculated yardage, allowing for a foot or more of fill over the top.
FIGURE 7.2 Irregular settlement after grading.
Excavation is done in the same manner as for a basement. Topsoil should be stripped off the area that is to be dug and regraded. The hole should be deep rather than wide, and might well be dug by a backhoe rather than a dozer, if one is on the job. A backhoe may dig a trench close along the wall, followed immediately by a dozer pushing the stones into it and regrading.
A backhoe is often more efficient than a dozer at breaking up a stubborn wall, as it can work out one stone at a time. However, it cannot transport the stone readily.
The rocks can be trucked away if burial is impractical because of shallow soil, trees, or landscaping. A front-end loader, a backhoe, or a big clamshell can break up the wall and load it. Trucks should preferably have bodies built to carry rock, or be so old and beat-up that damage will not matter.
Loading a wall is slow work. Even small stones may be hard to dig out when in groups, and big ones are hard to get securely in the bucket. Production in yards per hour may be pitifully low.
Retaining Walls. Masonry walls are frequently used to separate different ground levels. They may be required where the slope is too steep for plain earth, or used largely for the sake of appearance. In the first case, the wall may make up only part of the required rise, and an earth slope is continued from its top. Such walls must be strong and well founded if they are to give good service. They are subject to very heavy pressure from the dirt behind them, particularly if it slopes up from the top of the wall, and if it becomes saturated. Freezing will cause a push against the top of the wall and disruptive forces inside it. Tree roots can act to lift and overturn it.
Some cross-sections of retaining walls are shown in Fig. 7.3. The foundation must be adequate, or the wall will fail. If the ground under it is unstable because of its nature, recent placement without proper compaction, or frost heaves, the wall will break up or lean outward. It is therefore essential to found it below frost level on firm soil or rock. If the quality of the soil is questionable, a wide concrete footing slab may be poured.
FIGURE 7.3 Retaining-wall sections.
The thickness and strength of masonry required for a retaining wall are commonly underestimated. Results of under-strength construction are sometimes satisfactory, but often not. For safety, wall thickness at any point should be between one-third and two-thirds of the height of the wall above that point, and the top should be 6 to 9 inches (15.2 to 22.9 cm) thick. Minimum thickness is safe when reinforced concrete is used, when height is moderate, and the retained soil is well drained and stable.
Maximum thickness is required when a steep slope rises from the rear of the wall, and when the ground is very unstable. Another consideration is the strength of the masonry. Reinforced concrete is the strongest used. Plain poured concrete is considered stronger than concrete block, brick, or mortared stone. Dry stone walls have little resistance against thrust and should be kept low.
The push from dirt behind the wall can be minimized by keeping it well drained. A layer of gravel or other porous material should be placed along the rear face of the wall. A tile drain should be placed beneath the foundation, and there should also be “weep” holes through the wall itself.
Ground expands when the water in it freezes, and the surface slab formed in this manner can exert a considerable thrust. A slope or batter at the rear corner will deflect this pressure upward so the slab will slide on the wall instead of pushing it.
A vertical wall often has an appearance of overhanging. A backward lean or batter of ½ inch for each 1 foot (0.3 per meter) of height will counteract this. Such batter can be increased to any desired slope with some increase in stability. A face slope in a dry masonry wall may permit outward movement for some years before it becomes vertical or overhanging.
Geosynthetic Walls. The use of geosynthetics to build high earth retaining walls has been increasing as it is a more economical choice than concrete retaining walls. A comparison is shown in Fig. 7.4. There are two types of geosynthetics. One type is the geotextile-permeable woven or nonwoven synthetic fabric blanket, described in Chap. 3, for stabilizing ground for vehicle passage. The other type is called a geogrid, which is like a mesh with openings between strips of high-quality, strong polyethylene material.
Retaining walls built with geosynthetics may be 20 to 40 feet (6.1 to 12.2 m) high and can have a nearly vertical face, if that part is built with decorative face blocks or open block to allow viney growth. The geosynthetic reinforcing sheets, which are generally as deep into the fill as the wall is high, are laid horizontally on the earth as the fill is built up. The vertical spacing between sheets is dependent on their required strength according to the loads that will be applied on the ground above and back of the wall. That spacing may be from 1 to 3 feet (0.3 to 0.9 m). Polymers creep at stress levels on the order of one-third to one-half their strength. A wall designed for that level of stress will allow creep, whereas a design at one-tenth the strength level should avoid the problem of creep.
FIGURE 7.4 Comparison of retaining walls. (Courtesy of Hoechst Celanese Corporation.)
Poor drainage is a leading cause of retaining-wall failure. Therefore, the geogrid wall needs various provisions for drainage, as shown in Fig. 7.5. The mortarless, interlocking Keystones drain naturally (1). Surface runoff can be diverted away from the wall (2). Embankment flow can be directed to an outflow pipe (3). And the groundwater that may get into the reinforced zone can be directed to another outflow pipe (4).
Drainage. It is desirable that all areas be provided with sufficient surface slopes, proper subdrainage, or both, so that water will not stand anywhere and the ground will dry and firm rapidly after saturation. Particular care may be required to subdrain any soil touching basement walls or floor.
When the soil is porous sand or gravel and the water table is low, drainage is usually automatic and mistakes in gradient will show only briefly during rains. Impervious soils, however, demand care in shaping so that they will drain completely, not only when the job is completed but also after settlement of fills.
If pervious fill is placed on a relatively impervious native soil, the surface on the impervious soil should be shaped to drain, to avoid trapping underground water in pockets. If the native soil to be buried is pervious, it need not be graded for drainage, regardless of the type of fill, although shaping to avoid uneven depth of fill is still advisable.
Where areas are large, rainwater flowing on the surface may constitute a serious nuisance even if it does not erode the ground. At a price, such water can be caught in catch basins, then removed through underground pipes. Because of the expense of such an installation it is best to have it designed by someone familiar with the work. If this is not possible, pipe size should be figured in the same manner as culvert capacity, according to the maps and tables in Chap. 5.
FIGURE 7.5 Drainage for a geogrid wall. (Courtesy of Keystone Retaining Wall System, Inc. as published by Sweet’s Group, McGraw-Hill, Inc.)
If land tile is used, it will also function as a subdrain. However, care must be taken not to allow more surface water to enter it than it can easily handle, as the hydraulic pressure resulting from water standing in or over the inlets may force channels outside the tile, which will undermine or misalign them, with resultant impairment or destruction. If the important problem is surface water, concrete pipe or sewer tile with mortared joints is preferable.
All inlets should be protected with gratings firmly set in masonry. Lack of these may permit entrance of large objects or masses of material which will plug the drain. Gratings are usually larger in area than their pipe, to allow for partial clogging with leaves. The vertical or steeply sloped pipes up to the catch basin should have tight joints.
If backfill is not tamped in the trench made for the drain, it may settle and leave the grating standing up above the sod. This is unsightly, makes it vulnerable to breakage, and interferes with reception of water.
If a garage is below ground level, a catch basin in the driveway just outside it is necessary. This drain must be adequate, as its failure in a heavy storm will flood the garage and perhaps the basement.
Subdrainage. Land tile subdrains may be installed under lawns and gardens to correct saturated or oozing conditions, to speed up drying after rain, or to provide better growth conditions for plants.
Subdrains may be tied in with the tiling around the basement and with catch basin systems for surface runoff. They should drain to low areas when possible, as opening into storm water drains exposes them to damage from backed-up flood water.
Topsoil. Topsoil which has been salvaged in advance of digging may be spread as soon as the fill is graded off, or left piled until the building is finished. Immediate spreading provides a cleaner appearance, which is of particular value to buildings built for sale, but the topsoil is liable to become mixed with various sorts of waste, and to be severely packed by supply trucks.
Two-ton “toy” dozers or small compact loaders are good spreaders, as they are so light that they leave average topsoil in condition to be finished off by hand, where heavier machines compress it so that machine tillage is required. They also can maneuver among trees, retaining walls, and other obstacles with less danger of damage and far less loss of time than larger dozers.
A light wheel tractor with a front bucket or rear grader blade can do light grading.
Freshly spread topsoil or undisturbed field sod which is to be reworked into lawn is often loosened up with a rotary tiller. This machine leaves it soft and easy to work, and if the topsoil is thin will increase its usefulness by mixing in some subsoil.
It is not possible to state a general rule for the amount of topsoil needed around a building. Good topsoil has three important characteristics: it contains humus which absorbs water and doles it out to plants in dry weather, it contains a supply of available fertilizer, and it has a grain size and arrangement that is favorable to plants.
A lawn made with poor or too thin topsoil may be persuaded to grow vigorously by proper fertilizing. However, it will tend to burn out during dry spells unless it is shaded. It will dry out more readily if it is over gravel or sand subsoil than over fine-grained soil. The minimum topsoil depth for a lawn under most conditions is 2 inches (5.1 cm), and 4 inches (10.2 cm) is safer. However, benefits are obtained from greater depths, and it is common practice to use up whatever piles are around. If the original soil was thin, or had been lost, more must be trucked in.
Gardens, flowerbeds, and shrubs like to have about 8 inches (20 cm), and depths up to 2 feet (0.61 m) are recommended for some species.
If peat (humus) is obtainable locally at a low price, it may be spread on subsoil and mixed in by hand or machinery. With the addition of lime and fertilizer it may serve well as topsoil and might be much cheaper.
Converting Field to Lawn. The original lawn made around a new building may be partly or wholly at the grade of an existing field supporting a growth of mixed grass, wild flowers, and weeds. One way to make a lawn is to use a rotary tiller or a plow and a harrow to turn in the existing growth, and pulverize the soil for planting of grass seed. It is usually good practice to bury the old vegetation several weeks before planting. Decay of vegetable residues often temporarily deprives the soil of nitrogen to such an extent that the new crop cannot obtain enough for a start. Also, sprouts from roots of undesirable plants can be readily destroyed as they appear on bare ground, and can be reduced or eliminated before planting.
A less expensive method, which is usually satisfactory, is to pull out any brush, then mow the field repeatedly. It may be reduced in one operation from field length to lawn length, then kept short, but better results are apt to be obtained from starting with a high cut as if for hay, and progressively trimming shorter.
The effect of the cutting is to kill or place at a disadvantage plants that prefer to grow tall, and to encourage those which are not damaged by mowing. Most fields contain enough lawn-type grass and clover to take over the whole area within a year when encouraged by repeated cutting.
A similar effect may be obtained by moderate driving and parking of cars and trucks on the field. Field surface may be too rough for a lawn. Large inequalities may have to be cut or filled. Smaller ones can be rolled down, filled with dirt, or both.
A power lawn mower having heavy steel or rubber rolls will tend to flatten ground. Its effect is quite marked when inequalities are small and choppy and the soil is soft, and becomes negligible as the ground bakes in the summer.
A steel wheel roller, weighing from 3 to 5 tons (2700 to 4500 kg), of the type used in driveway construction and blacktop patching, is a very effective lawn flattener, but must be used when soil is in the right condition. If soil is too soft, it will make ridges, and probably get stuck. Heavy wet soils may pack so hard as to discourage growth.
Hollows may also be filled with topsoil. This may be applied in layers ¼ to ½ inch (6.4 to 12.7 mm) thick so that grass will grow up through them, or the fill made to the level of the surroundings on one lift, then new seed planted where necessary. Sometimes the seed is mixed with the topsoil prior to placing it.
Fills of more than an inch (centimeter) or two on bare ground, and any fill over grass, tend to settle noticeably so that the work will require doing over, although with less material, the following year. Overfilling enough to compensate for this settlement requires expert judgment. Firm tamping is sure to reduce this difficulty and may eliminate it, but might make it difficult for existing grass to push up through the new soil.
Erosion. A sloping lawn is vulnerable to severe damage from flowing water from the time the topsoil is spread until the grass has made a good root growth. The probability of such damage should be figured into cost estimates by both the owner and the contractor.
In some areas that are on more desert-like sloping land, as has been done in Afghanistan with the help of the United States Agency for International Development, the gully wash areas can be checked with built-up flat stone dams perpendicular to the water flow. Refer to Fig.7.6.
Danger of erosion can be reduced by mixing straw or lawn clippings into the surface. This necessitates a heavy addition of nitrogen fertilizer, and makes it harder to cover the seed.
FIGURE 7.6 Drainage Solution in Afghanistan.
In most developments, retention ponds to catch runoff from the construction site are required. The runoff will carry silt in it, which is undesirable for downstream properties. The use of Fair -cloth Skimmers, which float on the surface of the retention basin releasing the cleanest water instead of draining from the bottom as conventional outlets do, is one way to avoid the problem.
If a seeded surface is sprayed with asphalt emulsion, it will be held against ordinary erosion, without preventing growth of the seedlings.
It is sometimes possible to divert drainage into other areas. One section may be fixed up and seeded first, and water routed through the part which is only rough-graded. After grass is firmly established, drainage may be shifted to go over it, so the rough area can be smoothed and planted.
Sod. Drainageways and steep slopes can be protected with sod. This is cut out of existing lawns or mowed fields by means of hand tools or an engine-driven sod cutter, laid on freshly loosened and smoothed topsoil, and tamped into firm contact. It is sometimes fastened in place by driving pegs or thin stakes, or by pegging chickenwire firmly over the whole area..
Sod may be cut in strips 12 or 15 inches (30.5 or 38 cm) wide and 6 to 10 feet (1.83 to 3.05 m) long or in squares or rectangles of any convenient size. A depth of 1½ inches (38 mm) usually suffices to get practically all the roots.
It is essential that newly placed sod be thoroughly watered and tamped to establish its contact with the ground. It should be watered as necessary until it shows that it can take care of itself.
Trees are liable to destruction or damage from various causes during construction work. Trunks or branches may be broken or scraped by accidental contact with machines; roots may be dug away by ditching or lowering of grade, lessening the tree’s ability to obtain food and water and rendering it more vulnerable to uprooting by wind; its trunk may die because of dirt piled around it; or its roots may be drowned or suffocated by placing of fill.
In general, the larger and more valuable trees are less subject to fatal damage from collisions, although the scars they do get heal more slowly; but they are much more likely to die from root cutting or suffocating than younger and more adaptable specimens.
Bark Damage. Trees can be partly or wholly protected from collisions by wrapping with burlap or other cloth, and tying thin wood strips around the trunks and any particularly exposed branches. If used for anchors in pulling out machinery, trunks must have very heavy padding and thick wood pieces between the bark and the chain; and the chain loop should be fastened with a grab hook, bolt, or knot that will not slide. The choker effect obtained from round hooks or rings can readily crush bark and wood all around a tree so that it will be fatally injured.
If a tree is girdled by removing even a narrow ring of bark around the whole trunk, it will probably die. However, if the sapwood is not injured and the damage is kept shaded so that the wood will not dry out, young and vigorous trees may repair the cut by growing several inches of callus and new bark across the injury from top and bottom.
If the gap is wide, a skillful worker may be able to graft strips of bark across the injury. If circulation is established through these, they will serve to keep the tree alive and they will widen out so that the damage may heal over entirely.
Scars on trunks or branches should be promptly covered temporarily or left bare for self healing.
Ditching. Ditching on one side of a tree ordinarily does not injure it severely. However, it is best to keep the cut as far from the trunk as possible, thus reducing the number of roots lost, minimizing the danger of tearing the trunk, and making the digging easier.
If a hoe or dozer digs within two trunk diameters of the tree, the roots should be uncovered, and then cut by hand to avoid danger of splitting the trunk while tearing them up.
A close cut weakens the tree’s resistance against a wind that tends to tip it away from the ditch. If uprooting in that direction would cause it to fall on a building or across a highway, a tree expert should be consulted about the advisability of providing cable support, Fig. 7.7, or removing some of the upper branches.
Burial. A tree’s reaction to having its trunk buried varies with its species, health, and the nature of the dirt. Burial is fatal to the majority if the fill is deep enough or of such a nature that it will smother the bark and support organisms which will destroy it.
The fill also changes the air and water content of the topsoil and subsoil around the roots. Such changes may damage or kill the roots directly, or indirectly by changing the nature of the soil population.
The best defense a tree can muster is to put out new roots near the surface. The willow does this automatically, but the majority of temperate-zone trees do it with difficulty or not at all.
Trunk damage can be avoided by building a stone wall around the tree on the original ground, at a sufficient distance to allow free air circulation. See Fig. 7.8. The space inside is called the well. Sometimes the fill is made and part of it dug away by hand to make space for the wall. Or the wall may be built first, and the dirt placed around it.
The first method is expensive and offers some danger of damaging the tree with the digging tools. The second is subject to the danger of knocking over the wall while placing fill, or accidentally spilling dirt over it that will fill up the well.
In general, the most satisfactory technique is to build the wall first, and fill the well with easily removed material such as stones, wood scrap, or crumpled newspaper. Such items will prevent any appreciable amount of dirt from entering the hole, and are easily taken out when grading is complete.
Sometimes pebbles or crushed-stone collars are used to avoid unsightly or dangerous holes. These will usually allow sufficient air circulation when new, but are likely to plug up with dirt.
The fill should be pervious enough that water will not stand in it and in the holes. Tile may be laid to drain the tree wells, but a saturated fill is liable to kill the roots anyhow.
Root protection is more complex, and the results are less certain. Land tile is laid on the old surface of the ground or slightly below it, with lines 3 to 6 feet (0.91 to 1.83 m) apart. A 4- to 6-inch (10- to 15-cm) blanket of crushed stone is laid over the area and covered with hay. Pipe openings at each end of the fill or into wells should allow enough air circulation to preserve favorable conditions long enough to enable the tree to adjust to the changing conditions. Wire mesh must be placed across openings to keep animals out.
FIGURE 7.7 Temporary tree brace.
FIGURE 7.8 Tree trunk protection in fill.
If it is not economically feasible to take these precautions, the fill should be made of clean bank gravel or coarse sand. Trees may survive heavy additions of such open textured material.
Removal. Landscaping work may involve removal of trees. If they are to be destroyed, the job resembles the land clearing described in Chap. 1, except that interference from buildings, wires, valuable trees, and other obstacles is much more common.
If the ground is to be filled, trees may be cut as nearly flush with the ground as possible. This may also be done if the grade is not to be changed and the presence of the stump is considered less objectionable than the cost of removing it. If the grade is to be cut, stumps must be uprooted.
Most home landscaping involves the planning of a driveway. It may be a straight connection to a street a few feet (meters) away, or a long roadway involving considerable problems.
Short, straight drives can be as narrow as 8 feet (2.44 m) for use by passenger cars only, but 12 feet (3.66 m) is more comfortable. A long drive, one in a slot between walls, or any drive to be used by trucks, should be at least 12 feet (3.66 m) wide.
Curves should be 1 to 3 feet (0.3 to 0.9 m) wider than straightaways, the sharper turns requiring the greater width.
The entrance from the street should be 30 feet (9.1 m) wide at the curb, in order to permit turning into it from the near side of the street. An effort should be made to avoid entering the road through a deep cut, between large trees, or in or very near to a curve, as any of these features add to the danger of accident.
Sidehills. A long driveway of the farm or estate type may have to cross a hill slope. It is notched into it in the same way as a pioneer road. However, since it is a permanent improvement which should have a pleasant appearance, special procedures are followed.
Such a drive serves not only as an automobile route but also as a drainageway. Unless diversion ditches are made above it, whatever water flows down the slope will land in the driveway cut, and flow down it or across it. Unless an ample channel is provided, the drive may often resemble a streambed.
A driveway crossing a hillside below a long slope should be 14 to 16 feet (4.3 to 4.9 m) in width, including drive, shoulder, and gutter, but not the slopes. The gutter should be at least 3 feet (0.9 m) wide and deep enough to carry all the water. It can be relieved at frequent intervals by diagonal cross drains to the lower slope. The drive cross-section may be crowned or sloped oppositely to the hill, but should never slope with it. See Figs. 7.9 and 8.1.
The cheapest gutter is sod, and it may hold on quite steep slopes if well established. Temporary diversion ditches can be made with a plow to keep much of the water off until it is well established. Stone, concrete, and blacktop are water-resistant, but are subject to frost heaving unless on a stone or gravel base.
The slopes of the cut and fill should be topsoiled and seeded. They are often too steep for mowing.
Garage Level. Driveways offer minimum trouble in use and maintenance if they are nearly level. Unnecessary expense and inconvenience may be caused by placing the garage so that grades are created or exaggerated.
If the driveway is long, the garage should be at about the grade of the ground around its entrance after landscaping. If the drive is short, the garage should be at about street level.
It is unusual for a garage to be higher than the grade around it, but it is very common practice to place it under the building at basement level. In this case the driveway often must enter through a deep cut bordered by steep slopes or retaining walls, and usually descends more or less steeply as well.
A descending drive must level off several feet (meters) outside of the doors, and must not do it so abruptly as to cause a car’s bumpers to scrape when entering or leaving. Extra width is needed between walls. One or more grating drains should be provided outside the doors, with plenty of capacity. Drainage from a long drive or from the lawn should be diverted to other drains or channels. Failure to observe both of these precautions may result in a flooded garage or basement. It is also necessary to keep the area clear of leaves and trash that might block the drain.
If the drive is short, its slope will be determined by the difference in level between the garage and the street. Driveways as steep as 30 percent grade are in use, but they are both difficult and dangerous.
Any steep drive requires care in designing the vertical curves at each end so that the center of the car will not scrape on a convex curve, nor its bumpers hit on a concave one. There should be a parking place which is moderately level if possible.
In climates where freezing weather occurs, steep grades may become dangerous or impassable. Also, drifting snow may entirely fill a cut to a low garage, from which it will probably have to be dug or blown rather than plowed.
FIGURE 7.9 Hill slope driveway section.
Snow Melting. Heating pipes can be installed under driveways, walks, and outside stairways to prevent snow and ice from resting on them.
The preferred method is to lay wrought iron pipe or copper tubing in the pavement slab or immediately below it, and circulate an antifreeze solution heated by a heat exchanger in the building’s steam or hot water boiler. It can be turned on and off by hand, or by automatic controls operated by the weight of the snow, or by its interference with a light beam reflected off a polished surface to an electric eye.
The system must have ample capacity, or will occasionally do more harm than good. If it does not quite keep up with the snowfall, at the end of the storm it may leave the area covered by a layer of slush, which might then be frozen by an extreme drop in temperature and kept frozen until the weather has warmed slightly.
Electrically heated wires, available in hardware stores, may be placed on ice for emergency melting. Covering with cloth or paper increases effectiveness.
Turnarounds. A driveway which does not include a turning place requires that a car be backed out of it or into it. This is entirely impractical on long or curving drives, and is a nuisance and a danger in any case. Wherever property size permits, a turnaround should be provided.
The best way to lay one out is to have the people who are to use it make some trial turns, add a few feet to the space they require to allow for carelessness or a bigger car, and build the drive accordingly. If there is no opportunity to practice, any of the layouts shown in Fig. 7.10 should prove satisfactory.
Allowance should always be made for car overhang. This may be 2 feet (0.61 m) front, up to 4 feet (1.2 m) rear, and 8 inches (20.3 cm) at the sides. It is desirable to have a curbing that will keep the wheels about a foot away from vertical walls to protect the car from scraping at the side.
Extra space may be provided in a turnaround for parking, or to supply peace of mind to uncertain drivers.
Surfacing. Four inches of good bank gravel, crushed rock, shell, or similar materials should be stable enough for a building drive on well-drained soil. Under average conditions 6 inches (15.2 cm) is safer, and when the ground is soft and wet, 8 inches (20.3 cm) to 1 foot (0.3 m) or more may be required.
A stone fill underneath can be used to reduce the gravel requirement. Any flat stones near the surface should be set on edge so that they will not rock and disturb the top dressing.
If the driveway is long, it may pay to try to get by with a minimum depth and add more material to any soft spots as they develop. However, it is often necessary to dig away the softened gravel, as it mixed with mud underneath. If the driveway is short or the budget liberal, it is good practice to put down a safe depth in the first place.
These materials may be used for both the bulk and the surface of the drive, may be given a surface treatment, or may serve only as a base course.
If used alone, varying amounts of difficulty may be found with loose stones, gullies, dust, tracking small particles into buildings and cars, muddy surfaces, ruts or mudholes, or scattering on the grass, depending on the kind and quality of material used and the circumstances.
Calcium chloride, either scattered on the surface or mixed in, will prevent the drive from becoming dusty, and will help to hold it against washing and scattering. It should not be used in the immediate vicinity of the building, where it might do damage if tracked in.
A thin layer of loose pebbles or fine crushed rock makes an attractive surface for light and slow-moving traffic, but scatters under fast traffic. Snowplows are likely to move a large part of it to the lawn.
It is often necessary or desirable to regrade land in order to use it for farming. In arid regions, land is leveled to permit even distribution of irrigation water. In semiarid climates, sloping land may be terraced to hold rainfall behind dikes so that it will soak into the ground instead of flowing off.
Where the rainfall is adequate or excessive, terracing may be necessary to reduce washing of soil from cultivated slopes. Under any conditions of climate or soil, leveling may be desirable to allow use of large or high-speed machinery. Alone or in conjunction with underdrainage it may increase yields by eliminating burning out of crops on ridges and drowning in hollows.
Agricultural grading differs from other types of earthmoving in the large areas to be treated in proportion to the money available, the flexibility in engineering requirements to suit conditions and cost factors, and the problems relating to the handling of topsoil.
Cuts and fills are typically shallow, vertical movement of soil is slight, and horizontal movement is relatively great.
Terracing land is the grading process of interrupting slopes with ridges, channels, or benches, or combinations of them, in order to slow or stop the flow of rainwater and to prevent harmful soil erosion.
Terracing may serve to hold water on the slope so that it will soak into the ground; allow water to flow off it while keeping the loss of soil to a minimum, or to reduce slopes so as to make them more readily workable or irrigable.
Terrace Types. Three principal types of terrace are used. Each is constructed along level or contour lines. The ridge terrace, Fig. 7.11(A), is a ridge built of soil obtained from both sides. The channel terrace, (B), is a ridge constructed of dirt from the upper side only, and the channel formed by this excavation is an essential part of the structure. The bench terrace, (C), has a stair structure with steep risers separating relatively flat cultivated areas.
Ridge and channel terraces are usually built with sufficiently gentle slopes to allow farm machinery to work along or across them. Best results are obtained if farming operations are done parallel with their centerlines.
Ridge Terraces. The ridge or absorptive-type terrace is used primarily to conserve water in regions of deficient rainfall. Each ridge serves as a dam for a pond, which is deepest in the excavated area immediately above it. Water may also be impounded in the trough formed below this ridge by borrow of material.
A larger area and quantity of water can be held on slight gradients than on steep ones, by any one size of ridge. Not only is more water retained per yard of dirt used in the ridge, but also its distribution over the land is more uniform.
Too great a depth of water may drown out crops immediately above the ridge.
It is ordinarily not economical to construct terraces for water conservation alone on slopes over 3 percent, and structures for reducing soil erosion are more often of the channel or intermediate types.
Overflow channels may be provided to carry off rain in excess of that for which the system was designed. These should be protected like channel terrace spillways.
Channel Terraces. Channel or drainage-type terraces are essentially shallow diversion ditches which catch water flowing down a hill and lead it off to drainageways that have been protected against erosion.
The channel depends on the ridge of excavated material for much of its capacity. Its grade is flat, or nearly so, so that only extremely fine soil particles can be carried by the water it discharges.
Bench Terraces. The principal application of bench terraces in the United States is in connection with irrigation. If the original slope of the land is greater than that of the graded fields, each field will constitute a terrace, separated from fields above and below by comparatively steep slopes.
Benches may also be made in steep cultivated land by leaving narrow contour strips in grass or other permanent vegetation. Soil washing from the wider strips between will be caught by the grass and will tend to build up the low side of the cultivated piece, while its top is lowered by erosion. This process, often accelerated by plowing so as to throw dirt downhill, will ultimately result in gentle slopes separated by steep banks.
This work is ordinarily done by farmers without assistance from contractors.
Surveying. A terrace system must be carefully surveyed and planned before construction starts. The interval between terraces may be taken from the chart in Fig. 7.12, or better, determined after conference with soil conservation specialists.
Stakes are placed from top to bottom of the field at the selected intervals. From each of these a level line is run the full width of the field or area to be processed. These lines, known as contours or contour lines, will bend toward the high side of the slope in hollows, and away from it on ridges.
Each level line may be found by setting a level transit or laser at a point, then measuring its height above the ground. A marker or laser receptor is set at that height on a rod.
At various distances, the rod is moved up and down the slope until the marker is at instrument height when the rod is on the ground. A marking stake is placed.
A series of such readings indicates a level line. Rechecking is advisable.
Each line indicates the location of a terrace. However, sharp angles and extremely irregular lines are not desirable and can often be reduced or eliminated by minor adjustments, as in Fig. 7.13. Farming is simplified when adjoining terraces can be made parallel.
If an angle in a gully is eliminated by moving the line downhill, the terrace ridge will have to be built higher above ground level to preserve its grade, and water will be ponded behind it.
FIGURE 7.12 Channel terrace data.
FIGURE 7.13 Terrace line correction.
If the line is moved uphill to cut off a bend on a ridge, the channel will have to be dug more deeply to allow flow of water through it. However, such a ridge may be used as a divide or drainage head, in which case little water will be present.
Stakes are ordinarily used only for location guides, but may be marked with grades where the terrace is to be higher or the channel deeper than standard.
It is desirable that the top of the terrace system be also the top of the drainage area. The top terrace should serve the same width of ground as the terraces below it. If a larger area must be served because of flow from higher fields, the channel capacity must be increased proportionately, or some other type of intercepting drain used.
Grades. Ridge terraces usually have a level grade. Channel terraces may be level for the section most distant from the outlet, and slope increasingly to about a 
percent grade at the outlet. Drainage is normally from ridges toward hollows. Short terraces require less maintenance than long ones.
Outlets. The discharge from a terrace should be into a waterway that is capable of carrying the water directly down the slope, without eroding. Shallow depressions carrying a permanent sod are often satisfactory. These should have enough drop from side to center to ensure gathering of all water discharged from the channels, but should not concentrate enough flow at the center to cause erosive velocities.
The strength of the sod, as affected by fertilizing and grazing, and the condition of the soil will determine the maximum safe gradient for a meadow outlet. This is ordinarily 6 percent or less.
Sod should extend several feet above flow lines on the sides of the waterway. Steeper or narrower channels may be protected by ungrazed and uncut growth of grass, weeds, bushes, or trees.
For extreme conditions, a channel protected with check dams (Fig. 7.14), pavement, or other artificial structures may be required. Steep banks require sod or artificial flumes for the terrace discharges.
Permission of owners of land below the farm must be obtained if the terrace system alters the path or concentration of water on their properties.
Outlets must be completed and protected before terraces are built.
Construction. Terrace construction is primarily a matter of sidecasting. The work is commonly done with graders of either the powered or towed types. Bulldozers working at right angles to the terrace are also effective, particularly in the channel type.
Belt loaders, of both standard and special terracing models, give excellent results.
If the channel or cut depth is greater than that of the topsoil, a barren strip will be left which will yield poor crops. This damage may be reduced by overcutting the channel so as to leave it below grade, and blading some topsoil from above to cover it.
Maintenance. Terraces will serve their purpose best if plowing and cultivating are done along contour lines—that is, parallel with the terrace lines.
A terrace can be enlarged by plowing so that dead furrows are in the channels and lands on the ridges, as in Fig. 7.15.
Any accidental blocking of a channel or damage to a ridge should be repaired immediately, as it might cause the terrace to fail in a heavy rain, with possible destruction of the terraces below it.
Characteristics. So far as this discussion is concerned, a gully is a drainage channel that has become so deepened or enlarged that its banks are unstable and tend to extend destructively into surrounding land (Fig. 7.16).
Control of gullies is largely an agricultural problem, but may also be required to protect highways or structures.
Gullies are a sign of the beginning of a new cycle of erosion which tends to dissect smooth slopes or high levels of ground into tablelands separated by steep-walled channels or canyons. Unless controlled, it will eventually narrow such tables into peaks. Geologically, they are small examples of the type of stream erosion which carves rising land into mountains.
The new erosion cycle may be started by land rising so that steepening channels add to the velocity of flowing water; by lowering the outlet of a stream with the same result, or by reducing the resistance to erosion of the land.
Gullies are caused most frequently by the destruction of the vegetation which protects the land surface, although they may also be started by lowering of outlets, due to highway or stream channel work, or land slips.
The majority of gullies contain intermittent streams which flow only during or immediately after rains, or in wet seasons. Permanent streams are less often affected as their beds are unsuitable for agriculture. These are most apt to be raised and choked by silt deposits resulting from bad farming.
Growth. When a slope is covered with vegetation, whether sod, bushes, or forest, rainwater tends to move downward as a flowing sheet, and is only gradually gathered in definite drainage-ways. Its eroding action on the ground is slight, as it is held from contact with the dirt and its velocity is lowered by stems and roots that form a protective mat.
FIGURE 7.15 Plowing to preserve terrace.
FIGURE 7.16 Active gully. (Courtesy U.S. Department of Agriculture.)
If the vegetation is removed by plowing, disking, close grazing, or fire, the water comes in direct contact with the soil and tends to remove the surface particles. This effect is usually rather uniformly distributed at first, and is called sheet erosion. It can be reduced to slight proportions by proper farming, including contour plowing and cultivating, and terracing or returning to sod when necessary.
Erosion is most active where the amount or velocity of water is greatest, or the soil is least resistant. Such places tend to wash out more than the surrounding area, and then, being lower as a result, will catch the runoff from a larger area, increasing the quantity and velocity of water, and its eroding effect. The deepening of the channel therefore tends to build up forces which will make it deepen more rapidly.
In its early stages such a gully may be destroyed by plowing or harrowing, so that it is choked by clods and some of its water is diverted elsewhere. However, unless close-growing vegetation is planted, or weeds are allowed to grow, new storms will reform the channel or create new ones nearby, and they may eventually become too deep to be choked by plowing or even to be crossed by a plow.
Once a gully is formed, it enlarges by three separate processes. One of these is channel erosion—the scouring action of the water deepening the bottom. This is accompanied by the falling in of the sides as they are undermined.
The upper ends (heads) of gullies advance into the land by waterfall erosion, both along the main drainage line and branches which are acquired. Subsoil is often less resistant to erosion than topsoil. Water pouring into the gully will cut it into steep banks, undermining the topsoil and causing it to fall. The impact of the waterfall on the bottom gouges holes which accelerate channel erosion.
Waterfall erosion usually produces a gully with a U cross-section. It becomes less active as it approaches the head of drainage and the quantity of water is reduced.
If the subsoil is equal or superior to the topsoil in resistance, waterfalls will not develop, but the gully can progress by extensions of channel erosion.
The third major factor in extending gullies is sloughing off of soil due to alternate freezing and thawing, or following saturation by heavy rains. This process is most active on southern and eastern slopes, and will eat through a field with little regard to slopes or drainage lines.
Continued progress of either waterfall or sloughing erosion depends on sufficiently active channel erosion to remove the loosened dirt.
Once well established, a gully will continue to enlarge even if the surrounding land is planted in erosion-resistant vegetation, as the head and side slopes will undermine the surface.
A gully may advance downstream by channel erosion. More often, it deposits debris in a delta fan at or near its mouth, so that the land is built up. This process is destructive also as it buries topsoil under subsoil.
Damage. The damage from gullies includes actual destruction of farmland, cutting up fields so that they cannot be worked economically, lowering the water table so that crops dry up, undermining buildings, roads, and bridges, burial of lower lands under barren subsoil, and choking of streams with silt.
An individual gully can do damage amounting to many thousands of dollars, and the national loss from them is in the hundreds of millions. Their control is therefore of great importance.
Control measures after gullying has started may include diverting water to other drainageways, planting, breaking down walls, building check dams, and proper use of the affected land.
Diversion. Water entering the gully can sometimes be diverted by plowing an arc around its head, as in Fig. 7.17(A). The slice should be turned toward the gully to make a dam to back up the furrow.
More often it is necessary to build dams or to dig ditches. Dams are safer, as a new ditch or even a plow furrow may start a new gully unless watched and controlled.
Where diversion is not practical, waterfall erosion may be checked by conducting the water into an overhanging pipe or flume, as in (B).
FIGURE 7.17 Diversion of water from gully head.
If water can be permanently diverted from a gully, the walls will eventually break down into stable slopes, which can be covered and held by vegetation.
Planting. Gully growth may be checked by planting. Small gullies may be held by pasture plants or vines; larger ones require shrubs and trees.
The plants should be of types that can grow well in poor soil, have extensive root systems, and will not be injured by partial burial. If the ground is damp most of the year, willows are probably the most efficient, particularly as they will grow from poles or logs secured horizontally in the floor across the direction of flow, thus providing mechanical control while roots and stems are sprouting.
Black locust grows vigorously in poor soil, has a widespread root system, and yields a crop of fencepost material. Many of the pines do well in barren soil, but their roots are not as strong. Vines such as kudzu and honeysuckle are used successfully, though kudzu may take over all vegetation. Whatever plants are chosen, enough soil should be loosened to give them a good grip. Whenever possible, fertilizer, manure, or topsoil pockets should be provided to give them a good start. Animals should be fenced out of the area.
It is usually not practical to plant very steep walls. When vegetation is growing well on the bottom, soil caving from the walls will be held so that the slopes will gradually become less abrupt, and will allow growth of self-seeded plants.
It is often necessary to divert water or to reduce its velocity in the channel before planting can be successful.
Breaking Down Walls. The healing of a gully scar can be greatly accelerated by breaking down the walls into slopes gentle enough to support vegetation. When practical, it is desirable to have the slopes such that farm machinery can cross them in any direction.
Very small gullies can be broken down with plows and other farm implements. Somewhat larger ones can be reduced by graders or angle dozers working parallel with the edge. Big ones require dozers pushing the bank more or less straight into the gully, or a dragline pulling it from below.
FIGURE 7.18 Gully check dams. (Courtesy U.S. Department of Agriculture.)
Gullies may be of such large size—depths of 50 feet (15.2 m) or more—that it is not economically practical to grade them in, even with the largest machinery.
The new slopes produced are planted in somewhat the same manner as road banks.
Check Dams. In order to obtain permanent control, it is usually necessary to slow, stop, or reverse the process of channel erosion. An actively cutting channel will steepen and undermine its banks and the new grading or planting they support.
A channel interrupted by dams or other obstacles will tend to silt up to a higher gradient, reducing the height of its walls and encouraging plant growth. If flow is only occasional, close plantings of bushes in the channel and on its banks may be sufficient. These are often planted in lines across the gully, and protected against washing out by wire stretched across deeply driven posts.
Sod, combinations of cut brush and wire, or stone or logs can be used in building check dams. Some constructions are shown in Fig. 7.18.
In general, any structure which water cannot get under or around, which is tight enough to prevent dirt from going through it, and which is strong enough not to be washed out, will serve as a check dam.
Slope Patterns. Land leveling may be divided into six classes, according to the result obtained:
1. Spot grading
2. General downward slope away from water supply—for sprinklers
3. Uniform grade in direction of irrigation
4. Uniform grade in direction of irrigation and at right angles to it
5. Uniform grade in direction of irrigation and exactly level at right angles to it
6. Exact level
Spot grading consists in removing humps or filling hollows, without establishing a uniform grade in any direction. It is sometimes done in advance of better leveling for irrigation, and is of general use to make possible faster tillage and more even production.
If water distribution is to be by means of sprinklers, perfectly uniform slopes are not required. For water distribution, it is only necessary that the land have a general slope down from the source of water. In climates where deep freezing of soil occurs, the slope should be uniform enough to make possible drainage of sprinkler pipe laid at a fairly regular depth.
When the water reaches the individual plants by flowing on the surface of the ground, it is necessary to have an almost uniform slope in the direction of irrigation. The steepness of slope may be determined by the character of the soil, the crop to be planted, the original grade, and the rate of water use.
Economies may be affected on many plots by leveling only in the direction of irrigation, and following the original profile at right angles to it. This type of job is used chiefly in orchards which can readily be cultivated into ridges that will regulate water drifting across the field.
Choice between the fourth and fifth methods will depend largely upon economies in working over the natural grade. In very large fields, the two-way slope will facilitate movement of water through the cross-distribution pipes. The cross grade should be so slight that even light ridging will prevent sideward drift of the water.
Entirely level plots are usually limited to rice fields, and alfalfa and other crops which can tolerate flooding.
Flow and Absorption. The rate of water flow and absorption should balance, so that water will reach the lower end of the slope in sufficient quantity for a crop without flooding or running off. In practice this balance may be difficult to achieve, and provision is made for draining off excess water when necessary.
Increase of gradient will accelerate the flow and decrease the rate of penetration. Light sandy or gravelly soils absorb water rapidly and require steeper grades than clay, which may be almost waterproof unless freshly loosened.
The maximum gradient would be below that which will cause the soil to wash and gully during irrigation or heavy rains. The minimum is flat.
If the maximum practical gradient is not sufficient to move the water the length of the field, additional distribution lines can be installed. See Fig. 7.19. Very porous soils require pipes and sprinklers.
Figuring Gradients. Earthmoving should be kept to a minimum to save money and to conserve topsoil. If conditions justify the use of any one of several slopes, the one will be chosen which conforms most closely to the natural topography.
If the steepest possible gradient is so much flatter than the original grade that excessive earthmoving will be required, because of deep cuts at the top and high fills at the bottom, the field may be divided into two or more levels or benches. These will have the desired slope and will be separated by steep banks. A separate waterline is required for each bench.
The high corner or end of the field must be below the level of water in the ditch or its head in the standpipe.
The new gradient must usually be placed at a level, or levels, where cut and fill will balance, as cost may be greatly increased by bringing in borrow or dumping surplus soil. After the surrounding areas are largely under cultivation, borrow or disposal may not be possible.
Topsoil is not a problem in many arid valleys where soil is fertile to a considerable depth. However, when topsoil is thin and rests on layers of soil which are infertile or hard to work, or when the surface soil differs sharply in character from that underlying it, the cuts should be kept shallow.
FIGURE 7.19 One-level and bench grading.
Stakes. Stakes set in a square grid at 100-foot (30.5 m) intervals, and on high and low spots, are used in measuring the original surface and for marking the new grade. One or more benchmarks are set outside the grading area, as discussed in Chap. 2.
The new grades are marked on the stakes in any convenient manner. On fills, time may be saved by tying strips of cloth on grade lines to enable operators to see them without getting off their machines. Where soils are loose, two stakes may be used to advantage—one hammered down to ground level, the other left to project 2 or more feet (0.6 or more meters). Cut and fill are figured from the top of the lower stake and marked on the upper one.
Clearing. Much of the growth in arid regions is of a light and brittle character which breaks up during grading and mixes with the soil, and has value as a binder and a source of plant food that is more important than the slight difficulties it causes during finishing. This group includes the various kinds of sage, tumbleweed, and many of the smaller cacti.
However, larger shrubs and trees such as mesquite, greasewood, and acacia, the presence of which often indicates good soil, are tough and deeply rooted, and their removal requires heavy machinery. In thick stands of these plants, clearing is more expensive than grading.
Such growth can usually be piled and burned immediately after removal. The leaves and sapwood are resinous, and the dry soil sifts out of the piles so that they burn readily. Heavy trunks are more difficult to ignite, and if only a few are present, it may be easier to haul them to a dump.
Clearing methods and machinery are discussed in Chaps. 1 and 21.
Savings may be effected by cutting the trees flush with the ground wherever the fill will be deep enough to permit tilling over them. However this is not recommended, as it will prohibit the future use of pan breakers or other deep tillage tools and will add greatly to the expense of installing underdrains if they should become necessary. The same objection applies to burying logs in the fills.
Wind Damage. Clearing and grading should not be started until irrigation water is available. The native vegetation, even when very sparse, has some power to break the wind and hold the soil. The weathered ground surface usually has a crust which resists wind scour. These natural protections are destroyed by the work, and unless water can be put on and a holding crop started immediately, the best part of the soil may be blown away or piled into dunes that may be more costly to level than the original surface.
Wind damage during the work may often be avoided by choosing a season in which wind-storms are infrequent. If this is not possible, the final leveling and planing should follow immediately behind the rough grade, as a perfectly smooth surface is much more resistant to scour and dune formation than one having ridges or tracks of machinery on it.
If such a planed surface becomes roughened by wind, it should be replaned before the next storm, and kept flattened until a crop can be grown.
Machinery. Dozers are used to clear, to take the tops off ridges and dunes, to bevel steep slopes, and to fill in pits; for cut-and-fill work on short pushes; and for pusher work with scrapers.
A drag leveler can be used to smooth out rough spots wherever it is possible to walk the tractor over them. It can transport soil long distances, although its efficiency diminishes rapidly over 200 feet. Compared with the dozer, it has the advantage of making a wider cut, with little tendency toward scalloping, and has a greater transporting capacity and speed. Compared with scrapers, it has greater stability against overturning, smooths a wider area with each pass, cuts down and fills more quickly, and can be dumped promptly if the tractor gets stuck. It has a smaller transporting capacity and generally will not make as smooth a grade.
On any large area, the bulk of the dirt moving is most efficiently done with scrapers. Because of the width of most of these grading jobs, and the small slope of the land, these can be used in almost any pattern preferred by the supervisor or operators.
Grading. One technique is to produce a rough finish grade in the high corner of the field and to expand this grade as continuously as possible. Where necessary, spot grading operations are done beyond this area in order to secure fill or to dispose of surplus.
Economies may be effected by loading the scrapers toward adjoining depressions so that the soil pushed along in their efforts to load will fill them. If the soil is very loose, this may be more important than loading in the direction of the dump, but it is often possible to do both.
Fills are usually made in thin layers in order to get maximum compaction from hauling equipment, as rollers are seldom used. Tamping rollers will produce a more permanent grade where fills of more than a foot or two are required, but close competition for the work may not permit the necessary increase in price.
Rough-graded sections may be settled by flooding before doing finish work.
It is often possible to keep the short-range equipment, dozers and drag scrapers, working all the way through the job if the area contains a number of adjacent humps and hollows. The drag level can also do the light finishing more economically than the scrapers.
Finishing Off. As soon as any considerable section is rough-finished, grades are rechecked and additional stakes may be placed. These can be on 50-foot (15.2-m) centers both ways, or 50 feet (15.2 m) one way and 100 feet (30.4 m) the other, or at the intersection of diagonals across the original squares. Any changes of grade necessary at this stage can usually be made by the drag level.
If a small error has been made in balancing cut and fill, the gradient in the lower part of a field or bench may be increased or decreased slightly to change the quantity needed to balance. Large errors may require lifting or lowering the whole field, or one of its benches, or making arrangements for disposal or borrow outside the plot.
When the entire field, or a large section of it, has been brought as near the finish grade as is practical, all stakes are removed and the job is finished with a land plane. See Chap. 17. This will flatten ridges and hollows around stakes, plane off spill windrows, piles, and track marks, and even local inaccuracies at hitting the grade.
Planing also serves as a maintenance operation and is sometimes repeated after each harvest.
Distribution pipes are usually laid immediately after completion of leveling. Ditches should not be dug until the pipe is on the job, as drifting soil can fill it very rapidly. It is usual to lay and cement the pipe, to partially cover it by hand, and to allow it to cure. This fill is removed by hand where standpipes are installed. The ditch is then backfilled and graded over by machinery.
Alkali. All soils contain some salts, in both soluble and insoluble forms, and these are necessary for plant growth. In rainy climates the soluble salts tend to be leached out of the soil and carried away in underground water about as rapidly as they are made soluble by plant action and weathering, or added as fertilizer.
In dry climates where there is not sufficient rain to leach them effectively, they accumulate in the soil, accounting for the great richness and productivity of the land. However, in flat low areas and some other places, the salts, then known as alkali, may be concentrated so heavily that they kill plants instead of aiding them. Alkali may also appear as a surface crust where groundwater comes to the surface and evaporates.
Underground Pools. Where soaking rains are rare, natural underground drainage tends to be poorly developed or nonexistent. If such an area is irrigated, water absorbed in excess of that required by the crops will accumulate in a stagnant underground pond whose top may rise close to the surface.
This water will dissolve minerals on its way down and while lying underground, and usually becomes so alkaline that it injures or kills plants which absorb it. If it does not become loaded enough to do this, it still may injure plants by drowning their lower roots. Also, when the water table is near enough to the surface that capillary attraction will lift it to the surface—a short distance for granular soils, a long one for fine-grained soil—its evaporation will form an alkali crust.
When such a stagnant or semistagnant pool forms, the land above it usually becomes unfit for crops; and even if irrigation is stopped and the water slowly drains away, the alkali deposits in the soil may render it unusable. Artificial leaching would reestablish the underground water.
Drainage. The area can usually be put back in production by the installation of an adequate system of drains. These will serve to lower the water table below the trouble line, or to give the water enough flow toward the drains that it will not stay in the soil long enough to become alkaline.
Such drains preferably are deep, 6 to 7½ feet (1.8 to 2.3 m) being usual, and spaced from 75 to 800 feet (22.9 to 244 m). Close spacing is for impervious soils, wide for granular ones. However, for subdrainage purposes, the porosity of the soil cannot be judged from casual inspection, or even by analysis of samples. Heavy impervious clays often respond readily to tiling because they are filled with fissures, either open or sand-filled, which conduct the water. Many really tight soils will not require drainage because of their refusal to absorb the irrigation water.
Some irrigated lands are composed of alternating layers of heavy and porous soil, which are in the form of lenses tapering to nothing on each end, so that natural drainage must move through both types of soil. Ditching cuts and drains the porous lenses.
The tile lines which do most of the work of draining are called laterals, and the larger pipe into which they empty is called the baseline. Four- or 5-inch (10.2- or 12.7-cm) laterals and 6- or 8-inch (15.2- or 20.3-cm) baselines are usual. Sizes are ordinarily much smaller in proportion to acreage than in nonirrigated fields, but layouts are similar.
If the problem is water leaking from an adjoining canal, an intercepting drain should be placed parallel to the canal, at a distance of 50 to 70 feet (15.2 to 21.3 m). Water may leak under it if it is too close or too shallow.
It is best practice to lay all drainage tile on gravel, and under tar paper and gravel, as described in Chap. 5, as the effective life of the system will be many times that of plain tile. Since tiling is generally done in saturated land, a tile box should be used to avoid danger of cave-ins.
Moles. A mole or mole-ball, Fig. 7.20, is a ripper accessory that will open a drainage tunnel in a plastic soil. The type illustrated is a torpedo-shaped piece of iron, attached to the heel of the standard. As it is pulled through the ground, the mole presses the soil outward with great force, leaving an open tunnel with a firm lining. Seepage from the surface is aided by soil breakage.
FIGURE 7.20 Mole ball. (Courtesy of John Deere.)
Gradient is determined by surface slope and the route taken across it. It should be between 6 inches (0.15 m) and 2 feet (0.61 m) to 1,000 feet (305 m), and in no case should be steep enough to permit erosion in the tunnel. A piece of tile should be placed in each outlet to protect it from erosion or stoppage.
This device is effective only in soils which are damp and plastic enough to be molded, and not soft or loose enough to flow or cave into the passage. An uneven surface or stones in the soil make it difficult or impossible to run it at an even gradient.
Mole drains may not work at all, may stop up in a couple of weeks, or may give satisfactory service for years. Occasionally their good effects are accomplished as much or more by the incidental breaking of an impervious hardpan than by the drains themselves. The work is much more economical than tiling.
Leaching. After a field has been subdrained, alkali can be leached out of it by repeated soaking with irrigation water. This dissolves the chemicals and removes them through the tile lines.
