Figure 4. Retaining walls can be used to deal with more severe changes in grade. |
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Regrading the land surface of a construction site is done for a variety of reasons. Some sites require regrading to properly site the house. Regrading is sometimes done to handle surface runoff from adjacent properties. Of course, the easiest way for a builder to dispose of the soil excavated when the basement was dug is to spread it over part or all of the site. From a tree's perspective, with rare exception, little good results from the regrading that occurs during construction. As enumerated in Column 2 of Table 3, the natural surface and subsurface drainage patterns and water characteristics are altered, soils can be compacted, tree roots may be destroyed, and many physical and chemical characteristics of the soil may be altered.
The majority of tree feeder roots are located in the top four to eight inches of soil. When the soil grade is decreased, not only is the nutrient rich topsoil removed, but if trees are present large portions of their feeder roots are exposed or destroyed. The first, and most important guideline concerning decreasing soil grade when trees are present is "don't do it unless it is absolutely necessary." If possible, when an overall change in grade is necessary, obtain fill from another source rather than by decreasing the grade around existing trees to get needed fill.
When trees are present and soil grade must be decreased, the impact can be reduced by preserving the existing grade in a circular area around the tree equal in diameter to at least one-foot for every inch of stem diameter. A red maple 10 inches in diameter, for example, would be in the center of an undisturbed circular area of soil at least 10 feet in diameter. Where the grade is to be decreased along one side of the tree, a similar distance between the trunk and the grade cut should be maintained along that side.
Before the grade is reduced around trees that are to be retained, the roots should be root pruned to avoid tearing and shredding by grading equipment. How the root pruning is accomplished will depend on the size of the tree and the roots, the depth of grading, and the proximity of the grading to the tree's trunk. With relatively shallow grade reductions, root pruning can be accomplished using a nursery spade or large circular concrete saw. Root pruning in areas with deeper grade reductions may require retaining walls with roots being pruned with shears or saws. An alternative would be to establish the edge of the undisturbed area before grading by trenching around the tree somewhat beyond the edge of the undisturbed area with a backhoe and then clearing the soil by hand to the edge of the undisturbed area and hand pruning the roots.
After grading, stone or wood retaining walls can be used to retain the soil around the edge of the undisturbed area (Figure 4). Trees growing in undisturbed "islands" where the surrounding soil grade has been decreased may require fertilization and more frequent watering during drought periods than had the area not been regraded.
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Figure 4. Retaining walls can be used to deal with more severe changes in grade. |
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Where there are no existing trees, simply decreasing the soil grade by moving the surface soil elsewhere results in the removal of the topsoil. Topsoil has the most desirable characteristics for tree (and other plant) growth. Once the topsoil is removed, trees and other plants planted on the site will be planted in subsoil. The subsoil has far less desirable characteristics for plant growth including a higher clay content, poorer aeration, and higher soil pH. When the soil grade must be reduced in an area without trees, the growth of trees and other plants planted on the area in the future will generally benefit from the topsoil being graded to the side, with the grade reduction performed in the subsoil. The topsoil can then be returned and graded evenly over the area.
In certain situations decreases in soil grade can lead to the impounding of water at or near the soil surface. This can be a particular problem in areas where the topography is flat to concave, where the grading occurs to a depth that exposes the lower soil layers, or where soil from lower soil layers has been used as a surface application. Lower soil layers (termed horizons) generally have higher clay contents and much lower water infiltration and percolation rates. If water impoundment occurs in an area where trees are to be planted, species tolerant to the new, wetter conditions will need to be selected. If impoundment occurs in an area with existing trees, it may, depending on the species present and the change in soil moisture characteristics, result in the short- or long-term decline and death of some or all of the trees present. The best solution is to anticipate the problem and, if possible, modify the grading plans to minimize the impact on the trees. Utilizing some form of soil drainage technology, such as tiling, might be possible on a relatively large commercial property but would generally not be feasible on an individual residential lot.
The effects of increasing the soil grade depend on whether trees are present and on the nature and depth of the fill. If the grade is to be increased on an area without trees but where trees are to be planted, the soil added should maintain or improve the site's ability to grow trees. Ideally, a loamy textured topsoil should be brought in to increase soil grade. Adding additional topsoil onto the surface of an existing soil will not reduce the ability of most Ohio soils to support trees, providing the surface and subsurface drainage characteristics of the site are not significantly altered. The addition of a high quality, loamy topsoil will improve the ability of many soils to support vigorous tree growth.
On the other hand, the use of finer textured soil, such as clays, silty clays, clay loams, or silty loams, whether imported to the site or from on-site excavations, can create severe drainage problems. Except at very high water contents, water does not move well across abrupt boundaries between finer and coarser textured soils. As a result, placing finer textured soils, such as those found in the subsoil, on top of an existing topsoil, which is generally coarser textured, will often produce a perched water table. The water moves slowly into the finer fill but does not drain into the coarser topsoil buried below. The result is a surface layer of soil that maintains a very high water content for an extended period of time after rain. Few tree species will thrive and many will die in such a situation.
A technique sometimes used to reduce the perched water table effect when increasing grade with relatively fine textured fill is to create a gradual boundary or transition zone between the original soil and the fill. This is done by applying two inches of fine-textured fill to the existing soil and rototilling it in to a depth of four to six inches. Apply a second two inches of fine-textured soil and rototill it in to a depth of four to six inches. Then apply the remaining fill to bring the area up to the desired grade. The two two-inch applications and the accompanying rototilling creates a transition zone between the two soil textures that facilitates water movement between the differing soil textures (Figure 5).
Figure 5. Minor fills add no more than four inches of fill. Create a transition zone as shown if soil textures vary. |
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Just as with decreasing soil grade, increasing soil grade can channel increased amounts of surface and subsurface water into areas where trees are growing or are to be planted. The results and concerns are the same. If increased water is channeled into areas where trees are to be planted, choice of species will be limited to trees tolerant of the new, wetter conditions. If increased water is channeled into an area with existing trees, it may alter the soil air and water environment, and may, depending on the species present, result in the short- or long-term decline and death of some or all of the trees present. As noted earlier, the best solution to changes in grade that markedly alter water distribution on a property is to anticipate problems that might be created and modify the grading plan to minimize the impacts. While utilizing some form of soil drainage technology, such as tiling, may be possible on a large commercial property, it would be less likely on an individual residential lot.
When the soil grade is increased on an area where trees are present, besides the possibility of altering drainage patterns and modifying soil characteristics, there is the added concern of reducing the impact of the increased grade on the existing trees. Adding fill over tree roots reduces gas movement between the soil and the air and disturbs soil water movement, both of which can result in the decreased vigor or death of trees. Some species, particularly those that grow naturally along water courses and are subject to periodic siltation such as box elder, willow, and sycamore, will tolerate a moderate amount of fill. Others, particularly species associated with a climax forest such as sugar maple, American beech, and the lindens, or species not naturally subjected to siltation such as white oak, do not tolerate any amount of fill well.
When trees are present, the first, and most important guideline concerning increasing soil grade is "don't do it unless it is absolutely necessary." This is the same warning given earlier for decreasing the grade. If increasing the grade where trees are present is absolutely necessary, the approach to minimizing the impact of the fill on the trees varies, depending on the depth of fill applied.
Minor or Shallow Fill No More Than Four Inches Deep. Few species will tolerate fill of even a few inches placed against the main trunk. Besides reducing air exchange, such fill creates an environment conducive to the development of diseases such as the crown rots.
Four inches is commonly considered the maximum amount of fill that can be "safely" applied under an existing tree. Remember as noted earlier, tree species vary considerably in their ability to tolerate fill (Appendix). Where there is a question about the likelihood of a tree tolerating a planned fill, a certified arborist should certainly be consulted before the grading begins.
Several practices can minimize the impact of shallow fill. Whenever possible, fill should be coarser textured than the existing soil to facilitate water and air movement into and through the soil. If a finer textured fill is used, some hand tilling may be desirable to create the transition zone discussed earlier, but care must be taken not to till too deep or tree feeder roots will be damaged (Figure 5). Fill should be kept several feet away from the main stem of the tree and graded out to the desired depth. Under no circumstance should fill be placed against the trunk of the tree. And finally, grading within the tree's dripline area should drain away from the tree, or drainage should be directed away from the main stem by a berm.
Moderate to Severe Fill (More Than Four Inches). Few trees can confidently be predicted to thrive or even survive if more than four inches of fill are placed directly over their roots. As a result, various procedures and structures have been developed to reduce the impact of these deeper fills by protecting the trunk of the tree and facilitating air and water movement into and through the soil. Two techniques that have been used successfully are illustrated here. One is for moderate fills between four and 12 inches, and one is for deeper fills. These two techniques are presented for illustration only. They do emphasize the potential work and expense that can be involved in minimizing the impacts of deep fills on trees.
Anyone anticipating the need to minimize the impact of deep fills on trees should work closely with an experienced ISA Certified Arborist. It is important not only to select the most effective (and cost effective) method, but also to have a realistic evaluation of the likelihood of success. The probability will vary depending on the site conditions, the species of tree, and the condition of the tree.
A technique used with fills four to twelve inches deep is illustrated in Figure 6. This technique involves a well around the main stem of the tree, a soil berm to divert water away from the well, and vertical air holes in the soil. A well wide enough to protect the trunk and main root flair (along with anticipated growth) is constructed around the main stem of the tree with its bottom on the original grade. If necessary, a soil berm is placed on the uphill side of the well to divert water away from the well and prevent water from filling the well. Vertical holes two to four inches in diameter are drilled on approximately two-foot centers to a depth of one foot into the original soil. The holes should begin four feet out from the tree's trunk or one foot out from the well (whichever is further), and should extend at least to the tree's dripline. Each hole should be lined with a porus material such as clay or perforated pipe and be covered with a grate or filled with a coarse, inert material such as pea gravel (not limestone or dolomite) for safety and to exclude animals.
Figure 6. Side (A) and top (B) views of a procedure used to deal with moderate fills between four and 12 inches deep. |
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Major fills, deeper than 12 inches, generally require an even more elaborate and expensive solution such as the hub and spoke system illustrated in Figure 7. In this system the tree's trunk is surrounded by a well and the area under the dripline contains a hub-and-spoke system, of four-inch perforated drain tile, constructed at the original grade. The well, large enough to protect the trunk and main root flair (along with anticipated growth), is constructed around the main stem of the tree with its bottom on the original grade. The interconnected drain tile system consists of "spokes" (which are approximately two feet apart at mid radius), a "wheel" of drain tile under or slightly beyond the tree's dripline into which the spokes connect, and vertical drain tile connected to the wheel where the spokes connect and extending to the top of the fill. At the center of the "wheel," the "spokes" open into the well with their ends covered with soil filters or joined to a small "wheel" of drain tile circling the bottom of the well. The entire drainage system is connected to a positive drain outlet such as a slope or storm sewer (if permitted) to provide drainage. Drain tile openings to the air must be covered with a grate for safety, exclusion of animals, and to allow air and water movement.
Figure 7. A hub-and-spoke treatment to protect a tree from a fill in excess of 12 inches. |
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Soil is then carefully added to build the new grade, being careful to maintain the integrity of the tile system. If no more than 18 inches of soil is to be added, soil alone is used as fill. The soil for filling is preferably of a coarser texture than the underlying soil. If more than 18 inches of soil is required, inert gravel is used for the intermediate fill and is capped with 18 inches of soil. The gravel should be protected from rapid siltation by covering it with geotextile fabric. A soil berm is placed on the uphill side of the well to divert excess water from entering the well.