Land Grading and Stabilization

From Massachusetts Erosion and Sediment Control Guidelines for Urban and Suburban Areas

Definition

Using engineering techniques or vegetative practices, or a combination of both, to provide surface drainage and control erosion and sedimentation while reshaping and stabilizing the ground surface.

Where Practice Applies

This practice applies where the existing ground surface is regraded, new cut or fill slopes are created, or existing slopes or ground surfaces would otherwise be unstable or subject to erosion.

Planning Considerations

Provisions should be made to safely conduct surface runoff to storm drains, protected outlets, or to a stable watercourse to insure that the runoff will not damage slopes or other graded areas. Wherever possible runoff water should be diverted away from the top of cut and fill slopes to stable outlets or grade control structures. Waterways, diversions, grade stabilization structures, terraces, pipe drains, flumes, subsurface drains, use of vegetation or bioengineering, or rock fills are some of the practices that may find use in slope stabilization.

Cuts, Fills, and Slopes

Compaction can be a major factor in erosion control for fill slopes. In addition to other compaction controls required by the nature of the project, the minimum criterion recommended for successful erosion control on fill slopes is to compact the uppermost one foot of fill to at least 85 percent of the maximum unit weight (based on the modified AASHTO compaction test). This is usually accomplished by running heavy equipment over the fill.

On cut slopes ground water seepage causes undercutting and soil slippage. Controls can include: subsurface drains or a layer of crushed rock at the toe of the slope. Slopes steeper than 2:1 will usually require special stabilization measures such as a crushed rock or riprap layer, crib wall or revetment. Additionally, sod can be used to stabilize steep slopes instead of seeding where grades are not more than 2:1. Sod on slopes steeper than 3:1 should be pegged. Sandy soils present a special problem for the establishment of vegetation, especially in areas where the sand is deep and droughty. American beachgrass is one solution to this problem. It is usually established by hand planting.

Steeply sloped areas such as lakeshores and road banks involve three special considerations:

To insure reasonable success in stabilization, bank slopes should be 2:1 or flatter.
The toe of the slope must be protected from undercutting by mechanical means where necessary.
Water seeping from the face of the slope should be intercepted by a drainage system.

Borrow and Stockpile Areas

Borrow areas, as well as stockpile and spoil areas, must be stabilized. Borrow and stockpile areas present the same set of problems for the control of erosion and sedimentation as exposed cut and fill slopes. Runoff should be diverted from the face of the slopes which are exposed in the excavation process. The runoff must then be conveyed in stabilized channels to stable disposal points. Stockpiles should be shaped and seeded with temporary cover.

Where borrow areas are off the development site, a separate system for trapping sediment from the area is needed. After the excavation is complete, borrow areas should be regarded to insure proper drainage and to blend the borrow area with the surrounding topography. Stockpiled topsoil is then redistributed and permanent vegetative cover established.

Exposed Surfaces

Although erosion rates on steep exposed slopes are higher than on flat or gently sloping areas, all areas of exposed soil are vulnerable to erosion. If erosion control is ignored on larger areas of nearly flat or gently sloping land, it will be possible for significant amounts of soil to be eroded. Clearing, grading, and vegetative restabilization in these areas can be timed so that the extent of exposed area and the duration of exposure is minimized. These areas require prompt vegetative restabilization. Temporary seeding or mulching is required where larger areas will not be permanently stabilized within recommended time limits. Diversions, sediment barriers, or traps constructed on the lower side of large disturbed areas should be used to intercept and collect sediment.

Right-of-ways and parking areas that are being prepared for paving must be protected from rainfall and runoff. Control measures include:

Divert runoff to protect these areas before clearing and grading begins.
Properly compact the exposed surface area to reduce erosion potential.
Use crushed aggregate or temporary seeding to reduce erosion.
Gravel or stone filter berms should be used at intervals along a right-of-way to intercept runoff and direct it to stabilized areas, drainageways, or enclosed drainage system inlets.

Construction Areas and Eroding Areas

Types of plantings

When erosion or sediment control is of primary and immediate concern, these areas are usually initially stabilized by seeding grass cover. When necessary, the site should be prepared by seeding temporary vegetative cover. Jute netting or anchored mulch should be used in conjunction with seeding at critical locations where water concentrates.

Seeding mixtures

When dense plant cover is needed for erosion and sediment control, or for appearances, seedings of enduring herbaceous species should be used. See the Permanent Seeding and Temporary Seeding practices. One-half to one bushel of oats, or 1 to 1 1.2 bushels of rye should usually be added to the basic mixture for quick cover.

Design Recommendations

Cut or fill slopes which are to be vegetated should not be steeper than 2 horizontal to 1 vertical. If a slope is to be mowed, it should be 3: 1 or flatter. Slopes of materials not to be vegetated should be at the safe angle of repose for the materials encountered.
Provisions should be made to safely conduct surface water to storm drains or suitable natural water courses and to prevent surface runoff from damaging cut faces and fill slopes.
Terraces or diversions should be provided whenever the height of the cut or fill exceeds 20 feet. The "benches" should divide the slope face as equally as possible and should convey the water into stable outlets. Benches should be kept free of sediment during all phases of development.
Seeps or springs encountered during construction should be controlled by subsurface drains or other appropriate methods. Subsurface drainage should be provided in areas having a high water table, to intercept seepage that would affect slope stability, building foundations, or create undesirable wetness.
Excavations should not be made so close to property lines as to endanger adjoining property without supporting and protecting such property from erosion, sliding, settling, or cracking.
No fill should be placed where it will slide or wash onto off site properties or be placed adjacent to the bank of a channel so as to create bank failure or reduce the natural capacity of the stream. Fills should consist of material from cut areas, borrow pits, or other approved sources.
Protective slopes around buildings should be planned to slope away from foundations and water supply wells to lower areas, drainage channels, or waterways. The minimum horizontal length should be 10 feet, except where restricted by property lines. The minimum vertical fall of protective slopes should be 6 inches, except that the vertical fall at the high point at the upper end of a swale may be reduced to 3 inches, if a long slope toward a building or from a nearby high bank will not exist.
Minimum gradients should be 1/16 inch per foot (1/2 percent) for concrete or other impervious surfaces and 1/4 inch per foot (2 percent) for pervious surfaces.
Maximum gradient of protective slopes should be 2 1/2 inches per foot (21 percent) for a minimum of 4 feet away from all building walls, except where restricted by property lines.
All graded areas should be permanently stabilized immediately following final grading.
Site plans should show the location, slope, cut, fill, and finish elevation of the surfaces to be graded and the auxiliary practices for safe disposal of runoff water, slope stabilization, erosion control, and drainage such as waterways, lined, ditches, diversions, grade stabilization structures, retaining walls, and surface and subsurface drains.

Construction Recommendations

Areas to be graded should be cleared and grubbed of all timber, logs, brush, rubbish, and vegetable matter that will interfere with the grading operation. Topsoil should be stripped and stockpiled for use on critical disturbed areas for establishment of vegetation. Cut slopes to be topsoiled should be thoroughly scarified to a minimum depth of 3 inches prior to placement of topsoil.
Fill materials should be generally free of brush, miscellaneous debris, rocks, and stumps. Frozen materials or soft and easily compressible materials should not be used in fills intended to support buildings, parking lots, roads, conduits, or other structures.
Compaction of other fills should be to the density required to control sloughing, erosion or excessive moisture content.
Maximum thickness of fill layers prior to compaction should not exceed 9 inches or as specified by the design engineer.
All disturbed areas should be free draining, left with a neat and finished appearance, and should be protected from erosion.

Maintenance

All slopes should be checked periodically to see that vegetation is in good condition. Any rills or damage from erosion and animal burrowing should be repaired immediately to avoid further damage.
If seeps develop on the slopes, the area should be evaluated to determine if the seep will cause an unstable condition. Subsurface drains or a gravel mulch may be required to solve seep problems.
Diversions, berms, and waterways should be checked to see that they are functioning properly. Problems found during the inspections should be repaired promptly.
Areas requiring revegetation should be repaired immediately.
Slopes should be limed and fertilized as necessary to keep vegetation healthy.
Control undesirable vegetation such as weeds and woody growth to avoid bank stability problems in the future.

References

Massachusetts Department of Environmental Protection, Office of Watershed Management, Nonpoint SourceProgram, Massachusetts Nonpoint Source Management Manual, Boston, Massachusetts, June, 1993.

Minnick, E. L., and H. T. Marshall, Stormwater Management and Erosion Control for Urban and Developing Areas in New Hampshire, Rockingham County Conservation District, August 1992.