8. Slope Stabilization Measures (Vegetation and Mulches)

Slope stabilization includes revegetation and other measures to control surface erosion on road cut and fill slopes and on waste and borrow areas. Usually the objective is to establish a dense vegetative cover to reduce forces available for erosion and increase surface protection.

Considerable evidence indicates that surface erosion on severely disturbed soils such as road fills is highest immediately after disturbance and decreases rapidly over time. On granitic slopes in Idaho, approximately 80% of surface erosion occurred within the first year following disturbance (reference 16). This suggests two things: (a) it is mandatory that stabilization practices be applied immediately during and following construction; and (b) stabilization practices must provide rapid benefits. Thus, simply seeding disturbed areas may not be acceptable; transplanting or mulching may be required to achieve the desired results.

Stabilization of mass erosion is beyond the scope of this guideline; however, there is considerable evidence suggesting that deep-rooted vegetation (trees and shrubs) acts as a deterrent to mass erosion. Since deep-rooted vegetation also helps control surface erosion, its use is advocated for slope stabilization (reference 17).

Some suggested guidelines for slope stabilization are described below:

8.1 Revegetation

1. Site factors governing air and soil temperature, soil moisture, and fertility are important influences on revegetation success. Large variations in these factors can occur throughout the length of a road, particularly in low precipitation zones or in areas with prolonged dry seasons. Such differences are often magnified in mountainous areas. Thus it is important to tailor revegetation measures to the specific site factors. Consider elevation, aspect, rain shadow effects, ground-water seepage, soil and bedrock properties, and so forth, in evaluating site differences. Be sure to include vegetation in the evaluation, both as an indicator of site potential and to serve as a guide for species selection.
2. Site preparation is often an important prelude to seeding and planting. This might include various practices such as: (a) spreading previously stockpiled topsoil; (b) chaining, harrowing, disking or rolling to roughen the seedbed and break surface crusting; and (c) fertilizing.
3. It is desirable to conduct seeding operations before mulching to attain maximum benefit from the mulch. However, this is not possible in some locations (such as., areas with pronounced dry seasons) because the time of seeding or transplanting is critical. To illustrate, many locations on the west coast of the United States are influenced by a distinct Mediterranean climate causing a prolonged drought period in the summer. Seeding and planting operations during the drought period are usually failures; operations must take place in the fall to be successful. Transplanting through a previously applied mulch is often successful in these situations. Sometimes wattling (installing low barriers of soil and brush along the contour up and down the slope) is a successful substitute for mulching. Seeding, planting, or both, between the wattles can then be carried out at the proper time.
4. Species selection must be designed to meet local needs. Grasses have been most commonly used; however, forbs, shrubs, and trees alone or in combination should be considered. Legumes have particular benefits as nitrogen fixers as do some other plants. Deep-rooted plants including both trees and shrubs can help increase mass stability as well as reduce surface erosion. Rapid-growing, short-lived species (such as., some of the ryes and oats) are often desirable for nurse crops for slower-growing vegetation.
5. Fertilization should accompany most revegetation operations. Proper types and amounts of fertilizer should be based upon soil analyses or experience in the area. Additional amounts may be required if organic mulches are used.

8.2 Mulches

In some areas, vegetation response is rapid enough to provide slope protection during the initial high erosion period. However, it is usually necessary to supplement the protection during the interim with mulches. Mulching provides additional benefits by reducing surface soil temperatures, water losses from the soil, and soil crust formulation.

1. Many kinds of materials ranging from logging slash to peanut shells have been used for mulching. Type of material is not as important as the need to use sufficient amounts in close contact with the soil.
2. On steeper areas it is often necessary to anchor the mulch into the soil by covering it with netting material that is pinned in place, spraying adhesive chemicals (such as, liquid asphalt, various polymers) onto the mulch, or rolling it with a spike roller.
3. Machines have been developed that combine mulching material (straw or wood fiber are commonly used) with water, an adhesive, or both, and spray the mixture onto the slope. Usually, seed and sometimes fertilizers are added to the mixture to provide multi-benefits in one operation.

8.3 Other Practices

Other types of stabilization practices have also been used:

1. Recent development of polymers permits stabilization of disturbed areas by spraying the material on the surface.
2. Sometimes, revegetation practices increase infiltration of water into the surface sufficiently to increase mass erosion hazards to the point of failure (reference 11). In these cases, an impermeable material (such as, asphalt, certain polymers, or even plastic sheeting) may be required to stabilize surfaces.

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Last Modified: 07/05/2007