Engineering geology

Earth Slope Stabilization/Mitigation


Some of the Earth Slope stabilization techniques that are currently available in North America are illustrated here. We highlight simple methods that can be used safely in the absence of detailed soil or bedrock analysis or in low-risk situations. Some Earth Slope stabilization methods are very expensive and require significant time to implement. This is an overview of stabilization methods; many other methods are in use around the world. Professional advice is essential before, during, and after implementation (where possible), as is further literature consultation.

The stability of any slope will be improved if certain actions are carried out.
To be effective, the first one must identify the most important controlling process that is affecting the stability of the slope; second, one must determine the appropriate technique to be sufficiently applied to reduce the influence of that process. The mitigative prescription must be designed to fit the condition of the specific slope under study. For example, the installation of drainage pipes into a slope that has very little groundwater is pointless. Earth Slope stabilization efforts take place during construction or when stability problems develop unexpectedly the following construction.

Most slope engineering techniques require a detailed analysis of soil properties and a sound knowledge of the underlying soil and rock mechanics.

In any high-risk situation, where a landslide may endanger lives or adversely affect a property, a professional landslide expert such as a geotechnical or civil engineer should always be consulted before any stabilizing work is undertaken.

The following sections provide a general introduction to techniques that can be used to increase slope stability.

Excavation – Slope Stabilization

Figures 1a, 1b, and 1c provide a cross-sectional view, in schematic form, of general principles for slope excavation, showing the effects and consequences of where on a slope the excavation takes place. These graphics are general in nature, and a geotechnical engineer or other professional should always be consulted if possible.

Removal of soil from the head of a slide

This method reduces the driving force and thereby improves stability. This method is suitable only for cuts into deep soil where rotational landslides may occur. Also, it is ineffective on translational failures on long, uniform or planar slopes or on flow-type landslides.

Reducing the height of the slope

Reducing the height of a cut bank reduces the driving force on the failure plane by reducing the weight of the soil mass and commonly involves the creation of an access road above the main road and the forming of a lower slope by excavation.

Also, it is possible to excavate deeply and lower the main road surface if the right-of-way crosses the upper part of a landslide. This method is only moderately efficient in increasing stability, and a complete solution may involve additional modification of the land. Therefore, according to Chatwin, it usually increases the Factor of Safety by only 10 or 15 percent. (“Factor of Safety” in its simple definition is the ratio of the maximum strength of a piece of material or a part to the probable maximum load to be applied to it.)

Earth Slope Stabilization
Figure 1a. Illustration of the importance of water in the stability of a slope. (Graphic by Rex Baum, U.S. Geological Survey.)
Earth Slope Stabilization
Figure 1b. Illustration of the difference in stability of loading either the head or the toe of a slope. (Graphic by Rex Baum, U.S. Geological Survey.)
Earth Slope Stabilization
Figure 1c. Illustration of the importance of water in the stability of a slope. (Graphic by Rex Baum, U.S. Geological Survey.)

Backfilling with lightweight material

A technique related to height reduction is to excavate the upper soil and replace it with a lightweight backfill material such as woodchips or logging slash. Then, covered with a thin layer of coarse aggregate, the backfilled material can form a foundation for limited-use traffic (fig. 2a,b).

Earth-Slope-Stabilization
Figure 2a,b. Schematic and photograph of a lightweight backfill. There has been increased growth in the use of recycled tire shreds in civil engineering applications. Highway applications include using shredded tires as lightweight fill over weak soils in bridge embankments and retaining wall reinforcements or, in very cold climates, as insulation of the road base to resist frost heaves and as a high-permeability medium for edge drains. (Graphic from reference 11, photograph from U.S. Department of Transportation, Federal Highway Administration.)

Benches

Benches are a series of “steps” cut into a deep soil or rock face for the purpose of reducing the driving forces. They are mainly effective in reducing the incidence of shallow failures but generally are not very efficient in improving the overall slope stability for which other methods are recommended. Benches are useful in providing protection structures beneath rockfall-prone cliffs, for controlling surface drainage, or for providing a work area for installing drainpipe or other structures.(Figure 3).

Figure 3. Benches cut into a slope.

Flattening or reducing slope angle, or other slope modification

This reduces the weight of the material and reduces the possibility of stream/river undercutting or construction loading.

When not to excavate a slide mass

In some situations, removing the entire slide mass is an effective and economic solution. Generally, however, it is only practical on small slumps or small rotational failures. Large-scale excavation of larger landslide areas is usually not recommended for several reasons:

• Excavation is not always effective—for large planar failures, excavation may not cause movement to stop and may allow the landslide to expand.

• Excavation may trigger a larger landslide by removing the support provided by the toe of the landslide.

• Excavation may actually destabilize the ground farther upslope by undercutting, which weakens the slope.

• In deeper soils, especially soft clays, where there are two potential failure surfaces, one deep and one shallow, excavating down to the first failure surface might trigger a sudden slippage on the deeper failure surface. A stability analysis using soil strength data is advised and most always necessary for any major excavation project in deep clay soils.

Featured image MAA group; Geological Survey of Canada, Adapted of “The Landslide Handbook” By L. M. Highland, U.S. Geological Survey, and P. Bobrowsky

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