Forces on Slopes

To determine the causes of landslides we must examine slope stability, which can be expressed in terms of the forces that act on slopes. The stability of a slope expresses the relationship between driving forces, which move earth materials down a slope, and resisting forces, which oppose such movement. The most common driving force is the downslope component of the weight of the slope material, including anything superimposed on the slope, such as vegetation, fill material, or buildings. The most common resisting force is the strength, or the resistance to failure by sliding or flowing of the slope material acting along potential slip planes. Potential slip planes are geologic surfaces of weakness in the slope material; for example, foliation planes in a slope composed of schist, bedding planes in sedimentary rocks, and fractures in all rock types are potential slip planes.

Figure 1 – Curved slip plane. How rotational failure slumps may form a slip plane that approximates the arc of a circle
Figure 2 – Earthflow on soft shale near Santa Barbara, California. (Edward A. Keller)

Slope stability is evaluated by computing a safety factor (SF), defined as the ratio of the resisting forces to the driving forces. If the safety factor is greater than 1, the resisting forces exceed the driving forces, and the slope is considered stable (SF greater than 1.25 is preferred for stability). If the safety factor is less than 1, the driving forces exceed the resisting forces and a slope failure can be expected. Driving and resisting forces are not static: As local conditions change, these forces may change, increasing or decreasing the safety factor.

Figure 3 – Complex landslide at La Conchita, California (1995). This slide, which had an upper slump block and a lower flow, destroyed the three-story home in its path. (Edward A. Keller)

Driving and resisting forces on slopes are determined by the interrelationships of the following variables: – Type of earth materials – Slope angle and topography – Climate – Vegetation – Water – Time Driving and resisting forces are not static: As local conditions change, these forces may change, increasing or decreasing the factor of safety. For example, consider construction of a roadcut in the toe of a slope with a potential slip plane (Figure 4).

Figure 4 – Effects on slope stability of a roadcut in the toe of a slope

The roadcut reduces the driving forces on the slope because some of the slope material has been removed. However, the cut also reduces the resisting forces because the length of the slip plane is reduced, and the resisting force (shear strength) acts along this plane. If you examine Figure 4, you can see that only a small portion of the potential slide mass has been removed, while a relatively large portion of the length of the slip plane has been removed. Therefore, the overall effect of the roadcut is to lower the factor of safety, because the reduction of the driving force is small compared to the reduction of the resisting force. Factor of safety is commonly computed for natural slopes and slopes constructed as part of site development or highway construction.

Edward A. Keller

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