Slope Stabilization

This chapter pertains to the design of earth slopes as it relates to road construction. It particularly concerns slope stability and which slopes should be used under average conditions in cuts and embankments. Some of the subjects covered are geologic features that affect slope stability, soil mechanics, indicators of unstable slopes, types of slope failures, and slope stabilization. Road failures can exert a tremendous impact on mission success. It is vital that personnel engaged in road-building activities be aware of the basic principles of slope stability. They must understand how these principles are applied to construct stable roads through various geologic materials with specific conditions of slope and soil. Basic slope stability is illustrated by a description of the balance of forces that exist in undisturbed slopes, how these forces change as loads are applied, and how groundwater affects slope stability and causes road failure. GEOLOGIC FEATURES There are certain geologic features that have a profound effect on slope stability and that can consequently affect road construction in an area. Many of these geologic features can be observed in the field and may also be identified on topographic maps and aerial photographs. In some cases, the presence of these features may be located by comparing geologic and topographic maps. The following paragraphs describe geologic features that have a significant effect on slope stability and the techniques that may be used to identify them: Faults The geologic uplift that accompanies mountain building is evident in the mountainous regions throughout the world. Stresses built up in layers of rock by the warping that accompanies uplift is usually relieved by fracturing. These fractures may extend for great distances both laterally and vertically and are known as faults. Often the material on one side of the fault is displaced vertically relative to the other side; sometimes igneous material or serpentine may be intruded into faults. Faults are the focal point for stress relief and for intrusions of igneous rock and serpentine; therefore, fault zones usually contain rock that is fractured, crushed, or partly metamorphosed. It is extremely important to recognize that fault zones are zones of geologic weakness and, as such, are critical in road location. Faults often leave topographic clues to their location. An effort should be made to identify any faults in the vicinity of a proposed road location. The location of these fault zones is established by looking for— Saddles, or …