Behavior of Weakly Cemented Soil Slopes Under Static and Seismic Loading Conditions

Large surface and near surface deposits of weakly cemented soils occur in various geologic environments. These materials are typically composed of sand or silt size particles and can have unconfined compressive strengths from slightly greater than zero up to 690 kN/m2. After failure in an unconfined test, a weakly cemented soil exhibits brittle response and loses most, if not, all of its strength. In spite of their relatively low strength, however, vertical to near vertical slopes of heights in excess of 100 m are found in these materials. Examples may be found in the marine terrace sands along the Pacific Coast of California and Oregon, the windlaid ash deposits in central America, and the loess soils in the central United States and along the Yellow River in China.

Failure of a steep slope in cemented soil during a seismic event is usually spectacular because, if the strength of the soil is exceeded near the face of the slope, it collapses and cascades downward. Observations of such failures have been made in numerous cases, and loss of life and property is substantial where populated areas are involved. Interestingly, however, little is known about the cemented soils. This study represents an initial part of a comprehensive investigation of slope behavior in cemented soils, and it is directed toward: (1) documenting of the behavior of natural slopes in weakly cemented soils, (2) defining the engineering response of weakly cemented soils under static and dynamic loading, (3) establishing the stress conditions in steep slopes during static and dynamic loading, and (4) developing guidelines for evaluation of seismic stability of slopes in cemented soils.

To achieve these objectives, field observations of slope behavior in the weakly cemented soils have been conducted in Guatemala and in California, along the Pacific Coast, between San Francisco and Santa Cruz. A laboratory testing program of over 200 static and dynamic tests on artificially and naturally cemented sands was carried out, and static and dynamic finite element analyses of idealized vertical slopes in linear elastic material were performed.

The results of the field studies show that failure in a cemented soil slope initiates by tensile splitting in the upper part of the slope. The failure then progresses either by toppling of blocks in the upper part of the slope or by shear failure in the lower part of the slope. The likelihood of the tensile fracturing phenomenon is supported by the fact that the dynamic tensile strength of cemented soil decreases as a result of cyclic loading.

The finite element analyses indicate that the magnitude of the dynamic stresses in a slope is a function of both the size of the earthquake as well as the degree of matching between the natural period of the slope and the dominant period of the earthquake. The analyses are also used to develop nondimensionalized charts for evaluation of slope stability of vertical slopes in cemented soil.