Dynamic Soil Strength and Slope Stability

The laboratory method of testing cohesive soils to determine the effects of pulsating stresses, simulating the effects of earthquakes, provided adequate data for designing hydraulic structures made from these materials. The tests were performed on undisturbed fine-grained clay soil from the San Luis Dam foundation and on undisturbed and remolded fine-grained clayey and sandy clay soils from the first three reaches of the canal, to determine soil strength characteristics under dynamically applied stresses. Results of the dynamic tests on individual soils are compared to the normal soil strengths at equivalent strains. The maximum normal soil strength is assigned a value of 100%, and soil strengths under pulsating stress may be expressed as a percentage of this. The studies show that confining pressure does not appreciably affect the relationship between normal and dynamic soil strength. Results show the dynamic soil strength to be generally up to 15% greater than the normal strength for overconsolidated plastic clays. However, dynamic soil strength is less than the normal by about 10% for low density, wet of optimum water content, sandy clays, especially at low strains.