Cyclic strength and pore pressure generation characteristics of fine-grained soils

Liquefaction potential research during the past three decades emphasized clean sands, until liquefaction was observed in "somewhat cohesive" soils as a result of strong earthquakes in 1975 and 1976 in the People's Republic of China (PRC). A wide range of soil gradations and consistencies that have not been extensively researched may be associated with severe shear strength loss on undrained cyclic loading.; This study was sponsored by the US Army Corps of Engineers to address the urgent need to evaluate the hazard posed by fine-grained alluvial soils in the foundations of large earth dams. The research approach consisted of the following activities: (1) collect field occurrence data on fine-grained soil liquefaction; (2) conduct a reconnaissance cyclic triaxial test program on reconstituted specimens to evaluate the effects of grain size and index properties; (3) conduct cyclic hollow cylinder torsional simple shear tests on selected soil mixtures to correlate cyclic simple shear and cyclic triaxial strengths; and (4) develop pore pressure generation and residual strength relationships for use in post-earthquake stability analyses.; Nearly 500 cyclic triaxial tests were conducted on isotropically-consolidated, moist-compacted specimens prepared from 129 soil mixtures varying in fines content and type. Cyclic torsional shear tests were conducted on both isotropically- and anisotropically-consolidated, moist-compacted specimens of four fine sand-based soil mixtures. For a given void ratio, cyclic strength of the mixtures tested was reduced to a lower bound value when the fractional weight of silt- and clay-sized particles reached 24% to 30%. Cyclic strength increased with the addition of fines beyond this point. Plasticity index of the fines fraction exerts a negligible effect on cyclic strength at a given void ratio.; The relationship between cyclic triaxial strength and cyclic simple (torsional) shear strength was found to be dependent on soil type and density condition. Cyclic triaxial tests should not be used to determine in situ cyclic strength of sands that contain more than about 5% fines.; A procedure for seismic stability evaluation of earth dams is proposed, including the findings of this study regarding the effects of fines content and type on cyclic strength, pore pressure generation behavior and residual strength.