Cyclic stress-strain behavior of partially saturated soils

Although soils are generally assumed fully saturated below the groundwater table, they may be semi saturated near the state of full saturation under certain conditions. The situation of partial saturation may be caused by several factors, such as variation of water table level due to natural or manmade processes. It has been known in geotechnical engineering that the partial saturation of soil usually gives higher strength and lower compressibility. Additionally, it was shown in the recent studies that unsaturation of soil can also cause much greater amplification in ground motion then a fully saturated model and more resistance to liquefaction of sandy soils. Moreover, the resistance of sand to onset of liquefaction tends to increase with a reduction in the saturation ratio of soil specimens that was expressed in terms of the Skempton's pore pressure coefficient, B. Since the B-value is defined as the ratio of the induced pore water pressure to the applied effective confining stress, it can be simply obtained in the laboratory. The Skempton B-value method has been widely used to determine the state of saturation of laboratory soil specimens. In this study, in order to examine the stress-strain behavior and the strain softening response of partially saturated cohesive soil, a cyclic torsional shear apparatus were used, in which it was possible to perform cyclic shear tests on the specimens under undrained conditions. A series of the shear tests were conducted on the partially saturated-undisturbed clay samples. Therefore, the stress-strain properties were investigated to identify the saturation degree of near-saturated samples. Undisturbed clayey specimens were subjected to multi-staged cyclic torsional shear stress, and deformation characteristics of soils, such as the variations of the shear modulus and damping ratios were evaluated in terms of the saturation ratio. For the cyclic phase of torsional tests, the degradation curves were shown to move the right side with the decreasing Skempton B-value. It was also observed that initial values of the shear modulus were considerably affected the Skempton B-value. Copyright ASCE 2006.