Poisson Ratio and Elastic Moduli of Partly Saturated Sand during Undrained Cyclic Loading

Abstract A method is presented to determine the average Poisson’s ratio ( ν ) of partly saturated sand up to any hysteresis loop while conducting a series of consolidated undrained (CU) strain-controlled cyclic triaxial shear tests; neither air nor water was allowed to drain from the specimen while cyclic deviatoric stress was applied. The effect of the number of loading cycles and degree of saturation ( S r ) on the average Poisson’s ratio up to any hysteresis loop for partly saturated sand has been examined. The average value of Young’s modulus ( E ) for any loop has also been determined. Experiments were performed for (1) two different effective confining pressures ( σ ′ 3 ), namely, 100 and 300 kPa; (2) two different values of relative density (RD), namely, 40 and 60 percent; and (3) three different amplitudes of axial strain, namely, 0.5, 1.0, and 2.0 percent. The measurements reveal that, for the chosen sand with the selected relative densities, an increase in the number of loading cycles invariably increases the value of E because of soil densification, whereas it decreases the value of ν . An increase in S r invariably leads to an increase in ν , and for fully saturated sand, the value of ν approaches 0.5. The magnitude of E , however, was found to reach a maximum corresponding to a certain optimum value of S r , which was approximately 50–55 percent for the chosen sand. The existence of the optimum degree of saturation ( S r −opt ) is due to the maximum observed increase in contractive volumetric strain at S r = S r −opt , which leads to the maximum relative density of the specimen. An increase in the magnitude of σ ′ 3 as well as in RD leads to a reduction in the value of ν . On the other hand, an increase in the axial strain leads to an increase in the value of ν and a reduction in the magnitude of E . The results obtained from the present research were also compared with data reported in the literature, and the observed trends in the results were found to be generally in line with those reported by other researchers. The observations drawn from the present research will be useful for determining the response of partly saturated sand during cyclic loading, and the proposed method for evaluating Poisson’s ratio, based on the measurement of axial and volumetric strains, appears to be reasonably acceptable.