Study of the variation of B with Sr

Abstract

Skempton coefficient B is commonly used to verify the saturation of a sample before triaxial testing. This coefficient is obtained during undrained isotropic consolidation and is defined as the ratio between the increment of pore pressure u measured and the imposed increment of isotropic stress. This coefficient varies between 0 for dry soils and 1 for saturated soils. Many studies on liquefaction of unsaturated soils were published using Skempton coefficient B to represent saturation degree Sr of soil. On the first hand, this variation of B coefficient with saturation degree is mostly due to the compressibility of air in the pores. On the second hand, we also know that the presence of air as a fluid phase gives birth to suction after equilibrium is reached inside the sample. The higher the suction, the stiffer the soil skeleton. These two phenomena are opposite. Their effects in laboratory testing depend on the experimental apparatus. For example, if we consider an unsaturated triaxial device, we will have to take suction into account. On the contrary if we plan to break the menisci just before measuring B, suction equilibration will not occur. Experimental tests were performed to show the difference between these two cases and to study the equilibrium phase. Based on these observations, this article presents new relationships that permit to calculate saturation degree with a given Skempton coefficient with different hypotheses and with different experimental devices. These results are confronted to the commonly used relation given by Lade and the difference between all these calculations is studied.