Total stress ratio and shear strength parameters for an unsaturated compacted soil

Unsaturated natural soils and artificially compacted soils near the ground surface are commonly overconsolidated due to environmental changes. Defining the overconsolidation ratio with respect to an unsaturated soil is difficult. In this study, a total stress ratio, TSR, is suggested which is the static compaction pressure, divided by the current confining pressure. The laboratory tests were performed on an unsaturated compacted soil using a modified direct shear apparatus. This paper presents data on the shear strength parameters, phi', phi(b) and c' for a compacted unsaturated soil with a total stress ratio, TSR, of 1.0. The change in shear strength of an unsaturated soil, due to matric suction can be related to the soil-water characteristic curve. Several models for predicting of the shear strength of an unsaturated soil are proposed in geotechnical literature. Measured shear strength with respect to matric suction was compared with the predicted shear strength by using a soil-water characteristic curve model.