Roscoe and Hvorslev Surfaces for Unsaturated Silty Soil

The existence and characteristics of Roscoe and Hvorslev state boundary surfaces for unsaturated silty soil have been investigated by conducting controlled-suction triaxial tests on samples of an unsaturated compacted silty soil. The experimental tests were performed in a specially designed double-walled Bishop and Wesley triaxial apparatus. In the experiments, the soil samples were subjected to isotropic consolidation to a virgin state (under constant suction) followed by unloading to a predefined overconsolidation ratio (OCR). The Roscoe and Hvorslev state boundary surfaces for the unsaturated soil were studied by performing drained triaxial shearing tests on the samples with specified OCRs. The results show that the Roscoe and Hvorslev surfaces also exist for unsaturated silty soil. The location of these surfaces in the normalized space (q/pe '):(pnet/pe ') is dependent on the suction. The Roscoe surface shrinks and expands with decreasing and increasing suction, and the Hvorslev surfaces are nearly parallel for different suctions.