Advanced Triaxial Tests on Partially Saturated Soils Under Unconfined Conditions

A series of partially drained (pore air pressure drained and pore water pressure undrained), fully undrained (both pore air and water pressures undrained) and fully drained (both pore air water and pressures drained) triaxial tests were conducted in unconfined conditions to explore the mechanical behaviour of an unsaturated silty soil. The water contents of test specimens were kept at 10, 15, 20 and 25% with the respective degree of saturation of 24, 36, 49 and 59.5%. The void ratio was 1.10 with a dry density of 1.25 g/cm³ while maintaining the relative compaction of all specimens at 80%. During the constant water content (CW) and fully undrained (FU) tests, the peak shear strength was within an axial strain range of 0–2.75% and the volumetric strain of all specimens did not vary within this range. Under shearing infiltration (SI) and constant loading infiltration (CLI) conditions, it was revealed that a significant magnitude of matric suction still exists in the specimens at the peak/post-peak deviatoric stress which depicts that a slope can fail during a rainfall under an unsaturated condition due to negative pore water pressure (matric suction) in the soil. The results of the triaxial tests suggest that the peak shear strength increases with matric suction linearly up to a matric suction of 15 kPa and the rate of increase gradually decreases with further increase in matric suction. Moreover, the failure envelopes have revealed an existence of a critical suction value of 15 kPa corresponding to 27% water content (i.e. on the dry side of optimum water content) and the increase in shear strength due to suction becomes insignificant afterwards. It is also observed that the peak shear strength increases nonlinearly with the increase in matric suction with a good agreement between the failure envelopes for CW, FU and SI tests.