CONSTANT VOLUME SHEAR TESTS ON COMPACTED SILTY SOIL

Abstract

Most earth structures, such as embankments, roadways, foundations, slopes, and retaining walls, are commonly built above the groundwater table whenever possible, implying that the soil used in the construction of such structures is widely unsaturated in nature. The behavior of unsaturated soil is less predictable as compared to saturated soil. Usually, triaxial tests are carried out on unsaturated soils at various net axial stress and matric suction under constant confining pressure. There is limited research regarding the constant volume tests on unsaturated soil. In this study, a series of triaxial compression tests were performed under constant water content conditions using a double-cell triaxial test apparatus. The samples were prepared with an 80% degree of compaction and an optimum water content of 20%. All the soil specimens were consolidated isotropically under three different confining pressures of 100kPa, 300kPa, and 500kPa before being sheared with constant volume. The soil specimen's volume was controlled by manually altering the cell pressure during the shearing phase (depending on the volume change measurement). A pre-shear infiltration test was also carried out to understand the influence of matric suction in unsaturated soil when sheared with constant volume. The results show that the normally consolidated specimen exhibits anomalous behavior of sudden temporary drop in axial stress within the axial strain range of 0-2%, which can be attributed to the collapse of soil particles held together by suction.