Understanding stress paths of unconfined unsaturated soil under matric suction in stress state space

Simple drained, undrained and partially drained shear tests are frequently performed in soil mechanics laboratories. However, the test that simulates field conditions in which surface soil undergoes water infiltration followed by short- and long-term loading is rarely performed. Considering the fact, two series of tests have been performed in the present study. Series-1 replicates field conditions in which the surface layer of the soil experiences water infiltration followed by quick/sudden loading; the tests performed in this series are termed as pre-infiltration shearing tests. Whereas Series-2 replicates field conditions in which the surface layer of the soil experiences water infiltration followed by long term/slow loading and the tests performed in this series are termed as shearing infiltration tests. Water in these series was injected by reducing matric suction and conducted shearing in constant matric suction plane. The pore water pressure remained undrained in Series-1, during the shearing process, while in Series-2, it was maintained in a drained condition. The results indicated that reducing matric suction from 20 kPa to 0 kPa increased water infiltration by up to 37.5 cm, raised the degree of saturation by approximately 30%, and reduced peak deviatoric stress from about 28 kPa to 5 kPa. The soil behavior transitioned from dilation to compression, resulting in decreased stiffness and shear strength. Furthermore, matric suction effect on the stress path within the stress state space is examined and analyzed.