Effect of Matric Suction and CSR on Dynamic Response of Expansive Soil

The unsaturated shear behaviour of soil is governed by the presence of negative pore water pressure or matric suction (u<inf>a</inf>-u<inf>w</inf>). Based on the distribution of solid, water and air phases, the soil–water characteristics curve (SWCC) is divided into three zones such as (1) boundary effect zone, (2) transition zone and (3) residual suction zone. The dynamic shear behaviour of soil in these three zones is entirely different due to their particular suction characteristics. However, this aspect of expansive soil has not been explored yet. Expansive soil has a very wide range of suction variation from 0 to 10,00,000 kPa. Hence, the aim of the current study is to evaluate the dynamic response of Nagpur expansive soil subjected to variation of matric suction and cyclic stress ratio (CSR). A series of stress-controlled cyclic simple shear (CSS) tests were carried out on the compacted specimens of Nagpur expansive soil at varying CSR (0.1, 0.3, and 0.5) and degree of saturation (30, 50, 70, and 95%). SWCC of Nagpur soil was determined through the filter paper method, and the saturation values for CSS tests were chosen to cover all the three zones of SWCC. Shear modulus was found to increase with the increase in matric suction until residual suction zone arrived. However, it decreased with further increase in matric suction. Rate and magnitude of stiffness degradation were observed to reduce with the decrease in CSR for complete matric suction range of Nagpur expansive soil. Dissipated cumulative strain energy and cumulative cyclic shear strain were maximum for lowest matric suction specimens for all CSRs.