Response of Unsaturated Soils Under Monotonic and Dynamic Loading over Moderate Suction States

Abstract

The understanding of the mechanical behavior of the unsaturated soils has been significantly enhanced due to the recent advancement of technology and modifications to the conventional testing devices. However, the implementation of the concepts of unsaturated soil has lagged, mainly due to the risks associated with the development of civil infrastructure based on practically untested theories and the limited reliable prediction methods for compacted, expansive, residual, and collapsible soils, which requires reliable and comprehensive experimental data. Moreover, the tests on unsaturated soils are enormously time consuming and often exorbitantly expensive, which further hinders the use of unsaturated soil mechanics in practical applications. Additionally, the liquefaction of cohesionless soils has predominantly been considered only for saturated soils. However, the soils with high degree of saturation, yet partially saturated, which are mostly near the ground surface and have low overburden pressure, may liquefy during earthquakes. Since plenty of unsaturated cohesionless soils are present near high seismic activity zones, the susceptibility of these soils to liquefy needs to be studied. Addressing these problems has been the main objective of the present dissertation research. The characteristics of compacted soils were studied by performing an elaborate series of suction-controlled monotonic triaxial tests on compacted silty soil specimens following the hydrostatic compression (HC) and the conventional triaxial compression (CTC) stress paths at varying net mean stresses and matric suction levels. The multistage triaxial tests were performed on the silty soil specimens at varying suction levels using a new approach to address the issues related to soil variability and to significantly reduce the testing time while maintaining the reliability of the test results. The Barcelona Basic Model (BBM) framework was used to reproduce experimental test results both from suction-controlled single-stage and multistage triaxial tests. Hence the BBM was indirectly used to demonstrate the utility of multistage triaxial tests in replicating test results or predictions similar to single-stage triaxial testing. The influence of suction, unit weight, and wetting and drying of the compacted specimens of silty soil on the resilient modulus was also studied by performing a comprehensive series of suction-controlled repeated load triaxial (RLT) tests over a wide range of suction to address the issue of repeatability of RLT tests. In this dissertation research, the tendency of the soil in unsaturated condition to liquefy was studied by performing cyclic triaxial tests on suction-equilibrated silty sand specimens. A series of cyclic triaxial tests were performed in undrained conditions at varying suction states and cyclic stress ratios to assess the liquefaction potential of the soil at varying relative densities and matric suction levels. The findings from this dissertation research would assist in the development of future constitutive models for predicting the response of dilatant soils in unsaturated conditions subjected to monotonic and dynamic loading.