Undrained Monotonic Compression, Cyclic Triaxial and Cyclic Simple Shear Response of Natural Soils: Strength and Excess Pore Water Pressure Response

The mechanical response of soils is governed by several factors including loading and boundary conditions. Under undrained boundary conditions, the nature and magnitude of excess pore water pressure (PWP) control the evolution of effective confining pressure (p′) which in turn controls the evolution of shear stress. In this study, we investigate the shear strength and excess PWP response of natural soils under monotonic triaxial compression (TX), cyclic triaxial (CTX) and cyclic simple shear (CSS) testing conditions. The experimental study consisted of evaluating the undrained response of 31 natural soils collected from 10 locations (including 5 dams) in the Kutch region of India. The significance of the investigation lies in the fact that the region is seismically active with a proven history of devastating earthquakes. The most recent earthquake, the 2001 Bhuj earthquake, created large scale destruction with incidences of widespread earthquake liquefaction. The experimental investigation revealed that the undrained response of the soils at the in-situ density is controlled by both the fines content (FC) and plasticity index (PI). For cohesionless soils, FC governed the soil behaviour whereas for cohesive soils PI dominated the soil behaviour. Cohesionless soils exhibited intense strain softening (SS) under monotonic triaxial compression whereas cohesive soils displayed limited strain softening (LSS). Under CTX and CSS testing conditions, cohesionls soils exhibited very low liquefaction resistance (less than 10 cycles) whereas cohesive soils did not liquefy in 50 cycles. However, cohesive soils did exhibit significant degradation in cyclic strength, which was controlled by PI. The excess PWP was found to be contractive for all three conditions. For cyclic loading, PWP was found to be 30% higher for CSS conditions compared to the CTX conditions. Cyclic simple shear simulates the earthquake conditions better and should be considered for seismic and liquefaction analysis.