Investigation of Softening Instability Phenomena Under Simulated Infinite Slope Conditions in Centrifuge Tilting Table Tests
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Element test results reported in the literature under both triaxial and plane strain conditions indicate that loose saturated granular specimens can experience softening instability at stress ratios lower than what might otherwise be expected given the critical state friction angle of the soil. The region of potential softening instability in stress-space is often explained using the framework of the instability line. This phenomenon is particularly relevant to shallow slopes of 1 to 2 m depth. However, the practical realities of sample preparation for triaxial testing make performing tests below 20 to 30 kPa of confining stress exceptionally difficult. In this thesis, the development of a centrifuge tilt-table test device is described which aims to test the behaviour of loose granular slopes under stress paths of increasing slope inclination or increasing pore water pressure. A system of instrumentation including pore pressure transducers, inclinometers, displacement transducers, and high-resolution cameras was designed to monitor the behaviour of the slope model. The development of a system to provide a controlled groundwater level within the slope model proved to be particularly challenging. The results of two competing design concepts are presented for the water boundary condition and discussed. The centrifuge tilt-table is used to compare the physical response of a slope to the behaviour predicted by the infinite slope and softening instability models using scale model centrifuge testing. If softening instability is a rigorous concept, it should be the primary observed failure mechanism as it will occur at a stress state below the failure line. Tests were performed on loose Ottawa F110 sand at 1g, 20g and 40g and 60g. Deviatoric strain-softening was observed in loose dry sand. The softening instability event resulted in a rapid increase in shear strain at constant shear stress while the soil was at a stress state below the failure envelope. Any soil that can experience softening instability (i.e. granular, loose, saturated, and behaves undrained) will undergo two failures: one caused by deviatoric strain-softening (softening instability) and a second caused by shear failure at a larger slope angle.