Boundary and internal sensing of granular instability in dry sand tilt table tests

This work explores the phenomena of instability in low stress tilt-testing of dry sands using novel imbedded shear stress and strain sensors to capture quantitative data of constitutive behaviour leading up to and during instability. A unique strategy of testing dry soil enabled the investigation of instability in isolation from pore pressure consequences. Six shear stress and shear distortion sensors were distributed along the transparent sidewall and centre of the tilt-table which was then rotated until instability and failure. Shear strain measured next to the transparent sidewall were 50% of those measured along the table centerline. Friction reduction strategies were successful in significantly reducing, but not eliminating, this discrepancy. Soil layers tilted until reaching instability, unloaded, and subjected to a second tilt cycle only exhibit instability at angles in excess of the first instability event. This phenomenon, analogous to the Kaiser effect, reveals the importance of void ratio with respect to instability. Tilt-table testing, when equipped with internal shear distortion sensors, was shown to be able to capture the modulus degradation curve over the strain range of 10-5 to 10-1 providing a promising new experimental technique to explore the stiffness degradation curve at very low confining pressures.