Experimental study of hydro-mechanical behaviour of granular materials

Abstract

Inadequate knowledge of the behaviour of wet granular materials such as unsaturated soils or wet and sticky industrial bulk solids formed the basis of the current research project in which selected granular materials were experimentally characterised. The main objective was to investigate the hydro-mechanical behaviour. Specifically, the following were studied: effect of particle size, particle shape, drying-wetting cycles and void ratio on the water retention behaviour of granular materials, and effect of suction on the shear behaviour and flowability of granular materials. A total of 85 pressure plate tests, 13 triaxial compression tests with the axis translation technique used to control suction in unsaturated tests and 52 silo model tests were successfully conducted to respectively measure water retention characteristics, stress-strain and shear strength, and flowability of glass beads of high sphericity (~95% roundness) and Leighton Buzzard sand (~82% roundness). With these deliberately simple materials each considered factor was isolated and investigated at a time something hugely challenging to achieve with many unsaturated soils. Many pertinent features of hydro-mechanical behaviour observed for most soils were well captured with spherical glass beads meaning that particle-water interaction alone can produce the typical unsaturated behaviour and the particle size and shape significantly affected the behaviour. Further drying-wetting cycles did not alter the WRCs of both glass beads and sand except in the saturated capillary regime suggesting that factors other than the inert water-particle interaction through the liquid bridges are responsible for the discrepancy between the first drying-wetting cycle WRC and the subsequent drying-wetting cycles WRCs often observed in clayey soils. It’s discovered that the additional inter-particle bonding force introduced through the liquid bridges maintained by the matric suction serves to increase the stiffness, volumetric stability and shear strength of the material. The rate of increase of strength diminished with increase in matric suction. The study noted that the Beverloo law is valid for estimation of the dry mass discharge rate and that moisture alone is sufficient to maintain stable arching action depending on the hopper outlet. The study generated new knowledge in form of the effect of the material particle properties on the bulk hydro-mechanical behaviour of granular materials. An approach has also been proposed for estimating the flowability and minimum hopper outlet diameter for the wet noncohesive bulk solids.