Structural evolutions of MX80 bentonite pellet/powder mixtures under wetting and suction-controlled oedometer loading

Abstract

The compression behaviour of MX80 bentonite pellet/powder mixture was investigated by performing suction-controlled oedometer tests. The suction from 113 to 4.2 MPa was imposed by vapour equilibrium technique (VET), and zero suction by water circulation (WC). After instantaneously unloaded from various target vertical stresses, the structural observation was conducted by mercury intrusion porosimetry and micro-computed tomography. Results showed that during wetting, the intra-grain micro- and macro-pores increased due to the swelling of pellets and powder grains. During loading, the variation of intra-grain micro-pores was characterised by the loading effect, while that of intra-grain micro-pores was controlled by the suction and loading effects. The inter-grain pores, for the VET samples, were reduced by the swelling of pellets and powder grains during wetting on one hand, and by the movement of pellets and powder grains during loading on the other hand. For the WC samples, most inter-grain pores were closed during wetting, with a few pores in the top closed by further loading. The global compression of bentonite mixture samples during loading was mainly governed by the filling of inter-grain pores for the VET samples, but controlled by the compression of intra-grain pores for the WC samples.