Damage behaviour of MX80 bentonite pellet in mixture under suction-controlled oedometer compression

Abstract

In order to investigate the damage behaviour of MX80 bentonite pellet in mixture, suction-controlled oedometer tests were conducted using vapor equilibrium technique (VET). After wetted to different suctions (113 to 4.2 MPa) at a given vertical stress σ_v of 0.1 MPa, MX80 bentonite pellet/powder mixtures were loaded in steps till different σ_v values (0.1, 3.2 and 12.8 MPa), then instantaneously unloaded. The damage behaviour was studied on pellets using Micro-Computed Tomography (micro-CT) and Mercury Intrusion Porosimetry (MIP), due to the difficulty of extruding loose bentonite mixture as a whole at high suctions. MIP results showed that significant cracks occurred during wetting (suction decreasing), while cracking evolution during loading was more complex due to the coupled effects of suction and loading. At high suctions (s ≥ 57 MPa), cracks continuously propagated during loading, as opposed to the consistent crack closure at low suction (4.2 MPa). For intermediate suctions between 38 and 9 MPa, cracks propagated in the low stress range (0.1 to 3.2 MPa), while closed in the low stress range (3.2 to 12.8 MPa). In addition, micro-CT images revealed that due to wetting thin cracks preferentially occurred in the marginal area of pellets because wetting started from this area. By contrast, loading mainly generated extended cracks in pellets, which propagated from the centre to the border. Based on the experimental results, a damage coefficient D proposed to quantify the wetting- and loading-induced damage. This coefficient was defined as the ratio of the macro-pore volume to the total volume in pellet. It appears that D increased with the decrease of suction. During loading, D consistently increased at high suctions (s ≥ 57 MPa), as opposed to the lowest suction (4.2 MPa). However, at medium suctions (38 to 9 MPa), D increased at the low stress level, but decreased at the high stress level.