Shear strength characteristics of unsaturated compacted GMZ bentonite considering anisotropy

Abstract

Anisotropic microstructure would be generated in the bentonite block during unidirectional compactions. Working as buffer materials, the compacted bentonite will inevitably experience shearing processes during the long-term operation of geological repositories. In this paper, high-pressure triaxial tests were conducted on unsaturated compacted GMZ bentonite specimens with different water contents, dry densities and confining stresses, with the compaction surface of the specimens oriented in both horizontally (H-type) and vertically (V-type) configurations. Results demonstrate that an increase in water content leads to a reduction in both peak strength and residual strength, while higher confining stresses enhance these strength parameters. Normally consolidated and lightly over-consolidated bentonite specimens display substantial shear deformation, whereas heavily over-consolidated specimens tend to experience brittle failure. Water content plays a significant role in shaping both the critical state line (CSL) and the Hvorslev surface (HS), with increasing water content resulting in decreased slope parameters for both and an increased intercept parameter for the HS. Generally, V-type specimens demonstrate a steeper CSL and an outwardly extended HS in contrast to that of H-type specimens. The critical state ratio for V-type specimens is about 10 % higher, and the friction angle is 2.8° greater, than that of the H-type ones. Moreover, this difference appears to increase with increasing water content. The difference of the HS slope parameter between the two specimens is minor, while the intercept parameter is higher for the V-type specimens.