The shear strength of compacted bentonite is crucial for preventing tilting and damage of the waste canisters in deep geological repositories (DGRs). The shear strength evolution along the confined wetting path also needs to be investigated, given the long saturation time of the bentonite buffer. This study conducted direct shear tests on densely compacted Gaomiaozi bentonite after suction control under confined conditions to determine its peak shear strength and strength parameters. Furthermore, the shear strength evolution along the confined wetting path was modeled on the basis of the effective stress principle. The results show that, for a given dry density, the peak shear strength at a given vertical pressure and the strength parameters exhibit an overall decrease along the confined wetting path. Moreover, the peak shear strength of the specimen that underwent confined wetting was considerably lower than that of the as-compacted specimen with the same total suction, indicating that the suction value and microstructure codetermine the peak shear strength of compacted Gaomiaozi bentonite. For this reason, the peak shear strength in the as-compacted state and the dual-porosity water retention curves established along the confined wetting path were used to model the shear strength evolution along the confined wetting path. The substitution equation for the effective stress parameter chi was selected on the basis of the experimental evidence. Finally, the model parameters were calibrated from the shear strength evolution at a given vertical pressure, and they reasonably reproduced the shear strength evolution under other vertical pressures. These findings can be helpful for the design and safe operation of DGRs under extreme geological conditions.

The mechanical behavior and strength characteristics of unsaturated fine-grained soils with dual-porosity are of crucial importance in geotechnical designs. Nanyang fine-grained soils have been selected as typical dual-porosity structure soils to perform experimental tests under a wide range of suction and different initial densities while studying its stress-strain-strength properties constitute the main scope of this study. Axial translation and vapor equilibrium techniques are jointly employed to apply a wide suction range. Our data suggest that soil behavior transits from strain-hardening with shear-induced contraction to strain-softening with shear-induced dilation as suction and density increase. By exploiting a bi-modal soil-water retention curve (SWRC) explicitly separating capillarity and adsorption mechanisms, the shear strength is allowed to be analyzed in the capillary suction stress-shear stress space. The strength envelop exhibits bi-linear characteristics. Building upon these findings, we propose a bi-linear shear strength criterion specifically for dual-porosity fine-grained soils. We utilize the obtained test data to evaluate existing strength criteria based on effective stress and dual stress variables that consider the bi-modal SWRC characteristics. The comparison indicates that the proposed bi-linear shear strength criterion can more reasonably represent the variation of shear strength under a wide range of suction for unsaturated dual-porosity fine-grained soils.