Water‐weakening effect is one of the most important factors inducing large deformation and stability problems of shale strata in the Huangjiazhai Tunnel. The influence of water on shale with various water contents and its mechanism still needs more study. In this paper, the X‐ray diffraction and water absorption test were first conducted to determine the influence of mineral components on water‐rock interaction. Then, a series of rock mechanical tests, including uniaxial compressive test, Brazilian disk test, and triaxial compressive test, were carried out on shale under dried condition, wetted condition with various water contents, and saturated condition. For the uniaxial compression, tension, and triaxial compression, the softening coefficient could reach 0.38, 0.63, and 0.52, respectively. Based on the macroscale experiments, a numerical case study of tunnel excavation was investigated to evaluate the water effect. Finally, with the aid of scanning electron microscope and failure morphology, the mechanism of water‐weakening effect on shale was discussed from microscopic and macroscopic perspectives. The thickness of the double layer increases with the increase of water immersion time. This paper provides a set of useful data for reference in construction in shale strata, and a methodology to evaluate water‐weakening effect through laboratory and numerical modelling approaches.

The yield criterion of rock is significant for the identification of the whole failure process and the stability analysis of rock engineering. Studies of yield criteria from energy aspects are critical but rare. This paper investigated the yield criterion using strain energy and a modified compounded mobilized planes (CMP) model. The yield strain energy and strength criterion were formulated with different combinations of stress variables. Based on conventional triaxial and true triaxial tests, the evolution laws of mechanical parameters, including elastic parameters and strength parameter, were quantitatively determined. Accordingly, a new parameter correlation model, intermediate principal stress effect, and prerequisite for convex yield surface were investigated with theoretical and experimental approaches, respectively. With respect to previous criteria, the studied criterion has some advantages. Firstly, parameter correlations can be characterized with this criterion. Secondly, the shape of the yield surface is criterion-dependent and relies on rock properties, and can be applied to wide range of rocks. Thirdly, the estimation of yield stress is quasi-independent of the sample variability, which resulted in good agreement with test data. Fourthly, the criterion can be transformed into some classic criteria widely used in civil engineering. The criterion in this study offers an alternative viewpoint. In addition, it has high calculation precision and wide application for varied rocklike geomaterials.

Due to the weakness in mechanical properties of chlorite schist and the high in situ stress in Jinping II hydropower station, the rock mass surrounding the diversion tunnels located in chlorite schist was observed with extremely large deformations. This may significantly increase the risk of tunnel instability during excavation. In order to assess the stability of the diversion tunnels laboratory tests were carried out in association with the petrophysical properties, mechanical behaviors and water-weakening properties of chlorite schist. The continuous deformation of surrounding rock mass, the destruction of the support structure and a large-scale collapse induced by the weak chlorite schist and high in situ stress were analyzed. The distributions of compressive deformation in the excavation zone with large deformations were also studied. In this regard, two reinforcement schemes for the excavation of diversion tunnel bottom section were proposed accordingly. This study could offer theoretical basis for deep tunnel construction in similar geological conditions