Objective China’s hydropower resources are primarily concentrated in the western mountains and valleys, characterized by narrow ground space and significant altitude differences. Consequently, it is often more economical and efficient to locate hydropower station buildings within extremely deep caverns. However, the region’s typical geomorphic features, coupled with the strong environmental impact of “high geostress” and “high permeability”, pose numerous safety challenges during both construction and operation phases for these extremely deep caverns. Understanding the evolutionary patterns and long-term stability of the mechanical properties of surrounding rock is of paramount importance. A critical scientific challenge lies in elucidating the influence of water-rock coupling on the mechanical properties of rock under varying depths of stress, which currently leads to certain risks and inefficiencies in deep rock mass engineering. Methods To realistically simulate the coupled environment of water and rock at different depths, our research group independently developed a rock saturation test system under deep in-situ stress conditions. This system comprises five reactors, collectively named the deep environment simulation cavity, capable of replicating the in-situ stress environment of underground engineering at five distinct depths. Through a servo motor, rotary motion is converted into linear motion, facilitating the transfer of water pressure from the motor cavity to the kettle cavity for pressurization. Additionally, the pump transmission rate allows for adjustment of pressurization speed. Leveraging this self-designed and developed rock saturation test system under deep in-situ stress conditions, we conducted water softening tests on Jinping marble with saturation durations of 1, 23, 60, and 100 days. To provide comparative insights, we also conducted water softening tests on Jinping marble without pressure saturation for durations of 0, 1, 23, 60, and 100 days. Each group underwent five sets of triaxial compression tests at varying confining pressures, corresponding to pressures of 100, 1 000, 1 400, 1 800, and 2 400 meters at five different depths. All rock samples were extracted from the No. 8 borehole outside the D-scientific research cave, boasting the greatest buried depth in the Jinping Ⅱ (CJPL–Ⅱ) cavern group, at a depth of 2 400 meters. Results and Discussions The findings reveal that under saturated in-situ stress conditions, the triaxial strength of Jinping marble gradually diminishes with increasing saturation time, exhibiting nonlinear characteristics with a slowing decreasing trend over time. Furthermore, the degradation degree of Jinping marble increases gradually with time under identical depth environments, albeit with a diminishing upward trend. Moreover, the deterioration degree initially decreases and then increases with rising depth environments under identical saturation times, indicating a nonlinear change pattern. Comparing the effects of in-situ stress environmental saturation and non-pressure water saturation, it is evident that after 100 days of water saturation, the deterioration degree of peak strength of Jinping marble is approximately three times higher under deep in-situ stress conditions than under non-pressure water saturation. Additionally, other mechanical parameters such as cohesion and internal friction angle exhibit significantly greater deterioration under in-situ stress conditions compared to non-pressure water saturation. Moreover, the degradation of peak strength of Jinping marble demonstrates a nonlinear pattern, initially decreasing and then increasing with the increase in water-rock coupling environmental depth (100, 1 000, 1 400, 1 800, and 2 400 meters). During the investigation of the water softening mechanism of Jinping marble, X–ray diffraction (XRD) analysis was conducted using the DMAX–3C equipment at the Analysis and Testing Center of Sichuan University. Results indicated that the predominant components of Jinping marble were dolomite (CaMg(CO3)2) and calcite (CaCO3), with average contents of 92% and 8%, respectively. The low solubility of dolomite and calcite in a distilled water environment suggested minimal chemical dissolution. Furthermore, the absence of clay minerals in Jinping marble, coupled with its hydrostatic pressure state during testing, ruled out clay mineral softening, seepage erosion, and hydrodynamic scouring. Conclusions Based on experimental tests and compositional analysis, the water softening mechanism of Jinping marble in both in-situ stress and non-pressure saturated environments primarily involves frictional weakening and pore water pressure effects. The experimental findings suggest that the frictional weakening effect exacerbates the deterioration of Jinping marble under low confining pressure, while pore water pressure has the opposite effect. The interplay between these mechanisms results in a nonlinear trend wherein the strength deterioration of Jinping marble initially decreases and then increases with increasing depth environment. Furthermore, the variance in mechanical properties of water saturation between in-situ stress and non-pressure environments of Jinping marble is primarily attributed to pore water pressure. These findings underscore the pronounced water softening effect and its consequential impact on the mechanical properties of surrounding rock under in-situ stress conditions. Thus, in the long-term stability analysis of deep underground engineering, it is imperative to consider the softening characteristics of surrounding rock water and appropriately adjust rock mechanical parameters. This conclusion holds significant guiding implications for the construction and maintenance of Jinping underground engineering. © 2024 Sichuan University. All rights reserved.
