Abstract: Assessing the support strength of surrounding rock of high sidewall is of crucial significance for the surrounding rock stability in underground projects of high geostress. Due to high geostress and low strength-to-stress ratio, the underground powerhouse of Houziyan Hydropower Station experienced typical deformations and failures of surrounding rock during construction and excavation, including cracking of shotcrete layer, dislocation between rock anchor beam and anchor pier, and cracking of rock mass, posing severe threat to the safety of the underground cavern. By analyzing the data from multi-point displacement meter and anchor stress monitoring, we proposed the formula and method for assessing the prestressed supporting strength per unit area of prestressed anchor cables and anchor rods, and further designed corresponding reinforcement measures for sidewalls according to the assessment results. Our findings revealed that the deformation of the surrounding rock of Houziyan underground powerhouse was larger than that of the Jinping I-stage project in the same period, while the stress level of anchor cable was smaller, and the overall stress level of anchor rod was equivalent to that of Jinping I-stage project. The supporting strength of Houziyan's downstream sidewall was larger than that of Jinping's downstream sidewall, whereas that of upstream side wall lower than that of Jinping's downstream sidewall. In the later period, the deformation and displacement of surrounding rock after the reinforcement of the upstream and downstream sidewalls of Houziyan underground powerhouse was effectively controlled,and the deformation rate gradually reduced and finally stabilized,manifesting the effectiveness of the measures.

Key words: assessment of support strength, deformation of surrounding rock, high geostress, high sidewall, reinforcing support, Houziyan underground powerhouse