Abstract: The ratio of rock strength to geostress is a very important indicator of surrounding rock stability. Studying the relationship between the failure and strength-to-stress ratio of surrounding rock so as to reveal their characteristic connections is of great significance for the control of surrounding rock stability of large-scale underground hard rock cavern group. With the large-scale underground hard rock caverns of Jinping hydropower plant stage-I and Houziyan hydropower plant as case study, we established the relationship between failure of surrounding rock under high geo-stress condition and strength-to-stress ratio. Firstly the engineering geological conditions, inclusive of lithology, rock strength, and initial geostress, are summarized, and the types, quantities, and locations of failures during cavern group construction are statistically analyzed; then the surrounding rocks of underground caverns are partitioned according to strength-to-stress ratio. Moreover, in the light of the surrounding rock failure characteristics, the applicability of geostress grading standard modified based on the strength-to-stress ratio is verified, and the variation law of stress-induced failure of surrounding rock with strength-to-stress ratio is revealed. The research findings are conducive for the prediction of failure type of surrounding rocks of large-scale underground hard rock cavern group, and offer reference for targeted stability control measures.

Key words: large-scale underground cavern group, high geo-stress, hard rock strength, failure type of surrounding rock, ratio of rock strength to geostress