Abstract:

[Objective] The aim of this study is to address the issue of stability control of surrounding rocks during the construction of underground oil storage caverns. [Methods] We first clarified the types and manifestations of surrounding rock instability in caverns, and then applied the block theory to analyze the issues of block identification and stability caused by unfavorable combinations of structural planes. The key blocks identified during construction period were classified based on their geometric shapes, and key blocks requiring support were selected according to their morphological types. Subsequently, we focused on the identification of hazard-causing blocks during construction to analyze key issues such as the identification of cross-layer blocks (which only become fully exposed after multi-layer excavation in high-sidewall caverns), instability characteristics of surrounding rocks involving along-cavern joints, and potential instability risks at the intersections of caverns. [Results] Using block theory to determine whether different combinations of structural planes could form key blocks, followed by stability and support analysis, serves as a necessary supplement to the conventional approach relying on surrounding rock quality classification for support design. The geometric shapes of blocks were classified into three types: “regular-shaped”, “flat and shallow-buried”, and “sharp and deeply embedded”, with the “regular-shaped” blocks being the primary type requiring support. “Flat and shallow-buried” blocks were prone to spontaneous falling, “sharp and deeply embedded” blocks were less likely to become unstable, and “regular-shaped” blocks required support, thereby providing a basis for differentiated support during the construction period. Based on the distribution characteristics of blocks during the construction period, the key issues of the identification and control of hazard-causing blocks were summarized as follows: (1) cross-layer blocks were the main hazard sources during the construction of high-sidewall caverns. It was necessary to splice and compare geological sketches obtained from multiple excavation layers to analyze the cross-layer extension characteristics and intersections of structural planes to determine whether cross-layer blocks may form. (2) Along-cavern joints, due to their limited visible exposure and “concealed” characteristics, were prone to form collapse blocks when intersecting with other structural planes in hard rock sections, while in medium to soft rock sections, they may cause large-scale sliding instability. (3) At cavern intersections, the increase in free surfaces, along with fewer structural plane cuts, may still result in the formation of hazard-causing blocks, thereby increasing safety risks. [Conclusion] The findings advance the understanding of block identification and stability analysis during the construction of underground caverns. The proposed classification of block shapes and the summarized key issues in recognizing hazard-causing blocks can provide a reference for the stability control of surrounding rocks in similar cavern engineering projects.

Key words: underground cavern, stability of surrounding rocks, block identification, unfavorable combination of structural planes, identification of hazard-causing block