Abstract: To avoid support crushing again under the similar conditions of working face No. 1202(3) by improving upper limit in Gubei Coal Mine of Huainan, based on fracture characteristics of overlying strata, a structural mechanics model of overlying strata with the natural arch is established. The theoretical formula of the arch rise for both the low the high caving arches are deduced. Firstly, the influence of fracture location of the roof on the support load is studied. Then, the overlying strata loads under conditions of loose aquifer inside and outside the natural arch are calculated. Moreover, the criteria of support crushing for sliding instability and squash instability are determined. This study shows that it is easy to form the structure of high caving arch when the working face of improving upper limit is influenced by the water of loose aquifer. The volume weight of loose aquifer inside the high caving arch increases, and the contact forces between hinged rocks decrease. Additionally, the key layer is influenced by the water pressure of loose aquifer, which increase loads on the supports sharply. Therefore, the working face of improving upper limit easily leads to support crushing. However, the influencing factors of support crushing belong to both geological factors and mining technology factor. It should be mentioned that the blindly increasing working resistance of supports is not necessary to reduce the risk of support crushing, but the support cost is inevitable to increase rapidly. Based on the above analysis, the dimensionless sensitivity index is defined, and then the sensitivity of influencing factors of support crushing is analyzed. It aims at investigating the influencing factors of high sensitivity and preventing countermeasures of support crushing, such as controlling an appropriate mining height, forced caving for the roof of open-off cut, preventing an immediate roof suspending, hydrophobic step-down before mining, strengthening the rib of working face, improving support force, selecting reasonable equipment, and guaranteeing reasonable advance speed of working face. These measures provide some theoretical guidance for safe mining under the similar conditions of working face No. 1202(3).

Key words: working face of improving upper limit, natural arch structure, arch rise, critical support resistance of support crushing, sensitivity of support crushing