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).