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With the development of mining technique and the sharp reduceing of shallow mineral resources in ground surface, make mining environment become more and more complex, coal mining of hard roof is an important aspect. Such as, with the development of comprehensive mechanized coal mining technique, there are 40 percent of coalfaces belong to hard roof with intensive rock pressure, especially, coal faces of hard roof with thin-layer immediate roof distribute more widespread. So, it is important to study and learn hard roof controlling theory and technique. Based on analysis, the paper deeply studies the problem of hard roof by water-infusion softening method; the main conclusions are drawn as follows:1) Based on the chemical damage in water-rock interaction, the influence on rock strength of water is analyzed. Quoting crack tip model, divides rock fracture into three districts: the chemical damage district of crack tip; the chemical damage district of crack face; continuous damage district. Utilizing Dugdal model considering chemical damage, resolves the chemical damage in
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