Database and characteristics of underground in-situ stress distribution in Chinese coal mines

A total 1357 items of in-situ stress measurement data in Chinese underground coal mines were collected, which include the data obtained by hydraulic fracturing method with small borehole applied by Mining Branch,China Coal Research Institute,and those from overcoring and hydraulic fracturing methods by other companies and organiza- tions. The in-situ stress database of Chinese underground coal mines was established on the basis of those data,and the in-situ stress map for Chinese underground coal mines was drawn. The in-situ stress distribution features and its affect- ing factors were studied. The achievements are as follows:① the buried depth of measurement sites is an important factor affecting the in-situ stresses in underground coal mines. In general,the vertical stress increases with depth,as some discreteness of stress data exists. The maximum and minimum horizontal principal stresses also increase with depth,however,the discreteness of stress data is obviously greater than that of vertical stresses,and the increasing rate of horizontal stresses with depth is smaller than that of vertical stresses. ② The stress regime is mainly thrust type (σH > σh > σV ) in shallow coal mine,and normal type ( σV > σH > σh ) in coal mine with depth more than 1 000 m,while it is mainly slip type( σH > σV > σh ) with depth in between. ③ The ratios of horizontal stresses to vertical stresses,including those of maximum,minimum and average horizontal principal stresses to vertical stresses, the difference between the maximum and minimum horizontal principal stresses to vertical stresses, distribute in a broad range with a greater discreteness when depth is shallow. As the depth increases,the discreteness of these ratios gradually decreases with a tendency to approach constant values. ④ The stress statistical results of four strata with dif- ferent lithology,that is siltstone,fine sandstone,mud sandstone and mudstone,point out that,in general,the larger the rock strength,the higher the horizontal stresses the rocks bear. ⑤ The rocks with higher elastic modulus bear larger horizontal stresses. However,the soft and fractured rocks with lower elastic modulus bear less horizontal stresses.