In-situ stress measurements in underground coal mines and study on stress fields

In-situ stress testing methods frequently used in underground coal mines in China were introduced, including stress relief, hydraulic fracturing, geological structure information, earthquake focal mechanism and underground stress mapping. The stress data records obtained by the small borehole hydraulic fracturing testing rig used in underground coal mines were given more emphasis. Based on the testing data, the relationship between in-situ stresses and depth, and the changes of the ratio of the maximum horizontal principal stress to vertical stress were analyzed. There exist three types of in-situ stress fields. Depth, geological structures and rock properties are the main factors affecting in-situ stresses. Complicated geological conditions result in obvious scatter in testing data. However, the magnitude of in-situ stresses basically increase with the depth in the general trend; the increasing rate of horizontal stresses is larger than that of vertical stress in shallow sites, and gradually decrease as the depth increases.