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.
Coal measures are extremely complicated geological bodies. When compared with other geological materials, they have two distinct characteristics: firstly, they are cut by various discontinuities, such as joints and fractures, which sharply change the strength and deformation characteristics of them, and cause the great difference of strength between rock mass and a small rock block; secondly, there are active stresses in the coal measures, and the orientation and magnitude of stresses strongly influence deformation and damage characteristics of surrounding rock mass. Most coal mines in China are operated in underground, and there are a variety of deposition conditions of coal seams. The stress fields in the coal measures are complex and irregular, because of the mixed influence of faults, folds, subsided columns and so on. Deep mining brings about the unfavorable effects of high in-situ stresses, high temperature, high hydraulic pressure and violent mining disturbance.