Three typical deep coal mining areas, including Xinwen, Huainan, and Pingdingshan in China were chosen to take in-situ stress measurements by means of the hydrofracturing stress measurement rig with small borehole. 24 sites of stress measurements were carried out in 9 coal mines, with the maximum buried depth 1283 m, and the minimum 655 m. The stress measurement data point out that the types of in-situ stress fields in three coal mining areas are different. Xinwen coal mining area is intensively affected by tectonic movements with complex geological structures, so the in-situ stress fields are dominated by horizontal stresses, the tectonic stresses take absolute predominance, with the maximum horizontal principal stress up to 42.19 MPa, and the minimum horizontal principal stress up to 22.8 MPa. However, the in-situ stress fields are dominated by gravity stresses in Huainan and Pingdingshan coal mining areas. Based on the field data, the stress variation law with buried depth, and the relation between the specific values of the mean horizontal principal stresses to the vertical stresses and buried depth were analyzed. The in-situ stress measurement results were applied in roadway supporting design, and the roadway supporting states have been obviously improved.
It is an objective law that exploitation of coal resources develops from shallow to deep. The average coal mining depth increases by 8–12 m/a in Chinese coal mines, and several tens of coal mines have been developed at the depth more than 1000 m below the surface, with one deepest coal mine up to 1350 m. With the coal production increasing gradually, this rising trend in mining depth is speeded up. It is estimated that a lot of coal mines will get to the depth of 1000–1500 m in next twenty years.
Deep coal mining is seriously affected by high in-situ stresses, high geothermal temperature, high hydraulic stresses and intensive mining effects. The rising vertical stresses caused by gravity, complex and high tectonic stresses are common problems in deep coal mines. The geothermal temperature trends to rise with the mining depth, and the temperature change also causes the variation of in-situ stresses by the theory of thermal expansion and contraction. The deformation characteristics of coal and rock mass, on the condition of high in-situ stresses, have varied radically, and some characteristics, such as plasticity, rheological nature, dilatancy and impact, are shown obviously. A series of dynamic disasters, including pressure bump, outburst of coal and gas, gas explosion, water inrush, violent ground behaviour, large deformation of rock surrounding roadways, roof collapsing and wall spalling, determined by the condition of high stresses and the characteristics of rock and coal deformation, will bring about great threats to the safety and effectiveness of the coal mines.
The in-situ stress is the main acting force for the deep mining disasters. The values and directions of in-situ stresses play a key role on the stability of mining stopes, roadways and chambers, so in-situ stress measurements.