Characteristics of underground in-situ stress distribution in shallow coal mines and its applications

By means of the small borehole hydraulic fracturing measurement unit designed for underground coal mines, the 88 in-situ stress measurement sites were conducted in more than 10 coal mining fields located in the Shanxi, Shaanxi,Inner Mongolia,Ningxia and Xinjiang province or autonomous region. The shallowest site was only 38 m below the surface. The distribution features and variation trend of the in-situ stresses in shallow underground coal mines were analyzed based on the measurement data,and the comparison with those obtained in deep underground coal mines was made. The applications of the in-situ stress measurement results in a shallow coal mine were introduced through a case study. It is shown that the in-situ stress measurement results present large discreteness,and there is no obvious varia- tion discipline with buried depth,as the in-situ stresses in shallow coal mines are affected by a lot of factors. The in-si- tu stress pattern is mainly the type of σH >σh >σV with the dominant horizontal stresses. The ratio between the maxi- mum horizontal principal stresses and vertical principal stresses mostly focuses on the interval of 1 to 3;The ratio between the minimum horizontal principal stresses and vertical principal stresses is mainly on the interval of 0. 5 to 2. 0; The ratio between the mean horizontal principal stresses and vertical principal stresses mainly locates on the interval of 1. 0 to 2. 5. The ratio between the difference of the maximum and minimum principal stresses and vertical principal stresses presents large discreteness with the maximum value more than 2. The in-situ stress measurement results were applied to optimize coal pillar sizes and rock bolting design,the coal recovery rate was enhanced,and the rock bolting costs were decreased in the premise of entry safety and reinforcement effectiveness.