Stability level of tunnels that exist in an underground mine has a great influence on the safety, production and economic performance of the mine. Ensuring of stability for soft-rock tunnels is an important task for deep coal mines located in high in situ stress conditions. The aim of this study is to investigate the effect of tunnel shape and support pattern on the deformation, failure zone and stability around a tunnel located in a coal rock mass in China and to select an appropriate tunnel shape and a support pattern to provide stable stress-deformation conditions around the tunnel. Using the available information on stratigraphy, geological structures, in situ stress measurements and geo-mechanical properties of intact rock and discontinuity interfaces, a three-dimensional numerical model was built by using FLAC software to simulate the stress conditions around the tunnel in the coal rock mass. Analyses were conducted for several tunnel shapes and rock support patterns. Results obtained for the distribution of failure zones, and stress and displacement fields around the tunnel were compared to select the best tunnel shape and support pattern to achieve the optimum stability conditions.
Due to increasing demand and exploitation, shallow resources are decreasing each day, and many mines all over the world exploit resources located at deep formations with time. Mines at a depth of more than about 500 m may be called as deep mines. The depth of coal mine exploitation has increased at a rate of 8 to 12 meters per year in China, and 100 to 250 meters per ten years in eastern China . Recently, numerous coal mines in China have moved into the state of deep exploitation, some even more than 1000 m. Caitun Coal Mine in Shenyang with a depth of 1197 m, Zhaogezhuang Coal Mine in Kailuan with a depth of 1159 m, Zhangxiaolou Coal Mine in Xuzhou with a depth of 1100 m, Guanshan Coal Mine in Beipiao with a depth of 1059 m, Suncun Coal Mine in Xinwen with a depth of 1055 m , Huafeng Coal Mine in Xinwen with a depth of 1160 m , and Gucheng Coal Mine in Yanzhou with a depth of 1050 m are typical examples of mines in China that operate at depths of over 1000 m . Most of the coal resources are deposited in deep formations in China. The reserves of coal deposited deeper than 600 m and 1000 m are up to 73% and 53%, respectively. It is predicted that more and more coal mines will extend to depths of 1000-1500 m in the next 20 years in China . With increasing depth of exploitation, a series of problems, such as: (a) more complicated geological environment, (b) high in situ stresses, (c) high water flows, and (d) high earth temperatures are encountered. These features lead to: (a) difficult tunnel maintenance, (b) higher risk of rock burst, (c) difficult exploitation conditions, (d) reduction of safety, productivity and economic benefits, and (e) difficult ventilation design.