ثبات و تغییر شکل صخره های اطراف در ورودی حائل بدون ستون سمت GOB

We analyzed the stability of the retained gob-side entry in four different Chinese coal mining sites and evaluated the influencing factors of roadway deformation such as mining depth, support strength and area of gob-side hanging roof. It was found that the length of cantilever roof block above roadway has a major impact on the deformation, whereas the impact of mining depth is minor if the depth is less than 500 m. Minimum support resistance of 0.3 MPa is essential to effectively confine the deformation of a retained roadway. We performed physical experiments to further study the features of roof fracturing and their impact on roadway deformation under three typical immediate roof conditions, i.e., thick-immediate roof, thin-immediate roof and non-immediate roof. In addition, equations to calculate desired support resistance of filled gob-side wall were derived based on superimposed continuous laminate model. The results provide valuable theoretical and practical guidance for implementing pillarless gob-side entry retaining in engineering practices.

Gob-side entry retaining refers to maintaining either a maingate or tailgate behind mining face by constructing a fill-in sidewall on gob-side with special support to be reused for the next panel (Yuan, 2008). Pillarless gob-side entry retaining can effectively increase coal recovery, reduce roadway development, and mitigate outburst risk. Since the 1950s, pillarless roadway has been practiced worldwide and extensive research has been carried out on support resistance (He, 2000 and Xie et al., 2004), stability control principles (Zhang et al., 2003 and Ma and Zhang, 2004) and related technologies (Qi et al., 1999 and Jiang, 1993). Since 2005, gob-side entry retaining has been further developed as a key technology for integrated coal production and methane extraction in China (Yuan, 2008), which serves an important role in providing safe and long-term room for drilling and maintaining gas drainage boreholes. Nowadays, this coal extraction method has become a safe and efficient approach for mining gassy coal seams. The gob-side retained roadway has quite special deformation characteristics due to its special roof structure and stress change. The dynamic pressure loaded to a retained roadway is much higher than the abutment pressure ahead of mining face. This high dynamic pressure often results in excessive roadway deformation and rapid shrinkage. As the mining depth increases, maintaining a pillarless gob-side roadway becomes more and more difficult. Therefore it is crucial to clearly define the impact factors of deformation and the structural stability conditions of retained roadway to meet the requirement of complex mining environments.

The main factors that affect the stability of the retained gob-side entry can be listed as: the roof side cantilever length, roadway overburden depth and roadway support strength. Both roadway deformation and deformation rate increases with the length of cantilever roof block. When the roadway overburden depth is less than 500 m, the mining depth has little influence on the deformation. Minimum support resistance of 0.3 MPa is believed to be essential for efficient stability of retained roadway. (2) The roadway convergence and its impact factors can be correlated as follows: View the MathML sourceyv=0.727hLσd-1046.7,yh=0.0125hLσb-225.27 Turn MathJax on These equations indicate that roadway deformation can be considerably reduced by applying technical measures such as reducing the length of cantilever roof block and increasing the support resistance of surrounding rocks. (3) Physical experiments of roof fracturing under three immediate roof conditions were conducted. The results show that when the immediate roof thickness decreases, the caved position of main roof moves closer to the filled wall, and the angles of roof rotation and caving increase. (4) Equations to calculate desired support resistance of the filled wall were derived by adopting the block mechanics based on superimposed continuous laminate model. The equations indicate that the desired support resistance of the filled wall increases as the thickness of the immediate roof decreases. Promptly filling up the wall, improving the bearing capacity of filled wall roof and applying supplementary reinforcement in the roadway can considerably enhance the self-bearing capacity of the surrounding rock.