Large Deformation Mechanism and Three-Level Control Technology of Entry Retaining in Three-Soft Thick Coal Seams

Gob-side entry retaining (GER) has been widely used around the world. However, when the technology of gob-side filling and roadway retaining is applied to the mining of three-soft thick coal seam, large deformation of surrounding rock and failure of support structure are still unavoidable, especially the floor heave deformation, even the closure of the roadway section in serious cases. Taking 16,011 working face of Zhaogu No. 1 Coal Mine as the research object, mineral composition analysis showed that there are a lot of clay minerals in the surrounding rock, which shows strong swelling characteristics. The mechanical model of the overlying strata with long cantilever structure was constructed to explain the process of ground pressure transferring from two sides of roadway to the floor after retaining roadway, and then the variation and distribution law of stress on both sides of the floor with the working face promotion was obtained. With the aid of uncoupling support theory, this paper analyzed the floor heave mechanism of GER in three-soft thick coal seam, and pointed out that shear staggered floor heave was the main cause of floor heave deformation. Based on above analysis of large deformation mechanism of GER, the three-level control technology of surrounding rock in entry retaining in three-soft thick coal seam was put forward: directional pre-splitting decompression technology, constant resistance & large deformation anchor cable (CRLDAC) roof support technology and floor deformation control technology. The key parameters of the three-level control technology were determined by field industrial test. Monitoring data proved that the three-level control technology can effectively control the deformation of surrounding rock in retained roadway, in which roof-to-floor convergence had been reduced by 58%, and floor heave had been reduced by 65%, finally, the roadway had been retained successfully.