Assessment of Excavation Broken Zone around Gateways under Various Geological Conditions: A Case Study in Sichuan Province, China

To study common failure characteristics of gateways, a total of 55 typical gateways at coal mines, in Sichuan Province, China, were selected for investigating the rules of broken widths based on the ground-penetrating radar (GPR) technique and numerical model. Results indicated that the broken width values around the gateways were larger than 1.5 m, and those in the roof and high side wall were larger than those in the low side wall, as a whole. The width values had close relationships with the thickness of the coal seam and immediate roof, angle of the coal seam, and depth of the gateways. Furthermore, combined with the plastic zone of numerical models in 3-Dimensional Distinct Element Code (3DEC) and the broken width, we obtained the excavation broken zone (EBZ) cross-section diagram for each gateway and determined that the EBZ appeared to have a basically elliptical shape-with the long axis along the seam inclination direction and the short axis along the vertical direction of the rock layer-and that this elliptical shape was only slightly affected by the gateway cross-section shape. It was observed that the failure extent was greater in the seam inclination direction than in the vertical direction of the rock layer. Obviously, the gateways presented asymmetric failure characteristics and implied that an asymmetric support system should be provided when using bolts, cables, and shotcrete combined with steel mesh and steel belts. Such a support system could improve material parameters and form a combined arch structure in surrounding rocks, with arch crown and arch springing thicknesses that are larger in the roof and high side wall.