Waste rock filling in fully mechanized coal mining for goaf-side entry retaining in thin coal seam

Abstract

A large amount of waste rock is generated during thin seam coal mining operations underground. The waste rock material stacked on surface also adds to the environmental pollution. So it is important to reduce the amount of waste rock by exploring gainful utilization methods. This paper deals with an investigation undertaken to develop an alternate gob filling material in laboratory and evaluating its efficiency numerically as well as in field. A coal–waste rock separation system and a roadway layout system were designed, and a new technique of fully-mechanized goaf-side entry retaining and goaf filling back hydraulic supports was proposed. These methods helped realize the goal of green mining, which was featured by waste rock consumption in nearby region without any need to be elevated to ground surface. This study demonstrated that the two proposed mining methods, namely, caving mining method and mining with filling method, differed in mechanical environments where roadside filling body was located, deformation, and failure scope. The maximum principal stress concentration degree near the roadside filling body for caving mining method was high with a maximum value of 42.5 MPa because of key rock blocks in overburden. As a result, the surrounding rock of roadside filling body was severely deformed with a maximum displacement of 1117 mm. The entire roadside filling body moved toward the inside roadway and experienced plastic failure. After goaf filling, filling bodies in goaf, roof and floor strata, and roadside were combined to form an entire withstanding deformation. The motion space of overburden rock was limited, the adjustment range of surrounding rock stress near the roadside filling body was narrowed, the maximum principal stress decreased to 29.5 MPa, and the main roof did not rupture. Accordingly, the deformation of surrounding rocks of the roadside filling body was small, the maximum displacement of roof and floor was 457 mm, and the roadside filling body did not experience plastic failure. The study results enrich goaf-side entry retaining theory in thin coal seam and mechanized filling technology and provide a theoretical foundation for thin coal seam mining with difficulty in waste rock treatment.