This paper proposed a conceptual model of broken rock mass compaction based on elastic theory by simplifying this compressing process and assuming the connection is a similar cubic mass without huge void space. With this simplification, the stress-strain constitutive law is established. The compacted rock mass permeability evolution was modeled based on the cubic law of porosity and permeability. The mechanical model was coupled into permeability evolution model. The predicted permeability evolution by coupled model is similar with reported experimental results. The modeled permeability results were indirectly validated using the weak sandstone input parameters. Compared to intact rock mass, we found that the stress-strain curve of compacted rock mass takes a longer stress-strain path to reach the linearity due to the void space compaction resulted from the friction slipping and re-arrangement of grains. It was also found that the grain elastic modulus does not contribute to the overall bulk compaction and permeability reduction at the initial compaction stage and it will control the permeability evolution only after the broken rock got fully compacted when it can be treated as intact rock mass. The proposed models will potentially lay foundation for the future permeability and caving behavior characterizations using numerical simulation for complex gob area.
Long-wall mining is a very production-efficient coal mining technology with advantages including high recovery, safe working environment, operator-friendly roof management, easy mine management and highly-developed mechanization. In a typical long-wall operation, a working panel is usually more than 200 m in width and 2000 m in length. During the operation, the panel progressively moves forward as the coal is continuously removed and the immediate roof instantly caves behind the hydraulic supports. However, the overlying rock strata hang up and the span increases with mining advance. The overlying rock strata act like a beam and will break into large blocks until the span reaches to its limiting strength value. The broken rocks composed with immediate roof and overlying strata fill in the caved zone termed as “gob” or “goaf”. To meet the health and safety standards, gob gas control is being a challenging task due to its complexity of the gas flow behavior.
The gob with highly irregular shape and different granular broken rocks could reach 4-11 times the thickness of the mining height. (Karacan and Luxbacher, 2010) And the broken rocks would be gradually compressed by the overburden weight resulting from mining disturbance as shown in Figure 1.