Compaction characteristics of the caving zone in a longwall goaf: a review

Broken rock and coal—residual coal, plus material from the immediate roof and overlying strata—fill in the goaf, in an area termed the caving zone. Due to its high porosity and permeability, the caving zone contains gas and water, which may have originated from the mined coal seam, the adjacent unmined coal seam, or from any aquifer or surface river. Thus, studying the compaction characteristics of the caving zone can help understand gas and mine water drainage and identify steps to prevent spontaneous combustion of residual coal. The stability characteristics of the caving zone after mining are affect surface subsidence, as well as water and gas build-up and use. The caving zone is a potential underground storage of greenhouse gases. Therefore, the time–space relationship of caving zone compaction characteristics in the goaf has become an area for research focus in recent years; in this study, the formation, height determination, and compaction characteristics of a caving zone are examined. Reduction in block size and rearrangement of the fill are the main factors affecting the compaction process, as re-crushing and rearrangement of the broken coal and rock mass affect the secant modulus and pore size of the caving zone, causing the secant modulus to gradually increase and pore size to decrease. This in turn affects the macroscopic stress–strain curve and seepage characteristics of the caving zone. The strength and fracturing mode of the caving blocks are the main factors affecting the re-crushing and rearrangement of the caving blocks. The applicability and reliability of present research results and research methods are analyzed and the focus areas for future studies are identified. Using a combination of research methods, including theoretical analysis, laboratory testing, numerical simulation, and field measurement, the compaction characteristics of a caving zone in longwall goaf can be accurately calculated.