Surface subsidence is a serious problem during longwall coal mining. Grout injection into overburden fractures along the interfaces between weak and strong rock layers due to bed separation has been used as an effective longwall surface subsidence control technology. In this study, an overview of this technology was detailed. A new methodology to determine the injection horizon of overburden strata was developed by measuring the vertical flow along the injection borehole. The apparatus comprises an electromagnetic flowmeter, a cable, a lubricator, and a winch. An injection horizon was determined by a sudden decrease in the vertical flow profile. An injection borehole was successfully measured during the injection for an active longwall panel in China. It was found that the injection slurry flowed to a primary horizon, which was located below the thick and hard rock layer (key strata) above the injection borehole bottom. The results indicated that the key strata control the presence of the injection horizon, which provides a basis for the design of injection horizon and pressure for future longwall overburden grout injection.
In mining districts, longwall surface subsidence can cause a few environmental problems and hazards, including landscape modification, surface infrastructure damage, and waterlogging in the subsidence area (Grygierek and Kalisz, 2018; Luo et al., 2003; Thompson et al., 2011; Xuan and Xu, 2017). Recently in China, problems of surface subsidence have become increasingly serious because of rapid increases in coal production. Such problems are particularly serious in East China. For example, the Huaibei coal mining area is a densely populated area with more than 300 villages lying above the coal seams (Xuan and Xu, 2014). Residents of the area have long experienced serious issues relating to surface subsidence.
Backfilling technologies are effective subsidence control measures but have the limitations of high cost (∼$14.3 per ton of coal) and low production of <1 million tons per year (Xuan et al., 2013). Furthermore, the technology of isolated overburden grout injection has been developed, which involves the injection of fly ash slurry into bedding plane separations by pumping through surface boreholes as the longwall face advances (Xuan and Xu, 2017; Xu et al., 2015). Such bed separations are horizontal fractures that occur along the interfaces between weak and strong rock layers (typically referred to as key strata in China) during coal extraction (Palchik, 2005; Xuan et al., 2015).