Compaction Response of Mining-Induced Rock Masses to Longwall Overburden Isolated Grouting

Abstract

Surface subsidence in coal mine areas can cause serious geological hazards. After a coal seam is mined, the overlying rock layers fracture, collapse, and expand; the fractured and bulking rock masses are then continuously compacted under the action of overburden load, which eventually leads to surface subsidence. Overburden isolated grout filling via surface boreholes, and high-pressure grouting to mining-induced fissures under the hard rock layer, uses the grouting pressure to compact the lower fractured and bulking rock masses in advance, replacing the subsidence void and effectively controlling the surface subsidence. The characteristics of rock mass collapse, bulking, and compaction associated with mining and grouting are the key to the design of grouting parameters and surface subsidence control. In this paper, a theoretical model of the rock masses’ compactness during grouting injection is proposed, which determines the compaction of rock masses under the action of grouting filling. An experimental study was conducted to reproduce the grouting pressure evolution and the rock masses compaction in response to grout filling. The results indicated that the rock mass compaction was small in the no-pressure stage, and that the low-pressure and pressure-boost stages were key to generating the compaction effect of the grout filling. It was found that compaction grouting substantially increased the filled volume by transforming the fractured and bulking space of the rock masses into a filled space. Using engineering measurement data, the rock masses compaction law for grouting is verified. This paper provides a theoretical basis for the design of overburden grouting parameters and the evaluation of subsidence control effectiveness.