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.

The grout injection forms a compacted backfill in the separation chamber, and most of the grouting water seeps into the overburden pores. The initial saturation of overlying rock has been found to affect the seepage distribution of effluent in overlying rock. In the actual grouting process, the flow direction of grouting water in overlying rock and the initial saturation of overlying rock may affect whether the bled water will leak to the working face. In order to investigate the influence of initial saturation on the seepage law of isolated overburden grout injection, the present paper studies the saturation change process of bled water seepage at three different initial saturations (S_r = 29%, 51%, 73%). The water leakage of the working face was monitored with the self-developed 3D visual simulation experimental system, without considering the overlying rock of the injection layer. The results show that the bled water flows mainly in the horizontal direction, and the horizontal seepage velocity is about 10 times the vertical seepage velocity. The higher the initial saturation of the overburden, the larger the seepage range of the overburden, and the closer the seepage boundary is to the working face, the easier the bled water will leak into the working face. Therefore, the grouting design and process control can be carried out better. When the thickness of the selected isolation layer is insufficient, the bled water may appear on the working face, so this problem can be improved by increasing the thickness of the isolation layer.

Isolated overburden grout injection (IOGI) is an economical and efficient green mining technology. During the grout injection process, a large amount of slurry bled water will enter the underlying rock strata below the grouting layer. Under certain conditions, the slurry bled water may seep along the overlying rock layer into the water‐conducting fractured zone (WCFZ), resulting in the increase of water inrush in the working face, which affects the production safety and the grouting effectiveness. Therefore, it is urgent to establish the prediction model of working face water inrush and establish a safe grouting layer selection method to prevent the grouting water from entering the working face in the process of grouting filling. In this article, a new identification and prediction method of working face water inrush of IOGI were proposed. According to the characteristics of seepage boundary, a “semi‐ellipsoid” theoretical model of seepage boundary of slurry bled water was established, and a dimensionless Formula of the model parameters was constructed. The dimensionless Formula was solved based on the experimental results of influencing factors of seepage boundary. Based on the distribution characteristics of free water in grouting filling, the identification and prediction method of working face water inrush of IOGI were established. The prediction method was analyzed to the actual mine, and the correctness of the prediction method was verified. Using the proposed prediction method, a selection method of grouting layer (the distance between grouting layer and coal seam roof > seepage boundary |c| + the height of WCFZ H_d) was given to ensure that the slurry bled water will not enter the working face during the grout injection process. So as to ensure the effective implementation of grout injection work.