Stability evaluation of mined-out area and Borehole Grouting Treatment-a case study in China

In consideration of the practical demand for the reuse of mined-out areas as construction land, the assessment of foundation stability over mined-out areas has become a critical issue. Using a goaf in China as an example, this study investigates the overburden structure, void distribution law, and goaf treatment process through theoretical analysis, similar material simulation, and field measurements. The results show that the mining-induced voids in the compression zone are small and stable in distribution, whereas the voids in the tensile zone are subject to mutation, especially under external factors. Therefore, the tensile zone is the key processing position when dealing with long-wall goaf. To ensure accurate evaluation for the different positions of the goaf under the building, various methods are proposed. The compression zone in the middle of the goaf is evaluated using the additional stress method, which is modified under the grouting condition through theoretical analysis. The boundary tensile area is evaluated based on the inclination angle of the first key block of the masonry beam, ensuring evaluation accuracy. Furthermore, a void distribution model of longwall goaf is established. Under the guidance of the theoretical model, the goaf is reinforced by grouting and the optimal design of grouting materials is carried out. The final grouting test results show that the average strength of the gob after grouting treatment is high, which can ensure the stability of the gob, and the grouting treatment effect is good. This study provides a theoretical foundation and practical experience for similar goaf grouting treatments.