Plastic-Zone-Controlled Grouting Rehabilitation of Deep Soft-Rock Chambers under Ultra-High Stress: Mechanism and Application

Abstract To address the persistent large deformation and recurrent instability of the deep soft-rock chamber at the auxiliary shaft station of Yilan No. 3 Mine after repeated rehabilitation, this study systematically investigated the surrounding-rock instability mechanism and grouting-based synergistic rehabilitation method by integrating in-situ stress measurement, mineralogical analysis, theoretical calculation, numerical simulation, borehole imaging, grout-material testing, and field-scale industrial trials. The results show that the study area is located at a depth of approximately 700 m, where the maximum horizontal principal stress reaches 36.86 MPa, indicating an ultra-high-stress environment jointly controlled by tectonic and gravitational stresses. The surrounding rock contains relatively high clay mineral contents and exhibits pronounced water-sensitive softening and structural weakening. After excavation-induced unloading, a non-uniform plastic damage zone develops under the combined effects of principal stress redistribution and principal stress rotation. The theoretically calculated maximum depth of the dominant failure zone is approximately 2.5 m, while borehole imaging reveals a maximum fracture development depth of approximately 2.6 m in the arch-shoulder region, showing good agreement. The existing bolt-cable-steel set support system can restrict roadway contour deformation in the short term, but it cannot prevent the long-term expansion of the deep dominant failure zone. Based on the concepts of dominant failure zone coverage and structural reconstruction, a circumferential grouting rehabilitation method for forming a closed reinforcing structure is proposed. Grout-material tests indicate that a water-to-cement ratio of 0.8 and an additive dosage of 12% provide a favorable balance among injectability, stability, and long-term strength development. The field grouting parameters were determined as a grouting pressure of 8 MPa, a borehole depth of 3 m, and a borehole spacing of 1.5 m. Field-scale industrial trials show that, after grouting, the rates of floor heave and sidewall convergence in the test section were significantly reduced, and the deformation of the surrounding rock gradually stabilized. These findings provide a case-based reference for the long-term stability control and grouting rehabilitation design of deep soft-rock chambers under ultra-high in-situ stress.