Stability analysis of tunnel under coal seam goaf: Numerical and physical modeling

The goaf formed by coal seam mining would dramatically reduce the strength and stiffness of the ground in and around the goaf, which is not conducive to tunnel excavation near the mined-out area. By establishing the Finite differential method and Discrete element method coupling numerical analysis method and conducting similar model test, the influence of dip angle, thickness and distance of coal seam goaf on the stability of unsupported tunnel excavation is studied. The results show that when the tunnel under the goaf is exca-vated, the circumferential stress increment increases first and then decreases along the radial direction of the tunnel, the radial stress increment gradually decreases to zero along the radial direction of the tunnel, the displacement is approximately distributed in a trough shape, and the maximum displacement is at the top of the tunnel. The area of the stress loosening zone (SLZ) is negatively correlated with the dip angle and the distance of the goaf, and positively correlated with the thickness. The SLZ near the goaf is larger, and the peak value of the asymmetry is about 1.35. When the thickness of the mined-out area is 1.8-2.1 m, the SLZ and displacement around the tunnel increase sharply and then become stable. When the dip angle of the mined-out area is greater than 30 degrees or the distance exceeds 1.3 times tunnel diameter (D), the asymmetry converges. The research results of this paper are of great importance to the design and construction of tunnel support and the formulation of excavation schemes.