Influence of Overlying Goaf Dip Angle on Stability of Highway Tunnel during Excavation

In order to study the influence rule of the security and stability under different goaf dip angles during the tunnel excavation when the highway tunnel is approaching to the goaf, the computation models are built and simulated numerically with the finite difference software of FLAC, relying on the Tian Pingzhai tunnel of Dazhou-Wanzhou highway. Under the condition of the certain tunnel buried depth, thickness of coal seam and distance, a comparative analysis on the displacement and axial force of primary support resulted from the changement of the overlying goaf dip angle has been conducted. The impact on the displacement and deformation of surrounding rock for different goaf dip angles are also further explored. The simulation results indicate: (1) with the increase of the dip angle, the added forces acting on primary structure will decrease; and (2) when the dip angle is less than 5°, the impact on the added forces IS most significant. The changement of the overlying goaf dip angle brings remarkable impact to vault settlement and horizontal convergence, and the influence on left hance (which is close to the mined-out area) is greater than the right hance, but it has little impact on the displacement of arch bottom. It is suggested that Pre-firming and Treatment for mined-out areas should be used.