Stability Analysis and Support Optimization of A Tunnel Penetrating Through Mining Area in Close Proximity

Excavating tunnels within the influence range of goaf area poses significant technical challenges commonly encountered in underground construction projects. Considering the Baiyunshan Tunnel as the project backdrop, FLAC3D was utilized to analyze the stability of the tunnel under conditions that if the goaf area was filled with water or not. Then optimize the original support plan to suit different conditions. The results show that: Excavating the tunnel without grouting in diverse engineering conditions presented significant safety hazards. Stress concentration occurred at the left end of the goaf, resulting in a wide range of plastic zone above the tunnel and extensive damage to the tunnel. The vertical displacement of the tunnel floor was expected to surpass the prescribed control value that outlined in the pertinent guidelines. The presence of water in the goaf exacerbated the rock softening and damage. High pore water pressure posed a significant risk of water inflow into the tunnel’s crown. For the tunnel that underpassed dry goaf area, grouting was carried out within a range of 140° when the distance between the goaf and the tunnel was less than 4.5 meters. Grouting was applied to the entire surrounding area when the distance between the goaf and the tunnel was within the range of 4.5~9 meters. If the distance exceeded 9 meters, grouting was not required to meet safety regulations. During the construction of the tunnel underpassing water-bearing goaf area, it was necessary to extend the grouting range to cover a distance of 4.5~10 meters from the goaf to the tunnel. Other parts of the tunnel could be supported in the same way as the tunnel that underpassed dry goaf area. The field monitoring results indicated that the stability of the tunnel had been ensured.