Abstract: To study the influence of middle rock pillar on the stability of surrounding rock and the entire tunnel, a case study is conducted on tunnel on Xinjiu road in Jiulongpo district, Chongqing, the variation law of surface settlement induced by middle rock pillars with different reinforcement forms and heights is simulated using MADISGTS, and the simulation results are compared with the monitoring data. The results show the following. (1) The surface settlement curve of the tunnel with small clear distance is greatly affected by the tunnel depth. When the burial depth is small, the surface settlement curve after the excavation of double holes shows an asymmetric W shape, and the middle rock pillar plays an obvious role as a lever fulcrum between the first and the second holes. When the burial depth is large, the settlement curve shows a roughly symmetrical U shape, a common pressure arch is formed at a certain height above the two tunnels, the middle rock pillar is enclosed in the pressure arch, which leads to the weakening of the pivot effect of the middle rock pillar in the surface settlement curve, and the soil arch effect is obvious. (2) In the construction process of tunnel with small clear distance, the optimal reinforcement scheme of shallow buried tunnel is that the middle rock pillar is strengthened to the surface and bounded by the crown. The optimal reinforcement scheme of deep buried tunnel is reinforced to the crown with the sidewall as the boundary.

Key words: small clear distance highway tunnel, middle rock pillars, surface settlement