Numerical analysis of reasonable distance between tunnel faces for twin tunnel

Based on the MIDAS-GTS numerical analysis platform, the construction characteristics of the left and right holes of the newly-built twin tunnel with a large section and a small distance were systematically analyzed, and the evolution laws of the displacement changes of the tunnel and the mid-rock pillar under the left and right full-section tunnel method were studied. The results show that: (1) with the increase of the lag distance, the surface settlement decreases gradually. When the lag distance is greater than 2.0D, increasing the lag distance has little effect on the maximum settlement value. (2) The maximum value of the surface settlement trough occurs between the left and right tunnels, which is the superposition area affected by the two tunnels. (3) The increase of lag distance has little effect on the final horizontal displacement of arch waist and mid-rock pillar. Considering that the influence range of face excavation on the surrounding rock of the tunnel is within 15m, the lag distance should not be too large and should not be less than 15m. (4) With the decrease of the lag distance, the superposition effect of the twin tunnel becomes more and more obvious. With the increase of lag distance, this phenomenon begins to weaken obviously, and obvious separation phenomenon appears.