Abstract:

In order to solve the construction problem of small spacing tunnel in pebble layer, i.e. poor integrity and stability of surrounding rock, easy collapse in construction and great disturbance of surrounding rock, through FLAC 3D numerical simulation and field monitoring, the reasonable spacing distance for small spacing tunnel in pebble layer and the safety longitudinal distance between the exist and the following tunnel are analyzed. The results show that: 1) The soil surface settlement curve shows a Vshape pattern and the largest ground settlement is located at the top of middle rock pillar when the spacing between two tunnels is less than 6 meters. When the spacing is larger than 6 meters, the ground settlement corresponding to the tunnel vault is greater than the top area of middle rock pillar, and the soil surface settlement curve shows a Wshape pattern. 2) The pressure of surrounding rock after tunnel excavation exhibits an obvious superimposing effect and the tunnel is easy to collapse when the spacing is less than 6 meters (approximately equal to the tunnel span). The twin tunnel can be designed as two separated tunnels when the spacing distance is larger than 18 meters. 3) The construction of following tunnel has less influence on the existing tunnel with the increase of the longitudinal distance between the two working faces. When the longitudinal distance is greater than 30 meters(4.6time the tunnel span), the effect between the two working faces can be ignored.

Key words: pebble layer, small spacing tunnel, tunnel construction, clear distance, longitudinal distance