Construction Behavior of Double Down-side Drifts in a Weak Tunnel with Shallow Cover

Abstract

Confronted with accidents in a shallow-buried weak tunnel using the bench excavation method, such as great subsidence and cracks in the ground surface as well as those in the preliminary support, a double downside drifts construction method was presented. The drifts were used to detect geological conditions and reinforce the lower parts of the tunnel. Its construction procedures and load transiting mechanism were then described. Its Construction behavior was also studied by numerical simulation using software MIDAS. The results show that (1) double-side drifts can improve tunnel load, the key construction step is arch ring excavation and core soil is good to keep tunnel steady; (2) weak parts mainly locate at wall foot of drifts, wall foot and crown foot of tunnel, and the connections; (3) reinforcement of soil under the drifts has no apparent effect on improving rock deformation and support load. Advice on construction was proposed that main parts to be reinforced are drifts (its foot depth, connection parts with tunnel, and its corners) and core soil should be kept if rock is unsteady and needs reinforcing.