Recently many problems such as surrounding rock collapse

large-area settlement of ground surface etc have happened during soft rock and shallow buried tunnel construction of express highway. The stratum condition of a shallow buried zone of Guangfu Tunnel is very complex. The section is large and the lithology is weak. The action of conformation has much influence on the shallow buried zone. And its top length is very thin. It is very easy to cause cave-in collapse and fall of ground. In order to ensure the construction safety of shallow buried zone tunnels and reduce the stratum settlement and surrounding rock deformation caused by the excavation of tunnels

four construction projects are simulated by FLAC3D as much full section method

shot shoulder method

unilateral pilot tunnel method and double pilot tunnel method. The variation feature of ground surface settlement

surrounding rock circumjacent displacement and plastic zone during the four excavation projects are analyzed. These four construction projects are computed with finite element method. Some valuable conclusions are drawn: (1) the double pilot tunnel method may produce fewer vertical displacements and horizontal convergence and its stabilization is better. (2) the plastic zone radius for double pilot tunnel method is least. (3) it is the most reasonable that double pilot tunnel method is adopted in the shallow buried zone of Guangfu Tunne1.