Optimization Research of Construction Methods and Parameters of Large-span City Coastal Tunnel in Horizontal Layered Rockmass

A city coastal tunnel was a four-lane road tunnel. Maximum excavation width of tunnel reached to 19.51 m in Class IV surrounding rock. The surrounding rock of tunnel was medium-thick horizontal layered limestone. To obtain the mechanical parameters of transverse isotropic material, some tests, including rock mechanics test, on-site structural characteristics survey of rockmass, bearing plate test and numerical back analysis, field test of shear strength, were conducted. Then numerical simulation were adopted to study the reasonable excavation method and parameters of tunnel in Class IV surrounding rock. To verify effectiveness of the conclusion, stress of shotcrete and steel frame of primary support of tunnel were monitored on site. Research showed that normal elasticity modulus of rockmass was obviously less than transverse elasticity modulus, and it have remarkable transverse isotropy. Settlement of vault was the main deformation in horizontal layered surrounding rock. The failure characteristic of layered rockmass were rock-falling in spandrel and delaminating collapse of vault. Bench method could be employed in Class IV surrounding rock to speed up the construction progress. Stress test results of shotcrete and steel frame of support structure indicated the tunnel was safe under the optimized excavation method and parameters.