ABSTRACT

The super larger span tunnel is a common form of highway reconstruction and expansion projects in recent years. In order to determine the stability of tunnel structure of the two-step and three-section excavation method of the III-level surrounding rock mass of the super larger span highway, the field test method was adopted. Relying on the Laohushan Tunnel in Jinan, Shandong, China, the deformation and the structure performance of the super larger span tunnel in III-level surrounding rock mass are analyzed, and the safety of the tunnel and the support structure is evaluated on this basis. The results show that the maximum settlement of the arch section of the Grade III surrounding rock section is 12.5mm, and the maximum clearance convergence is 5.8mm. Both of them are much smaller than the design reserved deformation of 80mm. The maximum pressure of the surrounding rock is 0.091MPa, showing that the force acting on the supporting structure by surrounding rock mass is small. The inner and outer arched parts of the steel frame are subject to large stresses, and most of them are tensile stresses. The maximum stress of the steel frame is 283 MPa, and occurs at the inner side of right arch waist. Although the local stress exceeds the yield strength of the steel (235 MPa), it does not exceed its ultimate compressive strength of 400 MPa, and the tensile and compressive stress values of the other inner and outer parts do not exceed the yield strength. Mainly, the maximum stress appears on the left side wall, reaching 4.83 MPa, which is far less than the ultimate compressive strength of sprayed concrete (11.9 MPa). For super larger span highway tunnels, located in III-level surrounding rock mass, constructed by two-step and three-section excavation method, the initial support effectively controlled the tunnel deformation, the supporting structures were fully protected and the tunnel structure was stable. The super larger span tunnel is a common form in the road reconstruction and expansion project in recent years. In order to determine the stability of tunnel structure of the two steps and three excavation method of the III-level surrounding rock mass of the super larger span highway, the field test method was adopted. Relying on the Laohushan Tunnel, the deformation and the structure performance of the super larger span tunnel in III-level surrounding rock mass were analyzed. The results show that the maximum settlement of arch of the III-level surrounding rock mass is 12.5mm in super larger span highway tunnel, and the maximum clearance convergence is 5.8mm. Both of them are smaller than the design reserved deformation of 80mm. The maximum surrounding rock mass pressure is 0.091MPa, the force acting on the supporting structure by surrounding rock mass are small. The inner and outer arched parts of the steel frame bear larger stress, and are mostly tensile stress. The maximum stress on inner side of the steel frame is 283 MPa, and occurs at the right arch waist. The maximum stress on the outer side of the steel frame is184 MPa, and occurs at the vault. The steel frame plays an important role in the initial support, however the force does not reach the yield strength of the steel. The shotcrete is subjected to pressure, the maximum stress appears on the left side wall is 4.83 MPa, which is much smaller than the ultimate compressive strength of shotcrete of 25 MPa. So for super larger span highway tunnels, located in III-level surrounding rock mass, constructed by two-step and three-excavation method, the whole structure is stable.