Abstract:

Gongbei tunnel, with an excavation crosssection area of 340 m2, is an ultrashallow tunnel in coastal soft soil strata. The tunnel is to be constructed by bench method under the protection of the advance pipe curtain. In the paper, numerical study is made on the structural internal force and safety of the tunnel constructed by 5bench 15step method or 4bench 8step method by means of ANSYS program. Conclusions drawn are as follows: 1) Both the 5bench 15step method and the 4bench 8step method can meet the structural safety requirements of the tunnel; however, these two methods have different safety impacts during the removing of the temporary support; 2) If the tunnel is constructed by 4bench 8step method, the primary support and the secondary lining at the side walls and invert of the lower half of the tunnel will bear large internal force in the period from the removing of the temporary support to the installation of the third lining, therefore the 4bench 8step method has low safety performance; 3) After the excavation of each step, the primary support shall be installed timely and the secondary lining shall be installed as soon as possible, so as to minimize the crown settlement and the ground surface subsidence. 

Key words: ultrashallow cover, superlarge crosssection, coastal soft soil tunnel, construction method