拱北隧道为一座滨海软土隧道，开挖断面达340 m2，采用超前管幕预支护下的多台阶分部开挖方案。采用ANSYS软件对五台阶15分区方案和四台阶8分区方案的隧道结构受力规律及安全性进行了数值研究。研究结果表明： 1）采用五台阶15分区方案和四台阶8分区方案都能够满足结构安全要求，但在临时支撑拆除过程中，2种方案的施工安全性差异较大； 2）四台阶8分区方案在下半断面临时支撑拆除后、三次衬砌尚未施作时，下半断面边墙及仰拱处的初期支护和二次衬砌受力较大，施工安全性较低； 3）各部开挖后应及时施作初期支护，并应尽快施作第2层衬砌结构，以减小拱顶下沉和地表沉降。

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. 