Study of construction technology and mechanical effect of Guanjiao tunnel in shallow-buried sandy stratum in Xining-Golmud 2nd line

It is difficult to tunnelling in soft sandy stratum. To explore the key construction technology and study the mechanical effect of tunnelling in sandy stratum, numerical simulation and field experiments were carried out in sandy stratum section of Guanjiao tunnel. There existed about 450m long shallow-buried sandy stratum in the entry section of Guanjiao tunnel in the 2nd line between Xining and Golmud. Collapse had occurred during tunnelling using the initial three bench method; so four bench method was put forward. The excavation and support process of four bench method were simulated by means of FLAC3D numerical model, the deformation of surrounding rocks, ground settlement and stress distribution of primary support were analysed. And the superiority of the new method was proved by comparing the construction effect of three bench method. It was confirmed by actual construction and deformation measure that big arch-foot section, four bench excavation, long horizontal lead pipe-shed support, temporary transverse steel-support and shortening the distance of inverted arch closure were the key technologies of tunnelling in sandy stratum of Guanjiao tunnel. These methods could assure the safety of shallow-buried sandy stratum tunnel with big section. These could offer valuable experience to similar engineerings.