Study On Construction Risk Control Of Qiaojiashan Tunnel Excavated Through Tertiary Silty Clay Deposit

ABSTRACT:Qiaojiashan tunneling engineering is the controlling project of Datong-Xi’an railway passenger-dedicated line, where the tertiary silty clay deposit was encountered at section of DIK436+640～437+100and DIK437+585～438+170, which is notorious for its instability and long-time ground deformation after excavation, and thus brought high risk to Qiaojiashan tunnel construction, like for supporting structure damage due to large deformation of surrounding rock has been confronted. To solve the technical problem that tunneling through the tertiary silty clay formation and to propose controlling measures is of practical significance for guaranteeing tunnel constructing and operating safety.This paper reveals the weakness and rheology characteristics of rock mass of the tertiary silty clay formation by phase and microstructures analysis together with physical and mechanic test, and that rock properties are main contributions to large deformation of tunnel. In-site survey and analysis on deformation characteristic of surrounding rock and supporting structure at initial constructing stage were performed, based on which risk assessment was made on tunneling through the tertiary silty clay formation, almost all constructing risk factors were indicated, and constructing controlling measures were suggested to reduce risk, that is, through enhancing supporting strengthen and increasing allowance deformation to allow elastic liner perform certain deformation and bear more surrounding rock pressure as long as that liner stability and tunnel clearance were guaranteed, so that time-dependent deformation of surrounding rock would finish before installation of permanent lining.Several typical monitoring sections were chosen to verify the validity of the controlling measures proposed. Tunnel convergence, contact pressure between rock and elastic liner, mechanic and deformation characteristic of supporting structure were monitored in-site. The results denote that through optimization of supporting structure and excavation mode the tunnel supporting structure was in safety during constructing stage, and the tunnel clearance together with structural stability were guaranteed though large deformation still occurred in elastic liner. The achievement of this research provided a reference for the design and construction of tunnel excavated through similar rheological formation.31pictures,13charters and60references are presented in this paper.