Surrounding Rock Mass Deformation of Tunnel in Loess Constructed Using Three-Benches-Seven-Steps Excavation Method

The construction practice indicates that the good results could be achieved of large cross section tunnels constructed using Three-Benches and Seven-Steps tunnel excavation method.The change of stress and deformation within the rock mass during each step in the tunnel excavation has an important impact on the construction safety,supporting selection and construction quality control.It might also affect the stress in the lining structures after the construction.The case history of a large cross section tunnel construction in Xiangyangchun during Contract Two of Taixin Railway Line is presented.The Grade V rock mass improvement was required during tunneling.During the construction,the tunnel was instrumented using pressure cells,strain gages.The stress and deformation of the rock mass as well as the steel arch supported are monitored.The tunnel excavation sequence using Seven-Steps method was also simulated by using finite difference software.The shell elements were used to simulate the initial supports.The deformation of the surrounding rock mass and the stress change of the supporting structures during the construction are determined by both instrumentation and numerical analysis.