Research On Tunnel Face Stability And Pre-lining Loads In Soft Surrouding Rockmass

ABSTRACT:Tunnelling in the geological conditions of soft surrounding rock is one important issue of tunnel engineering for years. Deformation of tunnelling in loess, as a kind of special soft soil, is difficult to control for its rapid deformation rate and great displacement. This dissertation is supported by the projects of National Science Foundation of China named Reaserch on Interaction Mechanism of Pre-arching and Stability of Tunneling Faced in Weakly Cemented Sandstone With Water (NO.51178026). The research methods include theoretical analysis and numerical simulation and centrifuge model test. This paper investigates the effect of pre-tunnel on stability of tunnel face and the effect of strengthening tunnel face on tunnel lining stress and deformation of tunnel support. The main work and conclusions are described as follows.(1) Based on the Limit Equilibrium Theory, the minimum supporting force to balance the tunnel face, when the tunnel in loess excavated by full section method, decreases as the soil friction angle and the soil cohesion increase, and it increase as the soil density and the tunnel height increase.(2) The reasonable parameters of pre-lining are given through numerical simulation and orthogonal experimental design. The optimal length and overlapping length and thickness and elastic modulus of pre-lining are 8m、1m、0.4m、29.5GPa respectively.(3) The ratio of the soil pressure and initial soil pressure in tunnel vault decrease as the depth of tunnel and the thickness of pre-lining and the relative rigidity of the pre-lining with respect to the surrounding ground increase.(4) Comparing with the difference between strengthening the advance core soil with bolt and without bolt of the tunnel excavated by pre-tunnel method, through the centrifuge model test according to the ratio 1:60, the paper finds it can dramatically reduce the deformation of soil and improve the tunnel face stability and reduce the stress of pre-lining to strengthen the advance core soil.(5) Based on the Limit Equilibrium Theory, the tunnel excavated by full section method is safe when the depth of tunnel is less than 9m. Through the centrifuge model test according to the ratio 1:60, the tunnel excavated by pre-tunnel method is safe when the depth of tunnel is less than 6.7m. Considering the disturbance of the advance core soil during the centrifuge model test and the limitation of the theoretical, the findings obtained from the two methods are both reasonable.