Influence Of Construction Of Open-cut Tunnelling On Uplift Displacement Of The Underneath Metro Tunnel And Its Control Measures

Based on a practical project in Shenzhen, the influence of long excavating on uplift behavior of the underneath tunnel and the control measures for reducing the uplift displacement of the tunnel were systematically investigated. The major contents and results of this dissertation are as follows:(1) The parameters of hardening soil (HS) model and hardening soil model with small strain (HSS) for the typical soil layers in Shenzhen are obtained from the undisturbed soil test,the wave velocity test and the empirical method. Shear hardening of silt, clay and gravel clay, as well as slight over-consolidation for the gravel clay, are observed from the test results. In addition, lots of unweathered coarse grains in completely decomposed granite (CDG) lead to the distortion of stiffness parameters for CDG. It is found that HS model would still overestimate the rebound deformation of soil while HSS model predicts much accurately.(2) The influence of unloading ratio and tunnel position in horizontal on uplift displacement of the tunnel underneath the excavation are analyzed based on field measurement. It is found that the major displacement of the underneath tunnel is uplift displacement while horizontal displacement seems to be inconspicuous, relatively.Besides, statistic results indicate that the uplift displacement of the tunnel shows good linear relationship with the unloading ratio under foudation reinforcement, and the envelope curve connected by the uplift displacement of the tunnel at different horizontal positions of excavation basement closes to parabola.(3) Two-dimensional finite element method is used to study the influence of cover depth and horizontal position of tunnel on the uplift displacement during excavating. Then,the empirical formula is present for predicting the uplift displacement of the tunnel in similar projects. Study results show that the uplift displacement of the tunnel seems to be linear with the unloading ratio under the condition of low unloading ratio, then the relationship become nonlinear with the increase of the unloading ratio. Moreover, the nonlinear relationship is more obvious as the cover depth decreases. After excavating above the tunnel, the magnitude of uplift displacement of the tunnel is definitively affected by the unloading ratio and the soil condition under the tunnel. The envelope curve connected by the uplift displacement of the tunnel at different horizontal positions of excavation basement is consistent with the bottom heave of the excavation, which develops from straight type to parabolic type, and finally bimodal type with the increase of the unloading ratio.(4) The effectiveness of four measures, including foundation reinforcement,sectional excavation in longitudinal direction, grouting around the tunnel and installing anchor on the tunnel, on controling the uplift displacement of the tunnel are investigated through three-dimensional finite element method. The results show that it is effective to control the uplift displacement of the tunnel when the gate-type foundation reinforcement and the sectional excavation in longitudinal direction are adopted. The width of the gate-type foundation reinforcement could be 5 meters each side, while the bottom of the reinforced region is proper to be 3 meters under that of the tunnel. Longitudinal sectional excavation with island excavating method is benefit for reducing the uplift displacement of the tunnel. Moreover, the proper length of longitudinal direction for every excavating zone could be 100 meters, and it is reasonable to remain 50 meters between adjacent excavating zones as the back pressure. The longitudinal length of the earth island in every excavating zone is proper to be 20 meters. Numerical results also show that grouting around the tunnel could be very good for controlling the uplift displacement of the tunnel. However, if the homogeneity of grouting is not well handled, the effectiveness might be limited.Besides, the excessive grouting pressure even may cause excessive additional deformation of the tunnel and threaten the safety of tunnel structure,making this measure has limitation for application. In addition, the measure of installing anchor on the tunnel, which is not suitable to be used, is likely to enlarge the uplift displacement of the tunnel.