| Popis: |
Deformation measurements from 17 real cases of excavation spanning subway tunnels in soft soil were collected for examining the deformation reaction of the tunnels due to the above unloading. The investigation showed that (1) tunnel may be uplifted owing to the above excavation; (2) the deformation of the tunnel section due to above unloading exhibits vertical elongation and horizontal shortening, and the transversal shape of the tunnel changes from approximate circle to vertical ellipse; (3) the main influential factors include the excavation unloading ratio (N), the excavation area (A), and the excavation shape coefficient (α). It indicates that, with the increase of excavation depth and the increase of unloading ratio (N) of foundation pit, the rebound of tunnels may increase, and the higher the width of the excavation, the larger the range of vertical uplift that will be induced around the tunnel. For 12 out of the 17 examined cases, the vertical uplift measurements of the tunnels are smaller than the alarmed value of 10 mm, indicating that the maximum displacement of the tunnel can be controlled by purposed engineering measures, such as pre-reinforcement of the soil around the tunnel, installation of the anti-pulling piles around tunnel, and stacking around the bottom of the excavation. 2-D FEM modelling was also carried out to study the effect of excavation characters and the effectiveness of the control procedures. The combination procedure of soil improvement by cement mixing and anti-pulling pile installation is verified to be effective to reduce the adverse impact from above excavation on tunnels. |
