The Interaction Between Two Super-close Tunnels Constructed By Extra Large Diameter Slurry Shield

Because of the limitation of geological conditions and utilizing the underground space efficiently in cities, super-close tunnels constructed by extra large diameter slurry shield are inevitable. In case the new tunnel under construction is built after the existing one, the excavation will inevitably affect the stresses distribution and thereforethe deformation or settlement of the existing tunnel. This thesis was based on the Shanghai South Hongmei Road Tunnel Project, where super-close tunnels were constructed by extra large diameter slurry shield for a relatively long distance in Minghang Section. It is necessary to research the interaction between two super-close tunnels excavated by extra large diameter slurry shield and take proper measures to ensure the safety of this project. A three dimensional elastoplastic finite element model in ABAQUS software was established to analyze the construction process of a new tunnel constructed next to an existing one. Based on the analysis which shows how the stress and displacement of existing tunnel change with the construction process of the new tunnel, effective measures to reduce the adverse effects between the two tunnels were proposed. The results gained in this thesis will provide a reference for the design and construction of similar tunnel projects. The main works and results of this thesis are as follows:1. The disturbance of the surrounding soil due to the single tunnel construction by extra large diameter slurry shield was analyzed based on the field monitoring data, Meanwhile, the ground settlement due to the construction in soft clays was studied. The results show that the modified image displacement method can efficiently predict the ground settlement.2. Based on the numerical simulation by finite element analysis software ABAQUS, the mechanism of how ground settles was investigated, where several influencing factors were discussed such as soil condition, buried depth, and tunnel spacing. The results show that the larger the elastic modulus of soils, the less the disturbance will occur; the less the tunnel spacing, the larger the disturbance; besides, the influence of buried depthon the disturbance of surrounding soils is complex, in case the excavation surface is directly below soft soils, the soil layer above the tunnel axis has a tendency to "bump" compared to the surrounding soils.3. The mechanisms of how the ground settles and lining deformswith piles installed at tunnel axiswere also investigated by the finite element analysis software, where influencing factors such as pile spacing and pile length were discussed. The results show that the installation of the piles can effectively reduce the maximum settlement of the ground and linings, and the bending moment of linings. Secondly, the closer the pile, the less the interactive affect between two tunnels, however, a greater overall horizontal offset value of the lining is obtained. Moreover, shield tunnel construction will introduceless mutual influence to each other if longer pile length is used.4. The influence of a new tunnel under construction on the existing one was investigated by site monitoring data and compared with the numerical results. The results show that predicted and measured values are in good agreement and therefore the numerical model is suitable for predicting the soil disturbance during the tunneling process.