Tunnel Displacement Induced By Above Excavations In Soft Soils And Its Controlment

Under the complicated and sensitive construction environment in urban places, the excavations adjacent to tunnels become deeper, larger and even closer to tunnels. Any improper excavation construction plan will lead to serious consequence. People have to pay more and more attention to control soil movement by excavation and make efforts to assure tunnel’s safety. But the analysis of tunnel displacement induced by adjacent excavation is very complicated, which has to take excavation structures plan, soil reinforcement and construction technologies into accounts. And the relevant research is of insufficient. In order to solve these problems, Methods of theoretical derivation, in situ tests and3-D numerical analysis are employed in this paper. The main contributions of this thesis are described as followed:In order to predict tunnel displacement induced by soil unloading of above excavation, a semi-analytical method was proposed by this paper, which could take the rheological soil and tunnel stiffness into account. By application of Boussinesq’s solution and Mindlin’s solution, soil displacement was derived from soil unloading and tunnel forces. The soil-tunnel interaction was analyzed by the coupling conditions on displacement and forces between soil and tunnel. Visco-elastic model was employed to simulate the rheologic deformation of soils, by which the time effect of soil displacement could be taken in account. series of computer software were programmed based on this method. By applying the software, the tunnel stiffness, the excavation depth, the excavation area and the relative distance to tunnel were discussed in details. The effect of segmentation and layered excavation method were also investigated. Results of two case studies had showed a good agreement between the predicted tunnel displacement and measurements, which guaranteed the effectiveness of the analysis for future similar cases.At the same time, in order to study the effect of decreasing tunnel displacement by soil improvement and additional excavation structures,663-D fem models were set up for analysis, which could consider the interactions between the soil, excavation structures and tunnel. The Modified Cambridge Model was employed for soil constitutive. The tunnel displacement was investigated when soil reinforcement was applied at different places and with different bulk. The effect of decreasing tunnel displacement by dividing wall and cutting off wall was also studied. The tunnel displacements caused by three series of excavation technologies were compared to show the rationality of proposed construction method.Further more, full scale field tests were conducted to investigate the impact of deep soil mixing (DSM) columns installation and its controlments, since DSM columns installation of soil improvement will bring extra harmful tunnel displacement. In the tests, soil horizontal displacement, deep soil vertical displacement and excess pore water pressure by single and large number of DSM columns installation were studied thoroughly. The accumulation and dissipation of excess pore water pressure were analyzed and a prediction model was also set up based on the research. The soil horizontal displacement and different depth of soil vertical displacement induced by DSM columns installation were studied and soil horizontal displacement fluctuation effect was also found. The optimal DSM column installation technologies to control tunnel displacement were also provided based on the tests and relevant research, investigations of case applications guaranteed the effectiveness of the installation technologies.For case studies, the semi-analytical method,3D FEM method and in situ measurements were employed, which deal with tunnel and retaining wall displacements induced by actual construction technologies. The predicted tunnel displacements by the analytical method were close to measurements. And the program of displacement back analysis was set up to study the critical parameters of FEM models. By employing the factors of weighing function, displacement of tunnels and retaining walls could be taken into accounts at the same time for back analysis. The horizontal displacement development of retaining wall during soil removing and after the structure construction completion were also investigated. The numerical analysis and monitoring data showed that pit excavation had great impact on the tunnel displacement and the impact was superimposition effect.