| The foundation pit excavation and shield excavation are two main excavation activities in the urban construction in China. The soil'unloading function would change the initial stress state of the surrounding soil, which will bring adverse effect on the adjacent existing subway tunnel. Therefore, how to predict and control the longitudinal force and deformation of tunnel caused by adjacent excavation has an urgent and important significance in practice. There are manly influence factors, such as soil property, soil layer, retaining structure, stratum reinforcement, uplift pile and so on, which have a great impact on the deformation of existing tunnel. However, the present researchs on these aspects are still insufficient. In this dissertation, these issues were raised alone as a new research direction alone. By the means of theoretical analysis, numerical simulation, centrifuge model test and field measurement, a systematic and in-depth research was carried out to research the above issues. The main research content containing some aspects as following:(1) Based on the elastic layered half space soil model, a simplified theoretical analysis method was presented to solve the soil vertical displacement caused by the foundation pit construction. The proposed method could fully reflect the effect of soil unloading on excavation face, horizontal force of supporting structure, engineering dewatering and soil layer characteristics, thus the conclusion drawn from this method was more consistent with engineering in practice. Through a numerical example analysis, it can be concluded that the proposed method can reflect the concentration (or dispersion) of strain in layered foundation soil.(2) An integral coupling method is presented to estimate the interaction between tunnel and surrounding soil under the condition of excavation unloading. Based on this method, an elastic solution of longitudinal deformation of single tunnel and double tunnel caused by excavation in sandy soil was deduced firstly, and the validity of this calculation method is verified by an example of Centrifuge model test. Besides, an visco-elastic solution of tunnel'longitudinal deformation caused by excavation in clayey soil was deduced based on the three parameter rheological constitutive model of H-K, and the presented method was verified with measured data in site.(3) The presented method is compared with two-stage method through a case study. Research found that the coupling method proposed in this dissertation is an integral analysis method, so it does not need to rely on specific foundation model and various foundation parameters, it could reduce the number of parameters needed during calculation and simplifies the calculation process.(4) The presented method was applied in an engineering example, it could be found that the tunnel' deformation caused by the rheological properties of clayey soil can not be ignored. The time of tunnel' deformation tends to be stable is related with the rheological parameters of soft soil. In order to reduce the adverse effects on adjacent existing tunnel caused by the time effect of soft soil unloading, the main construction process, such as excavation, supporting, body structure and soil backfill should be finished before the rheological deformation tends to be stable.(5) Based on the mirror image method and soil mass' movement model of shield tunnel, an analytical method is presented to estimate the influence of shield tunneling under existing tunnel. Through the analysis of centrifuge test case, it could be found that the proposed method is very suitable for the analysis of the influence of shield tunneling on existing tunnel and rigid pie in the case of a lower formation loss rate. According to an engineering case study of shield tunneling under a rigid pipe, result shows that the analytical solution of the pipeline' settlement is basically the same with the measured data, which verifies the validity of the proposed method;(6) An example of the shield tunneling under an existing tunnel is studied by the proposed method. Research found that the vertical orthogonal relationship of two tunnels is the most advantageous position from the perspective of tunnel' deformation, while it is the most unfavorable relative position from the perspective of tunnel' shear and bending force. Increasing the longitudinal stiffness of existing tunnel can reduce the deformation while also significantly increases the peak moment on tunnel.(7) Soil reinforcement can improve the connectivity between retaining wall and soil mass, as well as the integrity of the foundation soil, thus the transmission of shear strain and unloading stress becomes more uniform within the range of soil reinforcement, which can not only reduce the longitudinal deformation of tunnel caused by adjacent excavation, but also control the relative tensile deformation of tunnel' cross section.(8) The finite difference software FLAC3D was used to investigate the mechanical properties and anti-uplift mechanism of uplift pile located in the rebound soil mass under the pit bottom. According to the relative displacement between pile and surrounding soil, the pile is divided into three parts as uplift section, stable section and embed section. Research found that the uplift pile can not only constrain the vertical displacement of soil around pile, but also cut off the transmission path of shear strain between the retaining wall foundation and tunnel cavern, which can significantly reduce the shear strain of soil mass around the tunnel cavern. |
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