Character Analysis For Shallow Tunnel Construction In Soft Soil Foundation

The soft soil is distributed extensivly in the southeast coastal area and inland of our country, which adds difficulty in tunnel construction because of its own characteristics, such as low strength, great deformation, bad water permeability and long time of lasting deformation. The soft soil has low shear strength and the soil around has low stability and it is easy to collapse after excavating, so it needs strong support structure. The soft soil has great deformation, which makes the deformation of soft soil tunnel also great after excavating and ground settlement hard to control. The oversized settlement is easy to cause road surface crack, even collapse, which affects the traffic safety. Using shallow tunnel construction method (STCM) in soft soil foundation is different from the traditional method, so the traditional process should be improved to control the deformation and keep soil stable more effectively. When using STCM in soft soil foundation, it is important to research the deformation behavior of soil around and seek reasonable construction process. The following works are done in the paper: First, the author takes a review on the methods of tunnel construction in soft soil foundation, the development of STCM and computational theories of the ground movement. Second, a brief introduction is made about the characteristics of soft soil and tunnel in soft soil,the characteristics and suitable condition of STCM. Based on the stratum of Fuzhou, the construction technologies of STCM in soft soil foundation are introduced. Then a few words are said about the calculation theory and method of FLAC-3D (Three Dimensional Fast Lagrangian Analysis of Continua), including the mathematical model description, the formula expression and the computation process. Next, the three-dimension model for shallow tunnel is set up by FLAC-3D and some researches are made about the effects of different factors for character of shallow tunnel construction. The stability of soil around after tunnel excavated is focused on in the paper, which considering construction methods, the length of excavation, the strength of reinforcement in vault and the condition of stratum. Ground settlements, stress distribution and plastic zone are analyzed. The contact interaction between soil and lining is taken into account. The constitutive model employed is Mohr-Coulomb plasticity with linear elasticity assumed within the yield strength surface. At last, a case is analyzed in this paper. It showed that the calculated results are similar the measured ones. It provides some useful conclusions for its design.