Mechanism Analysis Of Running Tunnel Upheaval Due To Top Excavation

The soil displacements are always the difficulties faced by the engineers during excavation of foundation ditch, especially which is strictly controlled while existing structure such as running tunnel beneath the excavation. These problems are related to soil consolidation, upheaval, stiffness of retain wall, method of excavation, soil stable mechanism, tunnel deformation and protection, construction plan & sequence, monitor, feedback analysis and construction controls etc,so there are not any systematic theory to be applied, but still in the phase of engineering practice. In this paper, integrate the engineering practice, based on the classical theory and feedback analysis, upheaval mechanism of running tunnel due to top excavation in the soft soil area is researched and semi-experience formula is proposed.This paper focuses on the normalization model of the soft and improved soil which influence the tunnel upheaval, and tunnel equivalent stiffness etc. Through FEM simulation, analyses all possible factors on the tunnel upheaval. By information-based feedback construction and data regression, the main factors affecting the tunnel upheaval is concluded. Thus summarize the main factors and establish the semi-experience formula, revise the parameters according to the projects. The followings show the main contents and the conclusions of the research:1. To control the soil displacement and the tunnel upheaval, in Shanghai area the soft soil usually be improved. Through establish the normalization soil constitutive-model, it's more convenient to use finite element for data analysis. On one hand, refers to the rheologic experiment date of the typical Shanghai soft soil, semi-experience modified theory is used to describe the non-linear rheological behaviors, On the other hand, based on the test of physic behaviors of the cement treated soil, through relationship analysis establish the the main parameters which affect elasticity modulus on the cement mixed soil; meanwhile, attained unload modulus of the improved soil through experiments on the soil sample. Lastly, adopt the normalized model for the common soil layer in Shanghai and the improved soil, only need to select suitable parameters separately, different soil's rheology can be simulated from this model.2. Study few questions in the FEM simulation. Make use of the secondary development function of ANSYS, the normalized soil model simulated as a sub- program input. Meanwhile, for the convenient of simulate the tunnel stiffness, the"equivalent continuous model"had constructed for bored tunnel. This model base on the similarity of the vertically deformation and the uniform continuous beams to simulate the whole tunnel or a certain section, beam unit and the tunnel have the equivalent stiffness. Use the anisotropic material properties setting in ANSYS to simulate the tunnel's horizontal and vertical stiffness.3. In the finite element analysis, adjust the related parameters to contrast analysis, some conclusion worked out: the main factor that affected the tunnel upheaval is the improvement property under the ditch. Not only the effect of the anti-pull pile increase with the improvement property; but the construction duration do not affect the soil deformation anymore which affect the soil deformation obviously in soft soil. The other main factors are part-excavation and stress release rate of the soil of the excavation face. Besides, the tunnel upheaval is insensitive to the change of the stiffness of the tunnel, because horizontal stiffness of tunnel is much larger than the soil, but the vertical stiffness is much less than the soil.4. Adopt the method of separate parameters, carry out regression on the feedback data, separate a single factor from the measured data, carry out regression analysis and obtain these conclusions: soil deformation is similar to elastically deformation after soil improvement; the tunnel upheaval and the distance of influence can be estimated as linear relationship; the value of tunnel upheaval and the excavation width is in inverse exponential proportion.5. According to the classic theory, integrate the major factors obtain from the finite element analysis and monitor date regression; establish the semi-experience formula to estimate tunnel upheaval. In computation, take into consideration of the equivalent modulus of tunnel, the affect of excavation character on unload stress distribution, the effect of soil improvement on the stress release rate under the excavation. Comparing the computation results from the finite element and the formula, revise the stress release ratio.