Research On Deformation Characteristics Of Soft Soil Foundation Pit Under Complicated Loads

With the development of China economy, an increasing number of buildings are being built in the soft soil area, and this is particularly common in the coastal areas with high-speed economic development. Therefore, it becomes quite essential to predict and control the deformation of the foundation pits, since buildings in the soft soil area are getting increasingly dense. This thesis, based on a case study of the soft soil foundation pits in Guangzhou, would carry out research on the deformation properties of the foundation pit excavation. Considering various complex loads such as the excavation effect of the soft soil and the moving vehicles on the pit heading, this study would adopt the numerical method. This thesis includes four parts as follows:First, the strength index of the soil decreased under unloading condition on the basis of our completed research. According to the experimental results, the slope of the critical state line of the unloading soil is smaller than the soil subjected to loading, which means its resistance decreased after unloading, the results of the test on a typical gray-black soft soil under two shear conditions (keep loading and unloading) showed the corresponding slope of the critical state line is 1.2 and 1.07 respectively. By discussing the influence depth of the unloading effect, we concluded that the depth of the numerical model should be not less than 3.33 times of the excavation depth.Second, following a review of the present regulations and some relevant research findings, this study will analyze the computational model of the over freight on the pit heading. Moreover, a study case indicates that, due to the differences between excavation problem and retaining wall problem in plane dimension, deep dimension and load diffuse methods, it is not appropriate to simulate the vehicles load with the equivalent thickness of soil layer method when an excavation supporting transect is taken for plane analysis.Third, this study will analyze different vehicle load models and dynamic coefficients. The vehicle dynamic loading effect can be properly considered by selecting and simplifying appropriate vehicle load models and conversing the dynamic vehicle load into pseudo-static load. This simplify method has practical value in practical engineering.Fourth,According to the analysis of several common soil constitutive model applicability in Foundation, Select the modified Cambridge model as the constitutive model of soil when analyse the excavation course of the deformation of the soft soil foundation pit. Through the test results of some typical gray-and-black sullage soft soils to determine the required model parameters, for example:λ=0.135,κ=0.028, M=1.2, When unloading, M=1.07. It suggests that the unloading effect of the soft soil should be considered with regard to the parameter of the soil constitutive model, which would be modified in the process of calculation.Fifth, finite element model was established on the research of the constitutive model, the vehicle load model and the unloading characteristic of soft soil. The deformation principle of the soil and the supporting structure were observed through a variety of loading conditions in the process of excavation. The results showed that the deformation of the supporting structure and the soil around the foundation pit would increase considering the unloading effect in the process of excavation compared with the general design in which the top of the foundation pit was overloaded by uniform force, the deformation was close to alarm value considering the static vehicle load and it would exceed the alarm value considering the dynamic vehicle load, the deformation would far exceed the alarm value when taking the above two factors into consideration.. Therefore, conventional design does not take into account unloading and vehicle load effect is unsafe for the foundation pit.Sixth, the results also showed that the horizontal displacement of the upper soil and retaining structure was highly impacted by the size and form of overloading of vehicles on the foundation pit, and would decrease along the depth of the foundation pit. It could be ignored at a certain depth. The unloading effect significantly affects the deformation and axial force of the supporting structure of the foundation pit, which can guide the design of supporting structure.