Study On The Time-space Effect And Control Of The Deformation Of Deep Foundation Pit In The Sea Reclamation

With the acceleration of urbanization in our country, sea reclamation has become an important way to increase the land resource reserves and expand the urban space, as a result of the growing contradiction between the expanding city construction and limited land resource. Along with the construction of high-rise and super high-rise building, many deep, big and complex foundation pits are excavated in the area of sea reclamation. However, because of the creep deformation of the thick marine sedimentary soft soil under the layer of the sea reclamation during the excavation, obvious space-time effect of the supporting structure and surrounding soils frequently occurs. Since the space-time effect affects the security of the construction seriously, the time-space deformation must be controlled. Considering that the creep properties of the marine sedimentary soft soil under the layer of the sea reclamation are key factors that affect the space-time deformation of the whole foundation pit, based on the well-known research achievements of the creep properties of soft soil,this thesis uses a combination method including experimental research, theoretical analysis and numerical simulation to study the creep properties and constitutive model of the marine sedimentary soft soil under the layer of the sea reclamation, and applies the research result to the control of the time-space effect of the deformation of deep and big foundation pits in the area of sea reclamation. In this dissertation, the main investigation work focuses on the following:①An systematic research was conducted on the basic properties and mineral components of the marine sedimentary soft soil and it is found out that high fines content, which reveals that high clay content and high mineral content of hydrophilic are important factors of the nonlinearcreep of the marine sedimentary soft soil. Adopting stress-control tri-axial rheometer, a series of consolidated undrained tests of the marine sedimentary soft soil of different depth under the corresponding confining pressure were conducted. Based on the experimental result, this thesis mainly analyzed the creep properties when the axial stress is both above and below the yield stress and how it varies with time, and got the function describing how the pore water pressure varies with time and stress.②By analyzing the linear visco-elastic and nonlinear visco-plastic creep properties of the marine sedimentary soft soil and adopting the standard linear model and the combination of standard linear model and emprical formula to describe the visco-elastic creep and the visco-plastic creep respectively, a nonlinear creep model was developed to describe the creep properties of the marine sedimentary soft soil. The parameters of the nonlinear creep model were identified by the MATLAB program of BFGS algorithm and a relatively high-precision result was obtained, which indicates that the developed creep model can well describe the creep properties of the marine sedimentary soft soil of different depth.③Based on finite difference theory, differential equations that describe the interaction of the nonlinear creep stress field and the penetrative stress field in Shenzhen soft soil were derived. A secondary development of the nonlinear constitutive equation in use of Microsoft Visual C++6.0 was accomplished, obtaining the Dynamic Linker Szsoftsoilmodel.dll. The result of numerical computation of the creep deformation under the confining pressure of 100 kPa, 200 kPa, 250 kPa is similar to the indoor creep test curve, which indicates that the secondarily developed program is right and forms the foundation of its engineering application.④Based on the 3D model of the practical layer, the creep model developed in preceding part of the thesis and the Mohr-Coulomb constitutive model were both adopted to calculate the horizontal displacement of retaining piles. By comparing the computation result and the monitoring data of the horizontal displacement, it is concluded that the computation result of the creep model better accords with the practical horizontal displacement than that of Mohr-Coulomb constitutive model, and that the secondary program that considering the creep of the soft soil is reasonable.⑤By applying the creep model of the soft soil to the research of the time-space effect of deep of a big foundation pit in the area of sea reclamation, the time and space effect of the horizontal displacement of the retaining piles, the ground settlement around foundation pit, bottom uplift, the loss of the cable’s pre-stress and the bending moment, the shearing force and the moment of the piles were analyzed. The numerical analysis with the creep model indicates: firstly, The time-space effect of stress and deformation lies not simply in that the the value of the stress and strain is small in the corner and big in the symcenter, it is also connected with the thickness of local layer, the underground water and sequence of excavation, etc. secondly, the eastern side of the foundation pit should be the key monitoring area because the value of the horizontal displacement of retaining piles and the ground settlement around foundation pit is the maximum among all the sides. Lastly, compared with the numerical model assuming the ground layers are horizontal, the numerical model according to the practical layer can better simulate the practuacl deformation of the foundation pit.⑥Taking the deformation control of the foundation pit at y=135m for instance, three deformation control schemes were proposed,according to the analysis of the main factors that affect the deformation of the foundation pit and environmental protection. Through the computation result of the creep model that considers these factors, the best deformation control scheme is the combination of expanding head of pre-stressed anchor and grouting outside the foundation pit.