Study On Deformation Control Methods Of Super-Large Foundation Pit In Soft Soil

The key of super-large foundation pit engineering in soft soil is to control the deformation, the deformation control is mainly related with the selection of supporting and bracing system and construction method, which involved many factors. therefore we need to do some research so as to acquire the reasonable design and construction parameters. With the background of main excavating foundation pit of Shanghai Nan railway station of South Square project ,this paper studied systemically the problems brought by supporting and bracing system and construction method through centrifuge model test, numerical analysis calculation combined with actual data, the creep deformation rules of slope during the construction process, factors that affect the creep deformation, dimension and function of pre-remained mound also with the stepped excavation method of clod are all involved in the problems. The research work done and results include as follows:(1) According to the characteristic of deformation of foundation pit in soft soil, problems brought by selection of supporting and bracing system and construction method were studied, thee keys of the deformation control of foundation pit in soft soil was presented,which were reasonable design and construction parameters .(2) The centrifuge model test was used to simulate and analyze the creep deformation of slope, based on the test, the creep equation was described by fractal dimension of equivalent stress and time according to the shear creep properties of soil, the calculated creep parameters were fitted combing with experimental creep curves of soil in Shanghai; the time hardening model was used to calculate the creep deformation,shear stress relaxation and creep strain change rules of slope during the construction process based on the combination of excavating slope and supporting, The results shows that it is feasible to adopt exponential creep equation to calculate the long-exposure creep of cutting slope in soft soil foundation pit. Besides, the variation rules of the creep of slope were calculated and analyzed under the influence of factors which include the amount and location of overload near the foundation pit,the width and height of leakageproof wall behind the supporting structure and whether it has middle board as a brace, these studies provide gist to the optimizing design of construction parameters. (3) In order to control the deformation of large excavated foundation pit during the construction process, in this paper, centrifuge model test and numerical calculation analysis was done aimed at the problems of the width of pre-remained mound,the construct order of clod and etc, which were brought by the selected construction method. The action mechanism of pre-remained mound in the deformation control of large excavated foundation pit in soft soil was studied, thereby the calculation formula of critical width and the relationship between horizontal deformation of wall and the mound width were deduced; the FEM was used to calculate and analyzed the construction order of central island clod and pre-remained mound through comparison of the deformation of foundation pit,the inter force and deformation of underground diaphragm wall, the results in a sense be guidance to the central island with partly top-down construction method in large excavated foundation pit in soft soil.(4) Based on the results of chapaters before,combined with much experience in projects, three designing and construction schemes were presented and somekey measures to control the deformations of super-large foundation pit engineering in soft soil were analyzed and summarized from two aspects,which are the control of influence to surroundings and the deformation of foundation pits.