| Deep excavation is widely used in the exploitation of city underground space.For an excavation which is located in the soil layers which contain confined water,dewatering is needed to lower the confined water table before excavation.Coupling effect between dewatering and excavation brings about a challenge to the construction and safety of a pit.In view of frequent use of deep excavation,it is necessary to investigate the environmental influence of a pit subjected to coupling effect between dewatering and excavation.Based on this investigation,the features of confined water table and deformation of soil layers of a pit could be revealed,and thus some useful guidance for practical engineering could be provided.The purpose of this dissertation is to grasp the features of confined water table,deformation of soil layers and deformation of retaining structure for a pit which is subjected to coupling effect of dewatering and excavation.Firstly,a modified consolidation theory,taking the coupling effect of excavation into account,was proposed and verified.And then the characteristics of drawdown and deformation of soil layers were studied when only dewatering was carried out in a pit with existing of waterproof structures.The influences of some relevant parameters were also investigated.Based on this,the responses of confined water table and deformation of soil layers were further studied when the coupling effect of excavation was included.The influences of the coupling effect,different soil constitutive models and different dewatering and excavation sequences were analyzed.Finally,the applicability and reliability of the proposed theory were examined by simulating two excavation cases.The main contents and conclusions of this dissertation are as follows:(1)A modified consolidation theory which considered the coupling effect of excavation was proposed and verified.This theory was realized by modifying existing consolidation theories.The modifications included adding the compressibility of pore water to the seepage equation,replacing point flux with boundary flux,applying a stress relief load and a phreatic water table boundary at the excavation surface.The theory was implemented numerically through COMSOL Multiphysics software.The numerical solutions were compared with some analytical solutions and field pumping test data.The results indicated that the proposed theory was reasonable and reliable.(2)Single-well and multi-well dewatering with existing of waterproof structures was simulated with the proposed theory.Two indexes were proposed to evaluate the drawdown effect and ground settlement induced by dewatering.They were the drawdown efficiency f_h and the external settlement coefficient f_?.The influences of some parameters on confined water table and deformation of soil layers were investigated in detail.The results of single-well dewatering showed that the drawdown curve and ground settlement curve were discontinuous due to the separation of diaphragm wall.The drawdown efficiency f_h was greater than that with no diaphragm wall,while f_?was smaller.The maximum stratum settlement occured at the top of confined aquifer.Different parameters had different influences.With the increase in shield ratio of diaphragm wall,f_h increased while f_?decreased.With the increase in the well screen length and the distance between well and diaphragm wall,both f_h and f_?increased.When the well screen was installed in an order of the center,bottom and top of the confined aquifer,f_?decreased gradually.Pumping rate and coefficient of permeability had little effect on f_h and f_?.A ranking of influence magnitude from high to low was shield ratio of diaphragm wall,well screen length,the distance between well and diaphragm wall,the installation location of well screen,pumping rate and coefficient of permeability.The results of multi-well dewatering showed that f_h and f_?were 2.5 times and 2.2 times the corresponding values of single-well dewatering respectively.When the wells were arranged in a uniform way,f_h was the highest while f_?was the minimum in comparison with other arrangements.Both f_h and f_?increased when more wells were used for dewatering.(3)The environmental influence of a pit subjected to coupling effect of dewatering and excavation was studied,and it was compared with the result of an uncoupled analysis which did not consider coupling effect.The influences of different soil constitutive models and different dewatering and excavation sequences were studied.The depressurization effect of pumping well was evaluated quantitatively.It was found that drawdown of confined aquifer was mainly affected by dewatering process.Maximum ground settlement occured at a distance of 0.55D_e behind the wall(D_e is excavation depth).The ground settlements induced by pre-dewatering excavation,dewatering and coupling of dewatering and excavation accounted for 7.8%,51.7%and 40.5%of the ultimate settlement respectively.When dewatering duration increased,leakage from the unconfined aquifer to the confined aquifer will lead to compression of the unconfined aquifer.Maximum wall deflection was about 0.23%D_e,of which the deflection induced by excavation and dewatering accounted for 92.3%and 7.7%respectively.The ground settlement,stratum settlement and wall deflection obtained from an uncoupled analysis were underestimated by 20%,9%and 31.6%compared to a coupled analysis.The influence of different soil constitutive models was investigated.It showed that soil constitutive model had little influence on confined water table.The soil deformation and retaining structure deformation computed by linear elastic model were smaller than that computed by Mohr-Coulomb model,while the result of modified Cam-Clay model was larger than that of Mohr-Coulomb model.Thus,linear elastic model was fit for a preliminary analysis.Mohr-Coulomb model was fit for a detailed analysis while modified Cam-Clay model was suitable for analyzing an important project.After analyzing the influence of different dewatering and excavation sequences,the results showed that multistep dewatering and excavation could relieve soil deformation.With the increase of drawdown,bottom heave was reduced.When drawdown increased to some extent,however,the bottom heave remained unchanged.(4)Two excavation cases that subjected to dewatering and excavation were simulated by using the proposed theory,and the applicability and reliability of the theory were examined.For the Shanghai Hongqiao transport hub excavation,computed confined water table and ground settlement agreed well with the measured values.Computed wall deflection showed a similar tendency to the measured one although there was some deviation between their magnitudes.For the Century Metropolitan excavation,computed confined water table,vertical displacement at wall top and ground settlement agreed well with the measured values.Computed wall deflection was close to the measured wall deflection above the excavation surface,alghough there was some deviation between them beneath the excavation surface.Computed heave of the tunnel was close to the measured value.However,the spatial characteristics of deformation for the tunnel were expecting for further research.According to the analyses of two excavation cases,the proposed theory in this dissertation was appropriate for analyzing the environmental influence of a deep pit that subjected to coupling effect of dewatering and excavation. |
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