Excavation-induced Deformation And Control In Soft Deposits

Deep excavations with complicated environment conditions have sprung up during urban underground space developing process; these deep excavations are usually surrounded by buildings and utilities. More severe restrictions are required for these deep excavations in soft soil deposits. To ensure safety of those adjacent structures, ground movement pattern and the influence of deep excavation on environment are needed to be predicted in advance, thus providing guidelines to take appropriate deformation control measures. This dissertation carried out researches on deformation influence and control issues of deep excavation on environment in Shanghai soft soil deposits, by means of theoretical analysis, numerical simulation as well as in situ test and field observation.Conventional analysis methods and softwares for deep excavation were summarized and discussed. According to advantage of explicit calculation method and limitation of complex numerical model establishment based on FLAC3D, a pre-processing program for FLAC3D was developed. Some technical issues for numerical simulation, including boundary and initial stress conditions, choice of constitutive model and corresponding parameters, interaction between retaining structure and ground, interaction between pile and ground, and judgement of calculation results, were discussed and the corresponding treatment principles were proposed.Theoretical analysis, in situ test and numerical simulation were employed to study slurry wall stability and ground movement during slurry trenching and concreting for diaphragm wall. Influence factors on slurry wall stability were classified and discussed, meanwhile, global and local instability of slurry wall were investigated by theoretical analysis. Deformation mechanism of slurry wall during different construction stages were analyzed by in situ test. New testing equipments and methods for soil pressure and bentonite slurry pressure were designed, and the test results provided parameters for numerical simulation. The slurry trenching and concreting process were dynamically simulated by the established numerical model, and horizontal stress distribution, lateral deflection of slurry wall and ground settlement were investigated. The influence of slurry wall improvement, guide wall, concrete pavement, and existing side wall on lateral deformation of slurry wall and ground settlement was studied by parametric analyses.An empirical ground movement estimation expression for any point outside excavation was established based on lateral deflection of retaining wall and ground surface settlement profiles. The influence of deep excavation on surrounding environment was divided into three influence zones, and an simplified zone chart and environment protection countermeasures were put forward. A deep beam model was established to investigate stress and deflection of building wall subjected to differential ground surface settlement, and a practical method for estimating building damage degree was put forward and verified. The influence of deep excavation on adjacent pile foundation was divided into four types, and the corresponding deformation control criteria and protection measures were presented.A whole process deformation control idea of"source restriction, path isolation, and object protection"was proposed; the corresponding control flow diagram was also established. Some deformation control measures for deep excavation commonly adopted in Shanghai soft soil deposits, including optimization and adjustment of retaining scheme, ground improvement inside excavation, ground improvement and isolation outside excavation, protection and strengthening of adjacent structures, were investigated, and an integrated deformation control system for deep excavation was established.The main findings of the study were applied to a deep excavation project for metro station in Shanghai and validated by the field observation. The types of the nearby piles were classified by the proposed influence zone chart, and the corresponding protection countermeasures were emphasized. The deformation influence of diaphragm wall and bored pile construction on the environment was investigated and analyzed, and the influence of the deep excavation on the nearby pile foundations was dynamically predicted by the established numerical model taking into account the existing pile foundations. The excavation was extensively monitored by the designed observation scheme, and the observed results agreed well with the predicted data, meanwhile, the measured lateral deflection of retaining structures were only half of the collected case histories in Shanghai soft soil deposits. The reliability of this study and the effectiveness of the integrated deformation measures were proved.