Deformation And Its Control Of Existing Shield Tunnel Induced By Unloading Of Adjacent Foundation Pit Excavation In Weak Stratum

At present,with the rapid development of urban rail transit and underground space utilization,the phenomenon of foundation pit excavation adjacent to existing metro tunnel is increasing.The prediction and control of tunnel deformation induced by excavation has become an urgent problem that must be faced and solved in urban construction.Under the supports of the National Key Research and Development Program of China(2016YFC0800200,2017YFC0805500),the Postgraduate Research & Practice Innovation Program of Jiangsu Province(No.KYCX17?0151)and the Scientific Research Foundation of Graduate School of Southeast University(No.YBJJ1791),on the basis of previous research,and in view of the current deficiencies of practical prediction methods of tunnel deformation and the imperfect research on the influenced zone of tunnel deformation,this thesis carried out a series of research using the methods of literature investigation,statistical analysis,numerical simulation and field test.The main contents and results are summarized as follows:(1)42 cases in which the foundation pit excavation influences the lateral tunnel and 33 cases in which the foundation pit excavation influences the underlying tunnel are collected,respectively.The typical stratum conditions,dimensions,relative position and structural deformation of foundation pit and tunnel in these cases are counted,and the influence of geological conditions,excavation depth,distance between foundation pit and tunnel on the vertical and horizontal displacement of tunnel are analyzed.Based on these analysis,the judgement criteria for the heave and settlement of lateral tunnel is clarified and the zoning of tunnel vertical displacement outside foundation pit is given,the prediction indexes for the horizontal and vertical displacement of lateral and underlying tunnel are put forward respectively,and then the empirical prediction formulas for the horizontal and vertical displacement of lateral and underlying tunnel under soft clay stratum,silt and silty sand stratum,sandy pebble and weathered rock stratum are given.(2)The FEM numerical models are established to analyse the influence of braced foundation pit excavation on deformation behaviors of lateral and underlying tunnel in soft silty clay.Then,through the analysis of tunnel deformation isoline,combined with the three-level tunnel deformation control standards of 20 mm,10 mm and 5 mm,the influenced zones of lateral and underlying tunnel deformation are divided respectively.And,according to the features of influenced zone,by defining determination parameters of influenced zone,a simplified description of the scope of influenced zone is realised.On these basis,the influence of excavation depth He and retaining wall lateral displacement ?hm on the determination parameters of influenced zone is studied,and then the determination method for influenced zone of lateral tunnel deformation under different He and ?hm is given.Similarly,the influence of excavation depth He on the determination parameters of influenced zone is analyzed,and then the prediction method for influenced zone of underlying tunnel deformation under different He is given.(3)According to the statistical results of tunnel deformation control measures,the most commonly-used soil improvement method is selected to study the control effects of soil improvement outside the pit and soil improvement inside the pit on lateral and underlying tunnel deformation in weak stratum,respectively.Considering the control effect and engineering economy,the recommended values of improvement strength,depth and width of soil outside the pit are given,and the recommended values of improvement form and strength of soil inside the pit are given.Then,by adopting the above recommended values,the control effect of soil improvement outside the pit and soil improvement inside the pit on the influenced zone of lateral and underlying tunnel is studied,respectively.(4)Relying on the two actual projecs in which the lateral and underlying tunnels are respectively affected by foundation pit excavation in weak stratum,the prediction methods of tunnel deformation and influenced zone obtained from the above research are used to predict and evaluate the possible tunnel deformation before construction,and relevant deformation control measures are taken according to the prediction and evaluation results.During construction,the deformation of existing tunnel is monitored,and the development law of tunnel deformation is analyzed based on the monitoring results.Furthermore,the reliability of the prediction methods is verified by comparing the predicted and measured results.