Mechanical Responses And Safety Control Of Composite Multi-layered Ground And Built Environment Due To Shield Tunnelling In Urban Areas

The core task of the shield tunneling in urban areas is to ensure the safety of the tunneling process itself and the built environment.It is particularly important to ensure the safety of the built environment nearby in view of its sensitivity to the ground deformation.The tunneling activities are often carried out in composite multi-layered ground in which the existing structures are always densely distributed.During tunneling,the ground often undergoes a complex loading and unloading process.Therefore,the ground and the built structures undergo a complex interaction accordingly.The ground deformation derived from the tunnelling propagates and interacted with the structures,thus causing risks in some extent along the deformation chain.How to effectively evaluate this deformaiton effect and make a precise control during the tunneling process is of paramount importance.This paper takes composite multi-layered ground as the research object,and takes the control of tunneling-induced deformation of the ground and the built environment as the key issue.The research methods including literature survey,theoretical analysis,numerical simulation and field test are all performed to invesetigate the tunneling effect and the control techniques.The tunneling effect includes the mechanical responses of composite multi-layered ground and the built environment due to tunneling.And the control techniques include the protecting measuers and the elaborative control techniques during tunneling.The main research work and conclusions are as follows:(1)Theoretical prediction for deformation of composite multi-layered ground due to tunneling has been proposed.Based on the state of different types of soils in engineering practice,the concept of composite multi-layered ground which means the overlap of soil layers and the composite of soil components is firstly proposed.After that,the in-plane deformation of composite multi-layered ground due to tunneling has been established based on the equivalent elastic theory and superimposed Loganathan-Poulos prediction theory.According to the parametric analysis,it is found that the presence of hard interlayer has an 'expanding effect' on the induced deformation,and the relatively softer interlayer has a 'contract effect' on the induced deformation.In view of the three-dimensional effects of the tunneling,a method for determining the gap parameter considering the longitudinal effects of tunneling parameters is proposed.This approach overcomes the limitation of calculating the gap parameter only considering the tunneling parameters at the current section.Besides,based on the equivalent elastic theory combined with the Mindlin's solution,the impact of six tunneling parameters on ground movments is predicted.These parameters are face pressure,the friction between soil and shield skin,linear decay tail grouting pressure,secondary grounting pressure considering the grouting,overloading during construction and volume loss away from the tunnel face.(2)Method for predicting the mechanical responsess of existing structures due to tunneling in composite multi-layered ground is proposed.According to the characteristics of the interaction between ground and existing structures,the structures can be divided into three categories including road and building structures,pipeline and subway structures and pile foundations.Among them,the mechanical responses of the pile foundation are emphatically studied.Based on the deformation prediction model of the composite multi-layered ground,bearing capacity loss of pile can be calculated comprehensively considering the weight of each soil layer,the lateral pressure coefficient of soil,the pile-soil friction coefficient,and the nonlinear stress distribution along the pile body.Depending on the degree of the bearing capacity loss of complete friction pile,the deformation ground can be divided into three typical zones including settlement zone,compression zone and tension zone.Besides,regarding the pile as the Euler-Bernoulli beam rest on the Pasternak foundation and considering the nonlinear change in the modulus of subgrade,the horizontal displacements and internal force of pile induced by tunneling are obtained.Results find that the presence of a hard interlayer can limit the displacements and significantly increase the bending moment of the pile.(3)The mechanism of the protecting strctures and the evaluation of mitigation efficiency due to tunneling are clarified.Considering the propagation characteristics of ground deformation due to tunneling,a prediction model for the mitigation efficiency of two protecting measures including horizontal grouting reinforcement and vertical mitigation pile was established.The mechanism of the two measures was clarified,and design recommendations were proposed for construction practice.Specifically,the mitigation efficiency of horizontal grouting reinforcement is firstly defined and the 'beam effect' is clarified.Considering the balance between mitigation efficiency and economical efficiency,an approach is suggested for determining the optimal parameters of grouted body.Besides,an analytical model of vertical mitigation pile considering the interaction between soil and pile is established based on the Melan solution.Results find that the variables including the position,dimensions and the mechanical parameters of the mitigation pile have significant influence on the mitigation efficiency of the pile.With the help of parameteric analysis,the optimal parameters of the mitigation pile can be further determined.(4)Refined control technique during tunneling process-Transparent Tunnelling Technique(3T)is put forward.Considering the empirical and hysteretic control of tunneling-induced ground deformation during the construction process,a theoretical framework and technical flow of Transparent Tunnelling Technique(3T)aiming at the refined process control were proposed and defined.It is clear that the core theory of this technique includes the determination theory of deformation standard,the prediction theory of deformation response,the measurement theory of deformation monitoring and the hybrid collaborative control theory of deformaiton.The technical flow includes three parts including the feedforward control before tunnelling,the hybrid collaborative control during tunnelling and evaluation and recovery control after tunnelling.The 3T unifies the control process,can realize the refined control of the deformation response and provides safety guarantee for the shield tunneling.This technique is successfully applied in the project in which the Qinghuayuan Tunnel of Beijing-Zhangjiakou High-speed Railway passes beneth the Zhichunlu Subway Station.