Engineering Response And Control Of Existing Tunnel Structure Parallelly Under-crossed By A Closely-Attached And Newly-Built Tunnel In Long Distance

ABSTRACT:With the large-scale construction of urban rail transit,the new tunnel is constructed inevitably passing through great amount existing structures in order to make the number of crossing projects increased gradually.The large deformation of existing structures will be caused byany slight disturbance.Especially,the crossing projects that parallelly attached to existing tunnel floor have a large influence range and less settlement controllability.It is very difficult for current control technology to ensure the safety of existing structure during construction.Therefore,this paper takes Olympic park station of Beijing subway line 15 crossing Datun road tunnel for example.Based on the field test、assessment and mornitoring,the influence and control of existing tunnel structure are studied using theoretical analysis,field test and numerical simulation.The research results are as following:(1)Based on analysis of crossing engineering cases,the main influence factors of engineering construction are refined,the risk assessment of the new tunnel beneath the existing tunnel is analyzed based on the four aspects:the angle between the new tunnel and the existing tunnel,the thickness of the soil between the new tunnel and the existing tunnel,construction scheme of new tunnel and the safety state of the existing tunnel.Finally,the key and difficult point of new station which cross the existing tunnel for a long distance in parallel and closely are put forward.(2)Based on the theory of elastic foundation beam,the new tunnel passes through the double-hole closed frame section of the existing tunnel as the research object.Two calculation models were established depending on if there is soil between these two parallel tunnels,which revealed the engineering response law of the existing tunnel along the longitudinal direction.Through the Matlab calculation,the defonnation and force law of the existing structure are obtained under the influence of stiffness change of existing structures,length of unsupported area of new tunnel,soil coefficient of subgrade reaction.The elastic response method is used to analyze the lateral response of the existing tunnel.The above conclusions provide theoretical support for the deformation control measures and deformation control indicators of this project.(3)A method for formulating the deformation control standard of existing tunnel structures is proposed.Firstly,the analytic hierarchy process and fuzzy comprehensive evaluation method are used to evaluate the comprehensive reduction factor of the existing tunnel bearing capacity.Subsequently,the total settlement and differential settlement of the different sections of the existing Datun Road tunnel are calculated by ABAQUS finite element software.Finally,the deformation control standard value is selected from the safety point of view,which ensure that the safety of the existing structure during the construction process provides a basis for dynamic control.(4)In order to realize the scientific and refined risk control of the whole process,this paper establishes a system risk control process by fault tree method,Kent model method,3D simulation monitoring and evaluation system method,which based on the deformation distributing principle.This process consists of four aspects:systematicrisk identification,scientific risk assessment,dynamic risk monitoring and timely and effective risk management.(5)A new type of underground excavation construction method is proposed,which is Single layer guide hole-large diameter pile method,and the corresponding pile-forming technology and supporting equipment are developed.This method can form a pre-support system as soon as possible,and quickly transfer external loads.Moreover,it can effectively control the deformation rate and final deformation value of the formation,and is suitable for underground excavation and tunneling through existing tunnel structures.Based on the displacement back analysis method,the dynamic prediction of the whole process of the project is implemented.The optimal formation parameters were determined by back analysis of each construction step.Through this method,the purpose of accurately predicting the deformation of the target section is achieved.The comparative analysis found that the prediction results are basically consistent with the field test results,Which indicate that the method can accurately guide the construction and verify the reliability of the new method.