Research On Mechanism And Control Of Existing Subway Tunnel Deformation Induced By Multi-crossing Of Shield Tunnel Underneath

In recent years,the urban rail transit develops rapidly in China.By the end of 2017,China had 136 subway lines and 3881.8km of subway lines.The mileage of subway lines was the highest in the world.Metro line networks are intensive and the operating mileage has ranked first in the world.Under this background,it’s inevitably for the construction of shield tunnelsto face the situation of tunneling through existing subways.In particular,with the increase in the number of tunnels near the comprehensive transportation hub,cases of double-line or even multi-line tunnels crossing existing operating tunnels have begun to appear.Multi-line crossing involves complicated cross-structure interactions,and the tunnel construction is repeatedly disturbed.It is easy to cause the existing operation tunnel to be deformed too much,and the existing subway operation has high requirements for the deformation of the track and the structure.A slight carelessness can cause structural damage,train derailment,and threaten people’s lives and property.In view of this,this article takes the third phase of the tunneling project of Shenzhen Metro Lines 7 and 9 as the background,focusing on more and more single-line and multi-line tunneling through existing tunnel projects in recent years.The effect of deformation and its control problem of existing tunnel induced by shield tunnel and tunnel tunneling is investigated.The main research work of the paper is as follows:(1)Based on the theory of ball contraction(expansion),combined with the virtual image technique,the formulae for the three-dimensional deformation of the formation caused by the loss of inhomogeneous stratum is deduced.A calculation method of three-dimensional deformation of the stratum caused by shield tunneling is established considering the load characteristics of shield tunnelling.The stress disturbance of the double-line and multi-tunnel tunnel excavation on the strata is analyzed,i.e.the memory of the soil.The additional stratum loss parameter is used to describe the weakening of the stratum parameters by the excavation of the pre-built tunnel,and the calculation method of ground displacement field caused by excavation of the shield multi-line is proposed.This method can reflect the spatial characteristics of shield excavation and the three-dimensional characteristics of formation deformation,which can be applied to the analysis of the influence of multi-tunnel construction on the formation deformation,and the conclusions obtained are more comprehensive and more in line with the actual site conditions.(2)Based on the multi-coupling analysis of "shield machine-soil-tunnel structure",the relationship matrix between natural formation and tunnel structure is deduced,the tunnel structure-soil interaction model and limit state are discussed,and the method for determining the tunnel structure and soil ultimate state is proposed.On this basis,the contact structure between the tunnel structure and the soil as well as the plastic contact state areintroduced and deformation analysis method for shield tunnel and tunnel group under existing tunnel considering the tunnel structure-discontinuous contact state of soil is established.This method avoids the defects of the existing deformation model of the elastic foundation beam,considers the spatial position change of the shield excavation and the influence of the construction load.At the same time,it improves the assumption that the soil and the tunnel structure always co-deform in the previous calculation method.(3)Based on the theoretical analysis method proposed in this paper,the influence of different parameters,such as stiffness ratio,crossing angle,and overburden thickness,on the deformation of the existing tunnel structure is studied.It has been found that the local void area is prone to occur when there is a large difference in the stiffness between the local layer and the structure,or when the thickness of the overburden is shallow.The void area is usually located directly above the midline of the newly built tunnel.At this time,the effect of discontinuous deformation in the void area should be considered.In addition,through the analysis of existing tunnels undercrossed by new twin tunnels,the basal stress zoning and bearing characteristics were ascertained,and the concept of the bearing pillars was proposed according to the stress distribution characteristics,which provided a theoretical basis for the reasonable setting of new tunnel spacing.(4)A simulation test device for tunnels and tunnels underneath existing tunnels was developed and a high-precision "drag-and-drop" method for automatic control of groundloss was proposed and adopted for the first time,which avoids the low precision control and difficult excavation speeds of conventional drainage methods.Through the tunnel and tunnel group under the existing tunnels,the centrifugal model test was conducted to test the contact state between the existing tunnel structure and the soil.The development process of the bottom void area of the tunnel structure was truly reproduced,and the impact of a single tunnel under the existing tunnel and mechanical response of surrounding rock was systematically analyzed.The influences of different new tunnel spacings and construction sequences on the deformation of existing tunnels are compared and analyzed.The reasonable tunneling sequence and distance setting of shield tunnels are discussed,and the optimal construction method for shield tunnels tunneling through existing tunnels is proposed.(5)Shield-based double-track and multi-line tunnels involved in the Phase III project of Shenzhen Metro have been excavationed under existing subway tunnels at close range.Through extensive field measurements and data analysis,the shield tunnelling process has been studied.The deformation and force changes of existing tunnels during the tunneling process of shields were studied.The settlement distribution rules of shield tunnels under double shields and multiple tunnels under the existing tunnels in Shenzhen area were summarized.(6)In view of the lack of a single control standard for the existing shield tunnel tunneling through multiple existing tunnels,a displacement distribution control method for shield tunnels crossing multiple existing tunnels was proposed.According to the construction process,the additional deformation control standard for tunnels was decomposed.During each shield tunneling construction,the single-tunnel crossing deformation control standard is formulated.As long as the deformation during each crossing is controlled,the additional deformation control of the entire structure can be achieved,making the construction of the shield tunnel group regulated.(7)Through extensive investigations of tunnel construction control cases at home and abroad,this paper discusses the deformation control measures and applicable conditions of existing tunnels from the perspectives of disturbance sources,propagation media,and protection objects.The deformation control effects of various typical protection measures were studied by means of numerical simulation.On this basis,various control measures were graded and evaluated,providing reference for making reasonable control measures during the process of shield crossing.Finally,based on the research results of this paper and previous engineering practice,we put forward the concept of "information is the premise,the assessment is the basis,the monitoring is the key,and the control is the core".This can provide guidance for the construction of single and multiple undercrossing projects.