Mechanical Behaviours Of Existing Tunnels Due To New Tunnels Construction Below

Recently,the urban rail transport develops rapidly in China.By the end of Dec 2019,the amount of subway station has been 3394 and the total mileage of rail transport has been 5590 km.The urban rail transport network not only constructs densely,but also develops to the deeper underground space.In this situation,it is inevitable for the new tunnel to excavate below the existing constructions.Due to the complex geological condition and extremely sensitive environment,it is difficult to control the deformation of existing tunnels.Once an accident occurs,the people's lives,property and social economy will be threatened seriously.Therefore,how to ensure the structural safety and operational safety has become a research hit.This paper analyses the mechanical behaviors of existing tunnels based on the cases of new tunnel construction below in Beijing.The key findings of this paper are as follows:(1)According to the 26 projects in Beijing,the basic characteristics of the engineering,in which a new tunnel excavates below an existing tunnel,statistically analyze.The deformation rules-two categories,two properties,three modes and six spatial locations-are summarized.The engineering projects can contain two categories: the construction below without clearance and with clearance;the deformation of existing tunnels can mainly divide into two properties: rigid and flexible;regardless of the property,the existing tunnel has three deformation modes: deflection,rotation and dislocation;due to the construction joints,the new tunnel and existing tunnel have six different spatial locations.The summaries proposed by this paper cover all the characteristics,which can provide a basis for the further research.(2)According to the centrifuge tests,the author independently creates a "push" device to simulate the tunnel excavation and to analyse the mechanical behaviours of existing tunnel by shallow mining method and shield method.Compared with the traditional method,the device can accurately control the volume loss rate and the excavation schedule.The summary of Section 1 is verified by the centrifuge tests.At the same time,the author found that the mechanical behaviours of existing tunnel are affected by the construction disturbance of interlayer soil below the existing tunnel;the construction joint or bolt joint on the existing tunnel;and the construction schedule of the new tunnel.(3)Based on the superposition method and the theory of elastic foundation beam,an analytical solution on the mechanical responses of the existing tunnel due to new tunnel construction below is derived.The parametric analysis is carried out.Compared with the traditional method,the proposed method can consider the fact that the bearing capacity of the interlayer soil is reduced due to the construction disturbance.The soil under the existing tunnel should be void,or the coefficient of subgrade reaction should be nonlinear.The proposed method can predict the deformation of existing tunnel more accurately.(4)The author uses the second derivative of ? function,the Timoshenko beam and two parameter Pasternak foundation model to simulate the longitudinal joints,the existing tunnel and the soil-tunnel interaction,respectively.An analytical solution is proposed to calculate the discontinuous deformation of the existing shield tunnel considering the longitudinal joints.In this method,the bending stiffness of the segment and the longitudinal joints can be utilized directly instead of obtaining the equivalent bending stiffness of the existing tunnel.This method not only provides a theoretical basis for predicting the discontinuous deformation of the existing tunnel,but also makes the deformation control standard more comprehensive.(5)Based on the three-dimension elastic model,an analytical solution on the soil deformation is proposed under four stress modes.The influence of the new tunnel construction schedule on the deformation of the existing tunnel is considered.For the first time,the author provides a longitudinal deformation curve of existing tunnel,a grouting reinforcement characteristic curve and a grouting uplift characteristic curve.Then the space-time characteristic curves of grouting reinforcement-grouting uplift-existing tunnel deformation are given.According to the space-time characteristic curves of this paper,the reasonable time and scope of grouting reinforcement and grouting lifting measures can be assured after the existing tunnel deformation control standard is defined.It can guide designers and constructors to work out safe and economic construction plans,and provide theoretical support for related research in the future.(6)According to three projects of different cross-sections of new tunnels in Beijing,it finds that the deformation of existing tunnel has four stages: advanced deformation,aggravated deformation,slow deformation and stable deformation.According to different control measures and construction methods,the first two stages can also have different rules.Especially,the influence of advanced deformation stage on the existing tunnel should not be ignored.According to the research results,the new tunnel construction should follow the principle: advanced forepoling,careful grouting,multiple excavation,strengthened support,frequent measurement and early closure.