| Increasing attention has been paid to the deformation of existing tunnels due to excavation of new tunnel.The new tunnels will inevitably affect the existing tunnels with more and more new tunnels excavation in the city.The protection standard of existing tunnels are harsh,the new tunnels excavation cause the disturbance of surrounding soil to produce free soil displacement,and the displacement of free soil causes the additional internal force and deformation of existing tunnels,which threatens the safety of them.Therefore,there is great significance to analysis the mechanism of new tunnel action on existing tunnel.In theory,a Double Pasternak elastic foundation model is proposed considering the soil-arch effect on existing tunnel,and the soil-arch effect is characterized by the unloading coefficient ?.It is assumed that the pressure of overburden on existing tunnel conforms to the Pasternak elastic foundation model.The theoretical solution of longitudinal deformation of existing tunnel was derived by two-stage method,then the displacement,shear force and bending moment of the existing tunnel longitudinal deformation were obtained through program.The different calculation models for deep and shallow buried tunnels are defined,which is the shallow tunnel adopts the Pasternak elastic foundation beam model,while the deep tunnel uses the Double Pasternak elastic foundation beam model.And the effects of the buried depth,soil loss rate,foundation modulus and shear modulus on the existing tunnels were analyzed.In terms of numerical simulation,the finite element was adopted to carry out 3D elastoplastic numerical simulation of the construction process of new tunnel in the background of two undercrossing engineering,and the variation law of vertical displacement,stress,bending moment and axial force of existing tunnel with increasing excavation step was analyzed.The theory is verified by comparing the finite element results,field measured values and theoretical values.In model test,the existing tunnel is simulated by rubber pipe by comparing steel pipe,hard PVC pipe,soft PVC pipe and rubber pipe,and the elastic modulus of rubber pipe is solved by the proposed unidirectional tensile test scheme.Barite powder,quartz sand and vaseline according to certain quality were used to simulate Ⅳ class surrounding rock,then thickness span ratio(both tunnels of layer thickness and the tunnel span ratio)0.5,1.0,1.5,2.0,2.5,diameters 39 mm,32mm,25 mm,wear under the new tunnel excavation influence on existing tunnel is studied.An experimental device was designed to study the influence of overlying soil layer on existing tunnels based on digital image correlation technology and laser ranging technology,and black paint were applied to tunnel section to precast artificial speckle,then using a CCD camera,image acquisition card,computer,video lamp and the corresponding image storage medium to constitute the DIC test system.Hard PVC pipe was extracted to simulate the excavation of a new tunnel.The influence law of new tunnel construction on overlying soil layer was obtained by studying the change of displacement field before and after tunnel failure.Another experimental device was designed to study the vertical displacement of the ground caused by the construction of new tunnel based on the laser ranging technique,the vertical displacement measurement system is composed of laser displacement sensor,dynamic signal test analysis system and computer,which can monitor the surface settlement caused by the excavation of new tunnel in real time,and study the change rule of overlying soil layer through the vertical displacement of the surface. |
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