Research On Deformation Prediction Method For Underpass Existing Tunnels Considering Reduction Of Subgrade Coefficient

In recent years,the utilization rate of shallow underground space in many cities has continued to improve.In order to build a more comprehensive underground rail transit network system,many subway lines are built in deeper underground spaces with greater burial depth.As a result,there will be more projects where new subway lines intersect with existing subway lines at the same level and intersect in space.The construction of new tunnels in the above projects has always been a hot issue in the underpass engineering for the structural stability and safe operation of existing tunnels.The thesis relies on the scientific and technological research project "Research on the Deformation Mechanism of Existing Structures in Urban Underpass Engineering"(project number: QN2021130)and the national key research and development plan "Construction Technology for Networked and Safe Expansion of Urban Underground Space"(project number: 2018YFC0808703)of Hebei Province’s higher education institutions.In response to the deformation prediction problem of existing tunnels in the construction of new shield tunneling projects,numerical simulation and theoretical analysis methods are comprehensively adopted,The influence range of new tunnel construction on existing tunnels during the underpass process was studied.Based on the elastic foundation beam method,the release law of interlayer soil stress was considered.By correcting the spring support stiffness of the lower soil layer of the existing tunnel,a theoretical prediction method for vertical deformation of underpass shield tunnels was proposed.The rationality of the proposed method was verified through numerical examples and engineering cases.The main research work and conclusions of the thesis are as follows:(1)Based on the theory of elastic foundation beam,a deformation prediction formula for existing tunnels was derived and an optimization calculation method was proposed for the parameters in the formula.The vertical surrounding rock pressure above the existing tunnel was determined based on the burial depth of the existing tunnel;The relationship between the stress release rate of the formation and the foundation coefficient was established,and the stress release rate was determined based on different formation parameters,thereby determining the reduction rate of the initial foundation coefficient after penetration.(2)Simulated the construction process of new tunnel underpass,analyzed the influence of multiple strata and structural parameters on the displacement curve of existing tunnels and the distribution of interlayer soil stress and plastic zone,and obtained the limited deformation range of existing tunnels and strata affected by underpass construction,as well as the fitting relationship with strata and structural parameters.Research has shown that the deformation range of existing tunnels is mainly related to the internal friction angle of the strata φ,cohesive force c is related to c,φ The deformation range of the existing tunnel gradually decreases with the increase of;The elastic modulus of the stratum and the stiffness of the tunnel segment have relatively little impact on the deformation range of the existing tunnel.The main factor affecting the distribution range of plastic zone in interlayer soil is also the internal friction angle φ and the cohesion c,while the stiffness of the new and old tunnel segments has no effect on the distribution of the plastic zone in the interlayer soil.Simulated the impact analysis of 0.3D～9D(D is the tunnel diameter)underpass distance on the deformation of existing tunnels.Research has shown that,while ensuring the normal use of new tunnels,the settlement deformation of existing tunnels shows a pattern of first increasing and then decreasing with the increase of underpass distance.(3)Comparing the stress release of the strata before and after underpass,it can be concluded that the larger the underpass distance,the less stress release of the interlayer soil,such as cohesion,internal friction angle,and stiffness of the newly constructed tunnel;The greater the stiffness of the stratum,the depth of the existing tunnel,and the stiffness of the existing tunnel,the more stress is released in the interlayer soil.A fitting relationship between the stratum,structural parameters,and stress release rate has been established.On a horizontal plane at the same geological depth,the closer the new tunnel is,the more stress is released,and the greater the deformation of the existing tunnel above it.The deformation value and stress release rate of the existing tunnel with a penetration distance of 9D are the smallest,and the impact of penetration construction on the disturbance of the existing tunnel is the smallest.(4)The basic validation of the theoretical prediction formula was obtained by substituting the model operating conditions with different distances of penetration into the theoretical method.Based on the double track shield tunnel underpass project,the underpass construction process was simulated.Analysis found that the displacement of the inverted arch bottom of the existing tunnel is slightly greater than the displacement of the track slab.The existence of the existing tunnel will not only reduce the upper surface settlement caused by the excavation of the new tunnel,but also reduce the longitudinal range of the upper surface settlement.The theoretical calculation results of the prediction method were obtained based on actual engineering parameters and compared with numerical simulation results,further enhancing the correctness of the theoretical method.Finally,based on the actual reinforcement measures of the project,a comprehensive displacement control measure was summarized.