| At present,the construction of subway tunnels using the shield method is becoming more and more normal.During the construction of the new tunnel,the shield crossing will cause the deformation and cracking of the existing tunnel segments,and there is a huge construction risk.In order to solve this problem,this paper first analyzes the engineering characteristics and interaction mechanism of shield tunneling through existing tunnels,considering the advantages and disadvantages of modified inertial usage and beam-spring model,and proposes an existing tunnel segment for the upper span of the shield in the composite rock and soil layer.The load system and calculation method of the composite rock and soil layer are then used to establish a three-dimensional numerical model to analyze the influence of different crossing angles and clear distances on the deformation of the existing tunnel.Finally,according to the stress and deformation characteristics of the existing tunnel on the composite rock and soil layer proposed targeted protection and control methods,the main research results are as follows:(1)After analyzing the advantages and disadvantages of modified idioms and beam-spring models,this paper proposes a segment load system and calculation method for shield tunnels in composite rock and soil layers that pass through existing tunnels.The modified idiom only considers the resistance of the strata on both sides.After the shield passes through,the vertical displacement trend of the existing tunnel is obvious.The passive resistance of the vault and the bottom of the vault caused by the displacement of the shield tunnel segments cannot be ignored.The beam-spring model exaggerates the active earth pressure at the bottom of the shield tunnel.It does not take into account that the lower rock layer has a weak restraint on the tangential deformation of the shield tunnel segments,and the tangential spring is set unreasonably.The calculated deformation at the bottom of the arch With the bending moment greater than the new method,the calculated axial force at the bottom of the arch is smaller than the new method.The new method is more in line with the force of the segment of the shield tunnel in the composite rock and soil layer that passes through the existing tunnel,and can provide a reference for similar projects.(2)Use MIDAS GTS NX finite element numerical calculation software to establish a three-dimensional finite element model to study the change of the maximum lateral convergence of existing tunnels with a clear distance of 3.0m,4.0m,5.0m and 6.0m under different crossing angles(30°,60° and 90°).The results show that under the same conditions of the crossing angle,as the crossing distance increases,the maximum value of the lateral convergence of the existing tunnel gradually decreases.Therefore,special attention should be paid to existing tunnels in composite rock and soil layers when the crossing distance is less than 4.0m,and prevention should be strengthened.(3)When the crossing angle is 30°,the overlap area between the new tunnel and the existing tunnel is the largest,and the unloading effect of the new tunnel excavation has the greatest impact.At this time,the longitudinal deformation curve of the existing tunnel has a larger offset value.When the crossing angle increases,the overlapping area of the new tunnel and the existing tunnel decreases,and the offset value of the longitudinal deformation curve of the existing tunnel decreases accordingly.In addition,when the composite rock-soil shield passes through an existing tunnel,because the arch bottom of the existing tunnel is in the lower rock layer,the rock bed coefficient is large,which can provide relatively large passive resistance,which is partially offset by the unloading effect of the new tunnel excavation.The lateral deformation of the arch bottom lining is small,which is in a safe range;the arch top is in the upper soil layer and is closer to the new tunnel.The excavation disturbance of the shield machine is greater,and the lateral convergence deformation is greater.In general,it presents the deformation characteristics of "large upper part and small lower part".According to the deformation characteristics of the existing tunnel when the composite rock-soil layer traverses the shield,three protection methods are proposed:steel ring reinforcement method,MJS method,and new tunnel weighting.The steel ring reinforcement method can effectively improve the rigidity and ultimate bearing capacity of the lining structure of the vault of the existing tunnel.The MJS method significantly improves the construction ability in all aspects,and effectively controls the impact range of the shield disturbance.The heaviness of the newly built tunnel can effectively reduce the uplift trend of the existing tunnel.The research results of this paper can provide technical reference for similar projects. |
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