Study of the effect of differential settlement due to different factors on the force characteristics of shield tunnels
Shield tunnels are susceptible to uneven settlement due to factors such as abrupt transitions between soft and hard underlying strata and localized overburden loading,which can lead to distress issues including misalignment between tunnel segments,bolt fractures,and water infiltration through joints,consequently imperiling tunnel safety.However,existing experimental studies often oversimplify longitudinal joints in tunnel models,making it difficult to capture the longitudinal deformation characteristics of shield tunnels.Additionally,there is a lack of simulation involving conditions such as abrupt variations between soft and hard underlying strata within tunnels.To this end,a novel'spring + bolt'joint that could simultaneously simulate tensile and flexural stiffness was proposed,and this joint was utilized to fabricate tunnel models that meet the similar longitudinal mechanical properties.The model test of the shield tunnel under the conditions of abrupt variations between soft and hard underlying strata and local stacking was then carried out to further explore the influence mechanism of differential settlement caused by different factors on the shield tunnel,and the feasibility of the test was verified through numerical simulations.The results indicate that:(1)compared with the uniform stratum,the longitudinal settlement curve of the tunnel exhibits significant nonlinearity when encountering abrupt transitions between soft and hard underlying strata.Both tunnel settlement and differential settlement experience notable increments,with more pronounced effects observed on the soft soil side compared to the hard soil side.Maximum settlement and settlement differential are concentrated at the interface of the soft soil lining within the abrupt transition zone.(2)The abrupt transition of underlying strata leads to an uneven longitudinal distribution of lateral convergence deformation within the tunnel,which reduces the convergence deformation of the lining on the soft soil side and significantly increases the convergence deformation of the lining on the hard soil side.(3)Localized overburden loading has a minor impact on the bending moment distribution of the segment but has a great influence on the maximum bending moment.(4)The internal force distribution of the tunnel structure within the underlying soft soil area is noticeably variable.The positive bending moment area of the tunnel lining increases significantly,while the negative bending moment area decreases slightly,and there is a marked elevation in the maximum bending moment value.Therefore,in view of the soft-to-hard abrupt strata,it is prudent to contemplate enhancing the lateral stiffness of tunnel linings on the side with hard soil through methods such as the application of bonded steel,aimed at managing their convergence deformation.Conversely,in view of tunnels situated on the side with soft soil,it is recommended to implement bolt retightening of tunnel segments and enhance joint waterproofing to mitigate the adverse effects stemming from the weakening of the underlying strata on the tunnel structure.
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