Mechanics Characteristic Research On Structure System Of Shallow-Buried Undercut Subway Tunnel In Loess Area

With the operation of metro line2in Xi’an, loess region come into a period of extensive use ofunderground space. The first time in the loess region subway construction and operation faces anumber of outstanding issues, especially in how to share the load in each part of the lining, how isthe effect that system anchor plays in shallow tunnels,the influence to the support system duringsmall space construction by shallow-buried undercut and the influence to the structure bydeterioration of rock conditions during subway operation. Such issues have respect to the tunnellink-up successfully and timely, as well as long-term safe operation.Therefore, paper chose intervaltunnel of Xi’an metro line2as study object, through systematic and comprehensive in-situ testingand theoretical analysis, model tests based on similarity theory, numerical simulation based on thefinite element theory, we have studied and summarized the issues above.Selecting typical testing cross-section in different section, we have monitored thesupporting structure’s strained-condition by testing element laid in structure and rock. Based onthe testing results we have analyzed the mechanics characteristic of early support and secondarylining in loess shallow subway tunnel, and determine he load ratio of various parts in bracingsystem. When Loess intensity is high, early support and secondary lining of the load ratio isabout1:0.9, when the intensity is low, the load ratio is up to1:1.23, the second lining notonly assumed by the early supporting delivery of the rock pressure, but also bear the earlysupport of the weight. So in leoss subway tunnel early support just as a reserve safety in theconstruction process, secondary lining should be used as the main load-bearing structure; it playsmore important with decrease of soil strength in supporting system.In the three-dimensional geo-mechanical test platform, for the loess rock features, wehave configured the testing materials including rock material, lining material and anchormaterial which can meet the test of loess subway tunnel. By drawing up11groups of testprogram in different rock moisture contents, different anchor locations and different anchorlengths, using FBG element to measure the stress of anchor and structure, we have carried outsystemic model tests about loess shallow subway tunnel. The results combined with fieldmeasurement and numerical simulation indicates that anchor system laid between spandrel and sidewall can provide effective long-term security reserve, delay the time of destructionduring the lining crack, they can also improve the construction safety in loess shallowsubway tunnel. Can be expected when the moment that rock produces ductile destruction,sprayed concrete grille is failure, the effect of anchor system will be more prominent.Through large-scale numerical simulation, building a three-dimensional numericalmodel, we have studied the effects of construction space of double-hole loess shallow subwaytunnel, then revealed the influence range between surface, stratum and adjacent liningstructure in three-dimensional. The vertical extent of excavation at2times the diameterof about12m range, the horizontal width of the bottom surface subsidence funnel-shapedcurve,the bottom width of12m,10m ground settlement to become the standard W-curve,surface, andare within10m of strata deformation greater impact.And we have found thatduring construction, the surface and stratum settlement goes through three stages as slowchange, rapid change and increasingly stable，such achievements can provide a theoreticalbasis for a series of key cultural relics above or below ground and ancient and modernarchitecture protection in Xi’an.Using numerical simulation, field measurement and indoor model test, we have had acomprehensive evaluation about rock change characteristics and supporting structure’s safetyafter loess rock flooding, we find that The impact of flooding range is about0.5times thediameter, the rock strength lower and deformation increase among flooding range,the rockappear loose collapse, the rock’s self-supporting capacity is reduced,which resulting in theformation pressure borne by rock transferred to the lining. After soaking the maximum axialforce and the maximum bending moment of the lining increased7%and15%before floodingrespectively.So during the process of loess metro operations the changes of rock within0.5times the diameter around the cavern should be closely monitored.