Mechanics Characteristic Research On Shallow-Buried Excavation Subway Tunnel With Large Cross-section Under Loads In Loess Area

In recent years，with the continuous development of Xi’an metro construction andsuperiority of shallow-buried excavation method，there will be increasing engineeringprojects of shallow-buried excavation subway with large cross-section in loess area.However，there is a world of difference in engineering characteristics between the subwaytunnels in loess area and those in general geological condition because of special physicaland mechanical properties of loess，and a further improvement and completion need to bedone for the design and construction of tunnels with large cross-section in loess area. On thebasis of the entity of Xi’an metro line3，a series of research on characteristics under loadsand stress characteristics of supporting structures therefore has been undertaken through themethods of theoretical analysis，field measurement and numerical simulation and so on. Themajor work has been summarized as follows：Firstly，widely investigation and collection of related reference has been carried out，and current fruit of research on tunnels in loess area and theories of surrounding rockpressure of tunnels have been got into summation and conclusion. Several methods havebeen applied into calculation of surrounding rock pressure in the paper based on subwayproject including rock pillars method，Protodyakonov’s mountain rock pressure theory，Terzaghi’s equations and specification of railway tunnel，etc.Secondly，comprehensive and systematic field tests has been conducted based on theXian-Chang section of Xi’an metro line3. A large number of raw experimental data hasbeen obtained from monitoring of stress state of supporting structures by laying outelements in surrounding wall and supporting system such as pressure cell，reinforcementmeter，surface strain meter，concrete strain meter and others.Then，according to the resultsof actual measurement and relevant analysis on surrounding rock pressure of tunnel andstress state of supporting structures，conclusions have been drawn as follows：contactpressure，on the whole，is high on the bottom of structure while low on the topï¼›lateralpressure is relatively large， and construction of dismantling temporary shoring andsecondary lining have more influence on pressure distribution. The axial force of grillearch，increasing rapidly in the initial stage，have an obvious supporting function when shotcrete has not reached its strength. It is significant that section steel with temporarycentral support is under stress，especially the right hole， which reveals that surroundingrock pressure is primary released by excavation of the right hole. Thus the support shouldbe closed in time when drift heading pilot tunnels are in stagger excavation. The proportionof loads resisted by primary support and secondary lining is35.32%å'Œ64.68%respectively，and secondary lining，as main carrying body，takes a majority of load. Thereis a big difference of internal forces in different position of concreteï¼›side wall and above itare under compression while inverted arch is under tension.Finally，simulation analysis has been conducted by applying finite element software inthis project. A comparison of theoretical calculation result and field test result has beenmade through the analysis on mechanical characteristics of supporting structures ofshallow-buried excavation subway tunnel with large cross-section in loess area fromtwo-dimensional model simulation on excavation and support of tunnel.The research results can provide the theoretical foundation and guidance for the designand construction of shallow-buried excavation subway tunnel with large cross-section inloess area and those similar underground works.