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Numerical analysis on excavation scheme for four-lane large-span shallow-buried loess tunnel

ZAN Wenbo;LAI Jinxing;QIU Junling;ZHANG Yuwei;School of Highway,Chang'an University;  
To guarantee the construction safety of four-lane shallow-buried loess tunnel and optimize construction schemes,four methods commonly used in large-section loess tunnel,named as three-bench seven-step excavation method(TSEM),central diaphragm,cross diaphragm,and double side-drift method(DSDM),were systematically compared and analyzed using the numerical simulation method.Compared with the common two-lane road tunnel,crown settlement significantly increases,horizontal convergence is relatively small and the arch foot has the tendency to outward extrusion deformation,so crown settlement should be the focus of deformation monitoring.The stratum vertical displacement increases linearly with the increase of depth,but it shows exponential increases within 4maway from the vault,which is closely related to the formation of plastic zone around the hole.Surrounding rock stress does not exceed the ultimate strength,stress of primary support is relatively large and it bears the most loadings coupled with surrounding rock,and loosening and excessive rock deformation must be controlled.Stress of secondary lining is relatively small and it provides safety storage.DSDM is recommended for the strict deformation requirement,giving priority to TSEM for allowing the large deformation.Conclusions can provide theoretical basis for the design and construction methods of loess tunnel.
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