It has been a focus of debate for a large time on construction methods for large-span loess tunnel.Reasonable construction method has much effect on stability of tunnel and construction schedule.Deformation and failure of surrounding rock are quite complex.Associating with the large-span loess tunnel of Zhengzhou- Xi'an high-speed passenger rail line in China,large scale model test with geometric proportion 1:20 is applied to study on dynamic mechanical behavior of various construction methods.They include full-face excavation with support and no support,and benching method with support.It is found that pre-deformation and stress accumulation take place ahead of working face.The effects of three construction methods are further studied,particularly in terms of tunnel displacement and stress changes.It is revealed that benching method transfers load to an unexcavated area,limits horizontal deformation,reduces stress concentration effectively,lengthens the distance between location of peak for stress concentration and working face,and consequently increases stability.The model test results not only supply theoretical foundation for determination of reasonable construction method,but also can act as reference for similar tunnel and underground engineering construction.
A section of the Nanliang high speed railway tunnel on Shijiazhuang-Taiyuan high-speed passenger railway line in China was instrumented and studied for its mechanical properties and performances. The cross section for the tunnel was300 m2and is classified as the largest cross section for railway tunnels in China. Through in situ experimental studies, mechanistic properties of the tunnel were identified, including the surrounding rock pressure, convergences along tunnel perimeter and safety of primary support and lining structure.Based on the field measured data, the surrounding rock pressure demand for large-span deep tunnel in hard rock is recommended as double peak type in the vertical direction and fold line type was recommended for horizontal pressure. The results suggested that Promojiyfakonov's theory was most close to the monitored value. Specific recommendations were also generated for the use of bolts in tunnel structures.Numerical simulation was used to evaluate the safety of the tunnel and it confirmed that the current design can satisfy the requirement of the current code.
HE Ben-guoZHU Yong-quanSUN Ming-leiLIU Hong-yanZHANG Zhi-qiang
