Study On Construction Parameter Optimization Of Large Cross-section Loess Tunnel Cao-jiagou

The design, construction and operation of large cross-section of loess tunnel oftenencounter a number of difficulties and problems due to its large area and unfavorableconditions of surrounding rocks. The loess tunnels at the present day are often constructedwith traditional methods which are featured by complex procedures and slow progress.Whereas the three-bench seven-step excavation method can display its unique advantagesunder such geological conditions. But the study on the optimization of construction parametersis far from adequate, which becomes an important topic for study in the project of largecross-section loess tunnel.This thesis summarizes the properties of loess, deformation features, surrounding rockpressure and major disasters of loess tunnel. Apart from the study on the definition andconstruction methods of a large cross-section tunnel, it makes comparison of the traditionalconstruction methods of loess tunnel and proves the reliability of three-bench seven-stepexcavation method applied in Cao-jiagou Tunnel project. Then it focuses on the process andmain points of three-bench seven-step excavation method. Drawing the experience of the largecross-section loess tunnel of the ongoing Cao-jiagou railway project, it takes numericalsimulation analysis on the supporting effects of three-bench seven-step excavation method.Moreover, it optimizes the staggered distances of construction sites with three-benchseven-step excavation method. The main conclusions are as follows:(1) The subsidence of surrounding rocks of the tunnel can effectively controlled due to theshort benches and timely enclosure of timbering. The subsidence mainly occurs after theexcavation of the upper bench and middle bench. The unleashing speed of the surroundingrock pressure can be controlled by increasing the construction pace of the upper bench, thusimproving the construction safety. (2) The subsidence and deformation of stratum can be effectively controlled byaugmenting appropriately the staggered distances of the benches. The staggered constructiondistance between the upper bench and the middle bench, as well as the staggered distancebetween the two pilots of middle bench, can greatly influence the subsidence and deformationof the surrounding rocks.(3) The suggestion on the reasonable ranges of staggered distances is as follows: thestaggered distance between the upper bench and middle bench is4~6m; the staggereddistance between the two pilots of the middle bench is2.5~3.5m; the staggered distancebetween the two pilots of the lower bench is2.5~3.5m; the staggered distance between thelower bench and the inverted arch ranges6~8m.