Abstract
Three-bench seven-step excavation method (TSEM) has been widely used in large-section loess tunnels for high-speed railway in China. As the most commonly applied pre-supports, pipe roofs and leading ductules are broadly used in the ground reinforcement of loess tunnels. Their application is to ensure face stability and prevent tunnel collapse during construction. This study focused on the impacts of the TSEM on the ground surface settlement (GSS), as well as the tunnel displacement characteristics for the high-speed railway tunnels with large cross-sections in loess ground. Furthermore, the reinforcement effects of the two kinds of pre-supports were compared in this study. In-situ tests for a total of 12 sections were conducted to reveal the GSSs and displacement characteristics for the shallow-buried large-section loess tunnels. The monitoring results showed that the excavation process plays a significant role on the GSSs and tunnel displacements. A maximum value was observed for the tunnel displacement rate at the excavation of the upper and middle benches, where the face instability or collapse were prone to occur. The GSS trough curves were deviated to the early excavation side, with no conformation to the Gauss distribution. After a series of comparisons, we concluded that the pre-reinforcement effect of the pipe roof is better than that of the leading ductule for the loess tunnels.
Highlights
In China, a total area of circa 640,000 km2 is covered by loess, which is about 6.6% of the territory of the country
We focus on the effect of the three-bench seven-step excavation method (TSEM) and two types of pre-supports used in the tunnel construction
This paper comprehensively presents an analysis of tunnel displacements and ground surface settlements during the construction of a large-section loess tunnel
Summary
In China, a total area of circa 640,000 km is covered by loess, which is about 6.6% of the territory of the country. The current literature suggests that a considerable proportion of ground displacement caused by excavation occurs before the installation of the primary support system which could cause tunnel instability or even collapse [6,7,8,9]. Wu et al [17] provided the optimal construction parameters of the combined pre-support technique of the pipe roof and grouting reinforcement for the shallow buried loess tunnels by using Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D) software. We focus on the effect of the three-bench seven-step excavation method (TSEM) and two types of pre-supports (i.e., pipe roof and leading ductule) used in the tunnel construction. Through analyzing the GSS, the tunnel arch settlement and horizontal convergence, the performances of two kinds of pre-supports for the ground reinforcement are compared. The research may shed a light on the behaviour of pipe roofs and leading ductules and serve as a practical reference for similar projects
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