Key Takeaways & Executive Findings
- •• Setting a transition section 6 m before a sudden rock mass change effectively controls deformation in layered strata. • Advance small pipe support significantly reduces tunnel arch settlement, while anchor bolts control horizontal convergence. • Adjusting anchor bolt angles is a cost-effective measure for asymmetric deformation in layered rock. • Recommended support optimization: flexible anchor bolt angles and increased advance small pipe support for rock stratum transitions.
Abstract
Layered rock masses represent complex geological formations characterized by pronounced anisotropy in strength. This study monitors stress/deformation during construction to summarize layered rock mass deformation and support stress characteristics based on Yunwushan Tunnel. Shale shows greater vault settlement and asymmetric support deformation than sandstone. The excavation was optimized by establishing a numerical model, analyzing the advanced support effect, and redesigning the anchor rod to control the asymmetric large deformation. The results show that: 1) It is effective to set a transition section before the sudden change of rock mass, and the optimal distance for setting the transition section is 6 m. 2) The implementation of advance small pipe support has been shown to effectively mitigate settlement in the tunnel arch, whereas anchor bolt support is effective in controlling the horizontal convergence of the surrounding rock. 3) Adjusting the angle of the anchor bolt is a cost-effective reinforcement method when facing asymmetric deformation. 4) It is recommended to flexibly adjust the angle of the anchor bolts and increase the advance small pipe support in mountain tunnel projects under the transformation of rock strata. These outcomes may serve as a valuable reference for the design and construction of similar engineering projects.
1. Introduction
Layered rock mass is a complex geological structure in underground cavern engineering [1−3]. The different failure modes of layered surrounding rocks are caused by differences in the physical and mechanical properties of the surrounding rock and the occurrence environment [4, 5]. Many scholars have studied the failure mechanism of layered surrounding rock through on-site monitoring [6], indoor experiments [7, 8], theoretical analysis [9], numerical simulation [10, 11], and other methods. The mode of failure is contingent upon several factors, including ground stress [12], the mechanical and physical attributes of the surrounding rock, the geometry of the rock layer [13], and the size and excavation technique of the tunnel. Large deformations, such as compression deformation, asymmetric deformation, and bottom plate uplift, are progressively revealed with the excavation of tunnels in stratified surrounding rocks [14−16]. Determining the mechanism of large deformations in layered surrounding rocks is key to implementing reasonable prevention and response measures.
The deformation of the surrounding rock affects the stability of tunnels. Currently, several countermeasures are available for large deformations of surrounding rocks, including strong support, yielding support, and advanced support. However, its high support stiffness inhibits the continuous development of tunnel deformation and failure [17], and the secondary stress redistribution process of the surrounding rock after tunnel excavation is prone to various large deformation accidents, yielding support that actively releases the stress of the surrounding rock has attracted attention [18, 19]. For fractured surrounding rock, advanced support can reinforce the surrounding rock in advance, which is beneficial for the stability of the surrounding rock during the period after tunnel excavation and before initial support, so that the surrounding rock will not be destabilized [20−22]. The selection of support methods is crucial for the safe construction of tunnels, asymmetric mechanical and large deformation caused by layered structure have long been recognized, and control measures have been extensively studied [23, 24]. Previous research has focused more on the deformation failure mechanism and stability of surrounding rock, and there is little research on the treatment measures of large deformation when the rock mass quality of the tunnel changes abruptly, and the support schemes need to be further improved.
Based on the Yunwushan Tunnel project of Chongqing Rail Transit Bi-tong Line, this study examines the optimization of excavation and support measures when the rock stratum changed from sandstone to shale. The deformation characteristics of the tunnel surrounding rock and the stress law of the support structure under different rock strata were summarized through on-site monitoring. FLAC3D was used to investigate the control effect of advanced small pipes and bolt support on...
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JIANG Hua, HU Hao, SHEN Qing-yun, ZHANG Ao (2026). Conversion of tunnel excavation methods and optimization of support measures under transformation of rock stratum: A case study. Journal of Central South University. https://doi.org/10.1007/s11771-026-6234-1
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Frequently Asked Questions
What is the optimal distance for setting a transition section in layered rock tunnels?
The study found that setting a transition section 6 m before the sudden change of rock mass is effective in controlling deformation.
Which support measure is most effective for controlling tunnel arch settlement?
Advance small pipe support is shown to effectively mitigate settlement in the tunnel arch.
How can asymmetric deformation in layered rock be controlled cost-effectively?
Adjusting the angle of the anchor bolt is a cost-effective reinforcement method for asymmetric deformation.
What are the recommended support measures for mountain tunnels under rock stratum transformation?
It is recommended to flexibly adjust the angle of anchor bolts and increase advance small pipe support.
What numerical method was used in this study?
FLAC3D was used to investigate the control effect of advanced small pipes and bolt support.
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