Design method of high prestressed support for shallow-buried large-span caverns

Abstract

The surrounding rock is prone to large-scale loosening and failure after the excavation of shallow large-span caverns because of the thin overlying strata and large cross-section span. The rational design of bolt support is very important to the safety control of surrounding rock as a common support means. The control mechanism and design method of bolt support for shallow-buried large-span caverns is carried out. The calculation method of bolt prestress and length based on arched failure and collapsed failure mode is established. The influence mechanism of different influencing factors on the bolt prestress and length is clarified. At the same time, the constant resistance energy-absorbing bolt with high strength and high toughness is developed, and the comparative test of mechanical properties is carried out. On this basis, the design method of high prestressed bolt support for shallow-buried large-span caverns is put forward, and the field test is carried out in Qingdao metro station in China. The monitoring results show that the maximum roof settlement is 6.8 mm after the new design method is adopted, and the effective control of the shallow-buried large-span caverns is realized. The research results can provide theoretical and technical support for the safety control of shallow-buried large-span caverns.

1. Introduction

The problem of traffic congestion is serious increasingly as the urban population increases [1 − 3]. In order to ease traffic jam and fully utilize urban underground space resources, large-span and large-section caverns are being designed and constructed [4−7].

Urban underground transportation caverns are generally buried shallowly, with the thin overlying strata and large cross-section span, the roof is prone to large-scale loosening and fracture after excavation [8−10]. If the surrounding rock support is not properly controlled, it will lead to accidents such as roof collapse and partial roof fall [11−14], so the stability of caverns will be seriously affected. Due to the strict settlement requirements of surface buildings, shallow-buried large-span caverns in urban underground traffic are faced with high requirements and difficulties in control. Many scholars have carried out a lot of research on the failure mode and stability control of shallow buried caverns.

In terms of theoretical research, ZHANG et al [15] analyzed the failure phenomena and rules during the excavation of shallow buried cavern, and revealed the characteristics of collapse failure of surrounding rock. YANG et al [16] established rigid body translational failure modes of shallow-buried cavern surrounding rock, and analyzed the rock pressure under the two kinds of failure modes. LIU et al [17] established a mechanical calculation model of load-bearing arch according to the characteristics of large-span cavern arched failure, and analyzed the influence mechanism of the thickness load-bearing arch. LI et al [18] analyzed the influence mechanism of large-span cavern under the condition of considering water pressure. LEI et al [19] established a failure mode for excavation asymmetric tunnels and derived an upper bound solution. Through the method of theoretical calculation, the above scholars have conducted further discussion on the failure characteristics and stress evolution law in shallow-buried large-span caverns under different conditions, but there are few theoretical studies on the prestressed support of caverns.

In terms of numerical simulation, JIANG et al [20] explored the influence of whether the bolt is prestressed on the reinforcement effect of tunnel surrounding rock. The results show that the failure occurs on the surface of surrounding rock without prestress. The failure occurs at the far end of the bolt after prestressing, whi