Abstract
The number of super-large-span tunnels is increasing in both new construction and reconstruction projects in China recently. In super-large-span tunneling engineering, the deformation properties and mechanical behaviors of tunnel portal structure are more complex than those of common tunnel due to the flatter shape and larger construction span. The mechanical behaviors of rock mass change in response to different sequential excavation methods and supporting parameters. The upper bench CD method has been gradually applied in the construction of super-large-span tunnels in China. In this paper, the design parameters for the supporting structure of super-large-span tunnel were studied by the field monitoring and numerical modeling in a case study of Laohushan tunnel. It was found that the crown settlement was larger than the clearance convergence, and the stress of arch was greater than that of the side wall in tunnel portal section. The invert structure was flat with small curvature. Therefore, the shotcrete was mainly subjected to tensile stress. The use of H200 × 200 steel rib with spacing of 60 cm and C25 shotcrete with thickness of 30 cm is recommended. The results of this paper provide basis for the development of design specifications and construction standards for super-large-span tunnels and provide reference for similar projects in the future.
Highlights
With the rapid development of national economy and the gradual growth of highway transportation volume, the existing single-hole two-lane tunnels are unable to meet the needs of transportation and travel in the economically developed areas and suburban megacities [1,2,3,4,5,6,7,8]
There is no system standard for the construction of superlarge-span highway tunnels, and the design principle of engineering analogy is still in use in the design of two-lane or three-lane tunnels. ere are many kinds of supporting forms of tunnel portal section, for example, primary supporting with two-layer secondary lining, two-layer primary supporting with high-strength secondary lining, and highstrength primary supporting with secondary lining
According to the “Design Rules for Highway Tunnels” [38], the safety factors of steel rib and shotcrete under the basic variable combination of QZH-II were 2.0, 3.6, and 2.4. e safety factor of the primary support structure of tunnel portal section was larger than the specified value. It showed that the design parameters could meet the structural safety. e results indicated that the secondary lining was less stressed and could be used as safety stock
Summary
With the rapid development of national economy and the gradual growth of highway transportation volume, the existing single-hole two-lane tunnels are unable to meet the needs of transportation and travel in the economically developed areas and suburban megacities [1,2,3,4,5,6,7,8]. A reasonable supporting method of four-lane highway tunnels was proposed by simulating the structural form and Advances in Civil Engineering supporting parameters of super-large-span tunnels [16,17,18,19]. The design parameters of super-large-span tunnel supporting structure were mainly studied for single-supporting parameter in a single-stratum environment through numerical simulation or model test. Li et al proposed to study the deformation of surrounding rock and the stress of supporting structure of the tunnel during construction with double-sided guide pit method and CD method through field tests, relying on the No 1 tunnel project of Qichong Village, Guiyang [39]. This paper uses the method of numerical simulation to analyze and calculate the primary supporting internal force and material safety factor to verify the safety and applicability of design support parameters in tunnel portal section. (a) Excavate the upper bench part 1 and timely construct structure II and the upper part of structure I (b) When part 1 is excavated 15–20 m ahead of part 2, excavate part 2 of the upper bench and construct the upper part of structure III in time (c) When part 2 is excavated 20–30 m ahead of part 3, excavate part 3 of the left and right lower guide pits and construct the lower part of structures I and III in time (d) Remove temporary support II, excavate part 4, timely construct support structure IV of invert, and fill lower part V of invert secondary lining (e) Construct superstructure V of secondary lining
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