Study of Deformation Behaviors and Mechanical Properties of Central Diaphragm in a Large‐Span Loess Tunnel by the Upper Bench CD Method

Abstract

The central diaphragm is often used to reduce the span to maintain the stability and safety of structure in the construction of large-span loess tunnel due to the structure complexities. In this paper, relying on the field monitoring and measurement for Wangcun tunnel in Huangling-Yan’an expressway expansion project, the crown settlement and horizontal convergence of primary support steel rib and central diaphragm steel rib during the construction are analyzed by the upper bench CD method. According to the internal force transfer, deformation coordination, and arch foot displacement between the two structures, the support system is regarded as the arch-beam fixed structure with three times of statically indeterminate and movable abutment under the loads, and the mechanical calculation model of sidewall steel rib and the central diaphragm structure bearing loads and deformation together is established. Finally, through the mechanical model mentioned above, the deformation characteristics of central diaphragm structure and the horizontal convergence in the upper bench of tunnel are calculated and analyzed. The research shows the following: (1) the accumulated settlement of sidewall steel rib in Part I is greater than that of the sidewall steel rib in Part II, and the accumulated settlement of each part at the support structure during the tunnel excavation is less than the reserved deformation of 150 mm specified in the tunnel excavation; (2) the settlement located at the waist and maximum excavation line position of central diaphragm is mainly affected by the excavation of Parts I and II in upper bench; (3) during the whole excavation process, the excavation of Part I and Part II has the greatest influence on the convergence at arch waist and the maximum excavation line position in Part I, and the convergence at the above two positions all experienced four stages of “convergence-expansion-convergence-gradual stability”; and (4) the errors between the horizontal convergence and the deformation of central diaphragm obtained by the mechanical model and the field monitoring data are between 12.7% and 27.5%. The calculated results are in good agreement with the actual situation. The research can provide a theoretical basis for the study of deformation and mechanical properties for support structure in the construction of large-span loess tunnel by the upper bench CD method.

Analysis for bearing performance and construction mechanical behavior of supporting structure for the large cross-section tunnel by half bench CD method

“…steel frame under the unit horizontal force; β 31 , β 32 , μ 31 , μ 32 , ν 31 and ν 32 are the rotation angle, horizontal displacement and vertical displacement of the arch foot section of the primary support and middle wall steel frame under the unit vertical force; β p1 , β p2 , μ p1 , μ p2 , ν p1 and ν p2 are the rotation angle, horizontal displacement and vertical displacement of the arch foot section of the primary support and middle wall steel frame under the surrounding rock load. It can be calculated according to the following Eqs 3 and 4 [16].…”

“…The new construction method not only simplifies the construction steps, but also effectively ensures the safety and stability of the tunnel structure in the construction process, and realizes the synchronous improvement of the safety and economy of tunnel engineering. With the gradual improvement of structural safety requirements in tunnel engineering practice, the mechanical response and deformation behavior of support structure in the excavation process are particularly important for the safety and stability of the whole structure, and gradually become the engineering difficulties in tunnel construction and the research focus of many experts and scholars [13][14][15][16].…”

Mechanical Characteristic and Length Optimization of System Anchor in Loess Tunnel Based on Field Measurement and Analytical Solution

“…e loess is mainly distributed in the provinces of Shanxi, Shaanxi, Gansu, Qinghai, Ningxia, and the northwest of Henan. With the implementation of China's Western Development Strategy and the Belt and the Road Strategic Plan, more and more high-speed railways and expressways have been or will be built in loess areas to stimulate economic potential and meet the needs of rapid increase of traffic volume [1][2][3].…”