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A better understanding of tunnel deformable supports: from analytical model to engineering application

Wu Kui, Wang Yuzhu, Zheng Xiaomeng, Zhao Nannan

Archives of Civil and Mechanical Engineering

2024

Vol. 24, no. 2

art. no. e114, 2024

The severe failure of strong supports occurred in the Xinhua tunnel that was a deep-buried tunnel excavated in squeezing ground. In order to address such problem, this study explores the possibility of applying deformable supports in this tunnel. The mechanical response of a circular “rockbolt and yielding lining” supported tunnel is studied from the perspective of the convergence-confinement method. The equations for calculating the elastic modulus, cohesion, and internal friction angle of bolted rock are provided firstly; the mechanical model of the bolted tunnel is established then, where the surrounding rock can be classified into the plastic bolted, elastic bolted, and elastic unbolted regions. The equations for constructing the ground reaction curve are provided considering rockbolt reinforcement and rock shear dilatancy. The reduced case that the surrounding rock does not generate the plastic region is discussed as well, if the support pressure is higher than the critical value. The required minimum support pressure is further determined following the assumptions of maximizing the utilization of rockbolt bearing capacity and generating no loosening rock pressure. The exact equations for determining the lining thickness and length of highly deformable elements are provided with the intention to ensure lining safety and accept rock displacement. Based on the consideration of shotcrete hardening property, the equation to calculate the yielding stress of highly deformable elements is provided. The equations for GRC in this study can be reduced to those without considering rockbolt reinforcement or rock shear dilatancy. The design model of yielding lining is well applied in the Xinhua tunnel. The analysis results show that the rock displaces for 382.6 mm and the lining generates a plastic displacement of 281.8 mm in the Xinhua tunnel using the strong supports, which have a good agreement with the field monitoring data. The required lining thickness is equal to 28 cm and the installation number and length of highly deformable elements are 9 and 41.3 cm, respectively, when the yielding lining is employed in the Xinhua tunnel. Finally, a parametric investigation is carried out, including the cohesion and internal friction angle of rock, rockbolt length, and initial ground stress. Some recommendations for the tunnel design are proposed.

A unified design model for estimating tunnel performance considering multiple excavation stoppages

Wu Kui, Xing Chenzhe, Yang Yuezong, Shao Zhushan, Zhao Nannan, Chu Zhaofei

Archives of Civil and Mechanical Engineering

2023

Vol. 23, no. 4

art. e260, 1--14

In tunnel construction, excavation stoppages are often encountered due to many irresistible factors. This study reveals the influence mechanism of excavation stoppages on tunnel mechanical response by using mathematical analytical method. Firstly, the mathematical expressions for stress release coefficient considering multiple excavation stoppages are redefined and provided, based on the previous stress release coefficient in the negative exponential function form. Secondly, the mechanical model of a deep circular lined tunnel considering excavation stoppages is established, and the unified analytical solutions for tunnel displacement and liner pressure during the unlined and lined (excavation and excavation stoppage) stages are provided, respectively. Furthermore, the proposed solutions are able to capture the average tunnel deformation and liner pressure in Rongjiawan tunnel (an excavation stoppage took place) and agree well with the numerical results. Finally, a parametric investigation is performed, including the influences of starting time and duration time of excavation stoppage, excavation rate and liner installation time. Results show that both the final tunnel displacement and liner pressure are unaffected by starting time and duration time of excavation stoppage. However, their convergence rates are significantly affected by these two excavation stoppage factors. A higher excavation rate before stoppage causes a larger tunnel displacement and a smaller liner pressure. The influence of re-excavation rate is too low to be paid attention. A higher liner pressure can be induced by an earlier liner installation. The findings in this study are helpful for the evaluation of excavation stoppage-caused influence in tunnels.