Failure mechanism and treatment measures of supporting structures at portal for a shallow buried and asymmetrically loaded tunnel with small clear-distance
Authors: Chen Hao , Lai Hongpeng , Huang Man , Wang Gang , Tang Qiang 
Published: June 2022

Abstract
The construction of tunnel portal not only has to face the challenges of complex geological conditions such as shallow buried and asymmetrically loaded, but also needs to consider the adverse effects of factors such as the layout form of the tunnel. Thus, the construction of portal has always been the difficulty and focus of tunnel engineering construction. Under the coupling of multiple adverse factors such as complex geological conditions and special layout form, the tunnel portal is prone to excessive deformation, supporting structures cracking and even collapse during the excavation. In this study, a shallow buried and asymmetrically loaded tunnel with small clear-distance in northwest China was taken as an engineering case. Aiming at the distresses of slope instability, peeling off and block falling of primary support concrete and cracking of secondary lining concrete in the tunnel portal construction, combined with field investigation, statistical analysis, numerical simulation and deformation monitoring, the failure mechanism of supporting structures was deeply studied, and the corresponding treatment measures were proposed. The research results indicated that the loose and broken gravel soil in the shallow buried section, asymmetrical loading, surface water infiltration, and short construction spacing between two tunnels were the main triggers of supporting structures failure. Affected by the topographic bias, the loose load generated by the surrounding rock on the deep buried side squeezed the entire tunnel to the shallow buried side after the portal excavation. And this deformation trend became more significant after the gravel soil was deteriorated by water immersion. The retaining wall produced a clockwise rotation deformation around the wall corner in the process of limiting the tunnel deviation, and the local wall body cracked due to the excessive tensile stress. The primary support concrete and secondary lining concrete produced excessive asymmetrical deformation because of the significant asymmetrical loading. The concrete with excessive deformation was cracked by obvious tensile or shear stress. The following tunnel excavation had a significant negative impact on the stability of the prior tunnel. Combining the failure mechanism of the supporting structures and the characteristics of the continuous development of cracks, the treatment measures of ‘stabilize the stratum first and then treat the cracks’ were proposed, including the backfilling and tamping the shallow buried side at tunnel portal, reinforcing the interlaid rock by ground surface grouting, setting intercepting ditch at the slope top, staggering a certain safe excavation distance between the following tunnel and the prior tunnel. The field monitoring and patrol inspection results indicated that the proposed treatment measures had achieved the expected results. The research results can provide corresponding construction experience and suggestions for similar projects in the future.