Safety assessment of unenclosed support in high geostress soft rock tunnels with bench method

Abstract

Ensuring the safe construction of tunnels in high geostress soft rock conditions is crucial and challenging. During the bench excavation, the unenclosed support with low load-bearing capacity may lead to failures. Traditional assessment methods often overlook this risk. This study presents a novel approach to assess the safety of unenclosed support during bench excavation, based on a comprehensive mechanical model. Load strength ratio and safety factor are defined to evaluate the local and overall structural safety. Through parameter analysis, the effects of support stiffness and closure time on the structural failure mechanism are clarified. Within the allowable deformation range, appropriately adjusting the support stiffness and closure time can effectively enhance structural safety. The proposed method is validated through its application in the Yuntunbao Tunnel project, providing valuable insights for the design and safety assessment of other tunnel projects.