Analysis of the Collapse Mechanism and Stabilization Optimization of the Composite Stratum at the Boundary between Prereinforced and Unreinforced Areas near a Shield Launching Area

Abstract Grouting reinforcement methods are usually employed in shield launching sections due to shallow burial depths and poor stratigraphic conditions. Hence, grouting reinforcement provides an increase in buried depth and better formation conditions. The length of prereinforcement is often determined by engineering experience. Excessive ground settlement, collapse, and even buried machine accidents may occur when shield tunneling occurs at the boundary between prereinforced and unreinforced areas. The case of the collapse of the composite stratum at the boundary between prereinforced and unreinforced areas near the shield launching area of the Shumuling shield tunnel in Changsha was analyzed in combination with monitoring data around the collapse area. Based on the slip-line theory and limit analysis method, the evolution pattern instability of the tunnel face soils around the collapse area was analyzed by means of FLAC3D5.0. This model provides a failure mechanism used to analyze the stability of the tunnel face of a shallow circular shield tunneling through the composite stratum at the boundary between prereinforced and unreinforced areas. The limit-supporting force that maintains the stability of the tunnel face was calculated. The rationale behind the proposed failure mechanism was validated by comparing the calculated value of the limit-supporting force with the monitoring value in the chamber of the earth press balance shield machine (EPB). Based on the proposed failure mechanism and considering the influence of the surrounding environment (nearby railroad), a reasonable reinforcement method was selected, and the design reinforcement scheme for the soil in the collapse area was optimized. The research results are expected to provide useful guidance for the stability analysis and safe construction of shield excavation at the boundary between prereinforced and unreinforced areas near shield launching areas.