Numerical analysis of volume loss caused by tunnel face instability in soft soils

This study investigates the critical support pressure at collapse on the tunnel face at different cover-to-diameter (\({C \mathord{\left/ {\vphantom {C D}} \right. \kern-0pt} D}\)) ratios. During shallow tunnelling, the cover-to-diameter ratios may change from a positive to a negative value. The collapse of tunnel face was modelled with the use of finite element analyse (FEA) with consideration of different soil strength parameters, cover-to-diameter ratios and support pressure distribution patterns. The numerical simulations were verified against published experimental data for different \({C \mathord{\left/ {\vphantom {C D}} \right. \kern-0pt} D}\) ratios, and the results agree well with each other. A design chart for determining the critical support pressure at collapse under undrained conditions is developed based on the FEA results. The effectiveness of the design chart in conjunction with volume loss predicted by Klar and Klein’s equation to predict volume loss at collapse failure for tunnelling in soft soils is demonstrated through two case histories.