Stability of Horseshoe Tunnel in Cohesive-Frictional Soil

Abstract

The stability of long horseshoe tunnels in cohesive-frictional soils is examined by using lower bound finite element limit analysis. Its stability is assessed by determining the magnitude of uniform internal normal pressure required to be applied along the tunnel peripheral face. If the pressure required to be applied to the tunnel peripheral face at a particular soil cover depth and soil condition is compressive in nature, then it indicates that the tunnel is unstable without the lining pressure at that particular condition. The analysis has been done for different combinations of (i) soil cover depth (H), (ii) ratio of tunnel height to width (h/B), (iii) peak soil friction angle ϕ, and (iv) γB/c where c refers to soil cohesion, γ is soil unit weight, and B is the tunnel width. The results are presented here in terms of normalized compressive internal pressure (σt /c) for both smooth and rough tunnel boundary conditions. The magnitude of σt /c appeared to (a) increase with an increase in γB/c, h/B, and H/B and (b) decrease with an increase in ϕ and roughness condition of the tunnel boundary. The results are presented in the form of dimensionless charts.