The Impact of Excess Pore Pressure on the Support Stability of a Shield When Tunnelling in Sand Stratum

When the large-diameter shield tunneling through the soft soil stratum, the pore pressure fluctuation would influence the tunnel stability, yet how it affected was still hard to figure out. Based on field tests data collected from a tunnel project in China, the distribution characteristics of excess pore pressure in sand stratum were obtained. By combining the limit equilibrium theory and variational calculus method, a modified formula of the limit support pressure was proposed, considering the effects of excess pore pressure and slope angle. The results showed that the excess pore water pressure had a big influence on the fracture angel of the sliding surface and the support pressure of the shield. If the effect of the pore pressure were taken into account, the support pressure should be set larger than the normal condition when the shield entered sand layer. The limit support pressure would decrease with the increase of the cohesive force and friction angle. Thus, effective reinforcement on the stratum should be adopted before the shield passing through.