Centrifuge modelling of tunnel face reinforcement using forepoling

This paper investigates the effect of forepoles on stability of tunnel face and unsupported length during tunnel excavation in clay beds. Forepoles were modelled using 1 mm diameter brass rods. The tests were conducted using 65 mm diameter model tunnels with a flexible face at 100g centrifuge environment. The unsupported length of the tunnel varied between 1 and 1.5 times the tunnel diameter in different tests. The results seem to suggest that forepoles influence to reduce the length of settlement trough ahead of the tunnel face. However, width of the settlement trough remained unaffected. Excess negative pore pressures after collapse were noted to decrease with distance ahead of the tunnel face and increase with depth from the surface up to the tunnel axis. However, scatter in the measured data points suggest that the tunnel stability depends not only on the unsupported length of the tunnel but also on the length of forepoles. It is difficult to include these effects in the simple plasticity solution framework wherein the soil structure interaction is ignored.