Soil Conditioning of Weathered Granite Soil used for EPB Shield TBM: A Laboratory Scale Study

Soil conditioning is one of the key factors for successfully excavating tunnels by utilizing the Earth Pressure-Balanced (EPB) shield Tunnel Boring Machine (TBM) for increasing the tunnel face stability and extraction efficiency of the excavated soils. Since the characteristics of weathered granite soil, abundant in the Korean peninsula (also in Japan, Hongkong and Singapore), is different from those of either sand or clay, conditioning agents applicable to either sand or clay cannot be directly used for the weathered granite soil. In this study, conditioning agents are mixed with the weathered granite soils and the properties of the resulting mixture are evaluated in a laboratory-scale experiment to derive and propose the most suitable conditioning agent as well as the most appropriate agent mix ratios. It was confirmed through an experimental study that the EPB shield TBM could be operated in good condition by injecting 22–67% foam depending on the water content of the excavated soils. In addition, it was also found that the range of particle size gradation of the weathered granite soils, under which the conditioning agent foam can be applicable, is wider than the existing application ranges proposed thus far for properly operating the EPB shield TBM.