Challenges related to face stability of EPB shield machines and the influence of the supporting medium on reuse of the excavation material

Abstract When using Earth Pressure Balance Shields in soft soil, the soil is used to support the tunnel face and the in‐situ or excavated soil, which usually does not have sufficient properties to support the tunnel face, is conditioned. For example, suspensions are used in fine‐grained soils, tenside foams in mixed‐grained soils or tenside foams with polymer additives and fine material suspensions in coarse‐grained soils, so that adequate support of the tunnel face with the conditioned soil is possible. Soil conditioning changes various geotechnical properties such as the water permeability and the density of the supporting medium and, if necessary, environmental parameters. The process‐specific changes to the geotechnical properties of the supporting medium can have partly a strong influence on the reuse of the excavated soil when used as a construction material or recyclate. This article therefore deals with the challenges to the face stability of Earth Pressure Balance Shields and the influence of the supporting medium on its reuse with regard to the geotechnical properties.