rheological characterisation of foam-conditioned sands in epb tunneling

The flow behaviour of soil–foam mixtures, used as support medium in closed-mode tunnelling with earth pressure balance shield machines, is an essential factor for the operation of the tunnel boring machine (tbm). on the one hand, a rather soft consistency is required providing a homogeneous face support pressure transfer to the tunnel face. high accuracy in face support regulation is crucial for settlement control, especially in sensitive environments, such as urban areas. on the other hand, a rather stiff consistency is preferable concerning transportation and disposal of the excavated ground to avoid additional treatments for landfilling, tipping or sewage management. so far, the flow behaviour of soil–foam mixtures has been investigated by index tests. most notably, the slump test, known from concrete technology, is widely applied on soil–foam mixtures. however, flow is actually a non-static phenomenon and cannot be expressed by a single parameter, which is derived from an equilibrium-state condition at rest. this contribution focuses on the rheology of soil–foam mixtures aiming at a better understanding of the flow behaviour and of the influences the individual components have on it during the excavation process. investigating the interaction between soil, water and foam provides optimisation strategies for the tbm performance.