Design Charts for Estimating Face Stand-Up Time in Soft Ground Tunnelling; TBM DiGs 2016, International Conference on Tunnel Boring Machines in Difficult Grounds; TBM DiGs 2016, International Conference on Tunnel Boring Machines in Difficult Grounds

ABSTRACT: Under unstable face conditions, inspections and maintenance work in the working chamber of EPBs or slurry shields have to be carried out under compressed air conditions. As hyperbaric interventions generally present operational disadvantages, the question often arises as to whether the face may be stable at least for the limited period of an intervention. In fact, if the ground exhibits a medium or low permeability, then the face – even if unstable in the long-term – may be stable for a practically significant period of time. Estimations of the stand-time of the face (i.e. the time lapsing from the beginning of the standstill until collapse) are thus important for assessing the feasibility of atmospheric interventions in the working chamber. Short-term stability can be traced back to excavation-induced negative excess pore pressures. Stability deteriorates over time due to excess pore pressure dissipation during the consolidation process. Instability occurs earlier or later depending on the permeability of the ground. In an on-going research project, the phenomenon of delayed failure was investigated in laboratory tests under controlled conditions; the experimental results can be reasonably reproduced using hydraulic-mechanical coupled spatial FEM analyses. Based upon a suite of such analyses, dimensionless design charts were worked out, which allow the stand-up time of a tunnel face to be estimated for a wide range of ground parameters and overburdens of between one and four tunnel diameters.