Comparison of static and transient face stability

Abstract

Tunnel construction projects in the western part of the Netherlands are typically built in soft, heterogeneous ground conditions with low stiffness and high groundwater tables. The tunnelling technique used is the ‘slurry shield’ method, which uses a tunnel-boring machine (TBM). The TBM consists of a steel cylinder with a cutting wheel at the front. At the face of the TBM, called bore front, a pressurised slurry is used as face support. The aim of a static tunnel face stability assessment is to determine the horizontal effective stress and pore water pressure acting at the tunnel face both during boring and a standstill. While the pore water pressure distribution along the tunnel is clear, the estimation of the horizontal effective stress distribution is difficult. At the moment many calculation models have been developed to calculate the latter, but none ever made it into a standard. The first part of this thesis consists of a comparative study between the Ruse-Vermeer (2002), Jancsecz-Steiner model (1984) and DIN4126+4085-model (2008). A theoretical background analysis has been performed for each model; by using a case study analysis, the effect of the models’ differences on the minimum support pressures has been determined. Thereafter, this theoretical analysis has shifted to a more ‘practical’ project analysis by including external aspects like project aspects, soil conditions and contract requirements. From the practical project analysis it resulted that at the entrance zones, the soil cover is limited and thus additional surcharge is required. The placement of this additional surcharge lead to consolidation in the subsoil which can subsequently influences the static face stability assessment. Within this research a model is proposed in order to incorporate a one-dimensional vertical consolidation analysis within face stability assessment. During tunnelling a continuous process of slurry penetration and excavation takes place and therefore, the interaction between the support fluid, advancement of the TBM and the soil needs to be taken into account. The combination of these two time-dependent effects can possibly lead to the build-up of excess pore pressures in front of the tunnel face which has a high influence on the minimum support pressure. The second part of this research investigates the possibility to include the time dependent effect of slurry infiltration and excavation within a face stability assessment e.g. transient face stability assessment. The last part of this research is based on the role of slurry TBM parameters on ground deformations. Due to overcutting and the tapered shape of the TBM, a joint (annulus) is present between the TBM and the soil formation. In this research it is assumed that the annulus is hydraulically connected to the face. This means that the annulus is pressurized with bentonite slurry originating from the face. Due to the slurry pressure gradient and hydraulic connection with the annulus, not at each location the exact geostatic stress can be reached and ground deformations will inevitably take place.