Investigations on transient support pressure transfer at the tunnel face during slurry shield drive part 1: Case A – Tool cutting depth exceeds shallow slurry penetration depth

Abstract

Stability of the tunnel face is a key design aspect for slurry shield tunnelling. Two requirements have to be fulfilled during excavation in order to achieve a stable tunnel face. The first requirement is a sufficient slurry pressure in the excavation chamber of the shield to balance the pore water pressure and the earth pressure. Moreover, the fraction of the slurry pressure, which exceeds the pore pressure, has to be transferred efficiently on the soil grains to avoid possible collapse of the tunnel face. The pressure transfer theory currently being employed in the practice expects a filter cake formation on the tunnel face or a penetration zone through which the pressure transfer is assured. However, this practical assessment approach was originally developed for diaphragm walls and therefore the excavation process is neglected. Hereby, it is assumed, that the transfer mechanism on the tunnel face is built-up immediately and steadily. This assumption is not valid for the tunnel face of the slurry shield since the transfer mechanism is periodically destroyed by cutting tools. This paper introduces a new approach for the implementation of the formation process in the analysis of the pressure transfer in case of a shallow slurry penetration depth smaller than the cutting depth of the tools. First, existing theories dealing with the pressure transfer are reviewed. Consequently, a new approach describing the support pressure transfer for slurry shields is derived and implemented in a numerical seepage model. Calculations are carried out in order to determine the efficient amount of the pressure transfer during excavation.