The impact of shallow cover on stability when tunnelling in soft soils

Abstract

Reducing the cover of shallow (metro) tunnels can lower construction costs by lowering cost of the station boxes, increase safety and lower operational cost in the long-term. For bored tunnels there are normally minimal depth requirements stemming from design and construction. The aim of this paper is to investigate the effects of the cover-to-diameter ratio C/D to the stability of tunnelling process. Several models to analyse the tunnel stability were investigated and were applied for a case study in a typical Dutch soil profile with soft Holocene soil layers. The range of the support pressures in TBM machines, especially in EPB, when tunnelling in soft soil is derived for varied C/D ratio in different soil conditions. Based on the analysis results, some designing optimizations are proposed for shallow tunnels in soft soil.