Tunnel Face Stability & New CPT Applications

Abstract

Nearly all tunnels bored in soft soils have encountered problems with the stability of the tunnel face. In several cases these problems led to an extended stand-still of the boring process. A better understanding of the face stability, and of the soil conditions around the tunnel boring machine, can reduce the risks during boring, and can avoid cost increases and delays. During the boring of tunnels in soft soils, the support pressure at the face should remain between a minimal and maximal support pressure, in order to avoid stability problems. Existing face stability models, however, do not incorporate the influence of the boring process. The excavation process may lead to excess pore pressures in front of the tunnel boring machine, which lower the stability of the face. These excess pore pressures have been incorporated in a new stability model, that can also deal with heterogeneity of the soil. This leads to a more reliable calculation of the required support pressure. Also, a number of new applications of the cone penetration test have been investigated, to see whether additional information about the soil conditions can be obtained. Instead of the traditional, vertical, sounding from the soil surface, a sounding can also be made from the tunnel boring machine in a horizontal direction. From laboratory investigations and theoretical models it has been found that the measurements depend slightly on the sounding direction. These differences can be corrected for, in order to use the existing (vertical) correlations also for horizontal cone penetration testing. Further, to gain information on the density ofsand layers, the excess pore pressures measured by a high-speed piezosounding have been compared to those obtained from regular piezosoundings. Although significant differences have been found in laboratory tests, no reliable relation with the density of the sand has been established. It has been shown that the other regular measurements can be reliably obtained at high speed as well as at the normal sounding rates.