ABSTRACT
Thessaloniki’s METRO line in Greece foresees the construction of twelve pumping stations for draining purposes of the TBM tunnels. The stations are to be excavated from within the TBM tunnels by removing/demolishing part of the precast lining. Six of the stations are situated in Tertiary fluvial deposits consisting of alternations between soft silts and loose silty sands and gravels of permeabilities in excess of 10-1 cm/sec. Trial drillings conducted showed water ingress exceeding 60m3/h accompanied by severe flow of sand and gravels that instantly gave rise to surficial settlements, thus necessitating the implementation of ground improvement. High pressure injections (i.e. 24bar) with accelerated micro-cement grout using self-drilling anchors installed at a triangular grid, determined through finite difference analysis, were adopted. Non-accelerated grouting with normal cement was unsuccessful as the grout could not infiltrate into the silty sands and the groundwater velocity led to complete washout.
