Stabilization of Running Granular Soils in TBM Alignment with Sodium Silicate Grout

Abstract

During the tunnel boring machine (TBM) excavation of the Portsmouth Forcemain Segment 2, in Portland, Oregon, the Lovat TBM encountered free flowing granular material including cobbles and boulders which resulted in excessive ground loss. Prior to stopping the TBM, the uncontrolled ground loss had resulted in a chimney opening up to the ground surface 24.4 m (80 ft) above the tunnel crown. Previous attempts at using polyurethane grouts from the face of the machine had been unsuccessful in controlling the ground loss associated with tunnel advancement. The tunneling contractor selected a grouting proposal which ultimately relied on sodium silicate grouting to stabilize the loose granular soils above the water table with an array of vertical and battered sleeve port pipes installed from a walking trail on the slope above the TBM. Drilling methods included a casing advancement system to deal with the expected cobbles and boulders in the tunnel alignment. Significant grout losses occurred in the previously loosened soils near the machine during placement of the cement-bentonite backfill grout around the sleeve port pipes. Following completion of the grouting program a 20.7 m (68 ft) length of the tunnel alignment was stabilized and the machine was advanced 12.2 m (40 ft) without measurable ground loss into a safe zone where the TBM was replaced with an open shield machine better able to deal with the boulders and cobbles in the alignment without excessive ground loss.