WASHINGTON METRO - JET GROUTING THE KEY TO NAVY YARD EAST SUCCESS

As part of the construction of the Washington DC metro twin 3390 ft long running tunnels were required to be driven in soft, waterlogged, ground of sand and silty gravel with some boulders. The 225 in diameter tbm selected to excavate the tunnels had to drive downhill on a 4% gradient towards the anacostia river during which time the drive would be hampered by two sewers, 18 ft and 14 ft diameter, perpendicular to the drive. In order to support the sewers, which had to remain in operation, the injection of chemical grout was carried out in the gravel layers between the metro tunnel and the sewers, a distance of some 4 ft, keeping the settlement within the 0.5 in tolerance required. The grout is quick-setting and is injected through 5 in diameter holes drilled in the sand. (TRRL)