The drilled shaft foundations for the units extend as much as 185 feet below high tide into the bedrock. Units EB1 and WB1 have down stands cast into their underside to match the position of the drilled shafts, so that when the units were lowered, they fit onto the shafts like lego blocks. The eastern two units will be founded on grout bags positioned on the underlying till. Founding immersed tube tunnel units on drilled shafts is unusual, but it is necessary on this contract because units EB2 and WB2 straddle the Red Line subway tunnel, with just six feet clearance. No load from the immersed units may be applied to the Red Line Tunnel.
Extensive instrumentation allows monitoring of the subway tunnels during construction.
An Emergency Response Plan is in place which includes rigid safety requirements, one of which is the installation of bulkhead doors in the tunnels to be closed in the unlikely event that flooding ever occurs. Cut off walls have been constructed at both the west end and east end of the units, and, together with bulkheads to be constructed on top of the ends of the units, will retain the water of the Fort Point Channel and allow the ends of the tunnel to be dewatered simultaneously. The foundations of the new Dorchester Avenue Bridge were also cast integrally on the west two immersed tubes while they were still in the basin. The Dorchester Avenue bridge and the Ventilation Building are currently under construction. The last two of the six giant tubes are being built within the dewatered casting basin, prior to the basin being flooded for the last time and the two units floated out and immersed onto their six feet diameter drilled shaft foundations.