INNOVATIVE CONSTRUCTION FOR TUNNELS UNDER THE FORT POINT CHANNEL ON BOSTON’S CENTRAL ARTERY PROJECT

In constructing the cut and cover tunnels below the Fort Point Channel to extend I-90 into South Boston and to Logan Airport, several unique and innovative construction methods have been adapted to meet the site’s particular geologic and existing structures conditions. To cross under the main 300-ft. wide channel, six huge concrete immersed tube tunnels have been constructed and a massive 60-ft. deep by 1000-ft. long basin excavated immediately adjacent to the channel. Because the basin is deep below groundwater, special depressurization was required to dewater underlying glacial till soils, but fully effective recharging was necessary on one side to prevent depressurization beneath a nearby manufacturing plant. The 35-ft. deep trench dredged for the immersed tubes had to be delicately excavated because it is within 5 ft. of the crown of twin, 85 year old subway tunnels that run up the axis of the channel. Deep drilled shaft foundations penetrating into bedrock were installed to support the tunnels throughout their length, the drilling of which required very accurate location control. Once across the Fort Point Channel, the largest U.S. installation of deep soil mixing was needed to stabilize deep soft clay deposit for continuing the tunnels with cut and cover construction. Through a 700-ft. long area, the deep soil mixing was used to depths of 100 to 130 ft. to restrain about 50-ft. of unbalanced lateral loads from adjacent land and to serve as the permanent foundation for the tunnels. Further south, at a shallower part of the tunnel, a 40-ft. deep berm of soil-cement was formed on one side of the tunnel to laterally restrain a 1000-ft. long section of tunnel.