Dulles East/West Automated People Mover
Atkinson/Clark/Shea (ACS), a joint venture of Atkinson Construction, Clark Construction Group, LLC, and J.F. Shea Co., Inc., performed excavation and support work for the Dulles East/West Automated People Mover (APM) project. Encompassing more than 6,775 linear feet of rock excavation and averaging a depth of 50 feet, this project is one of the largest and most challenging that the team has ever completed.
The joint venture team performed the support of excavation and cast-in-place concrete for the cut-and-cover tunnels and stations. In addition to the tunnels, the Dulles Airport East/West APM Tunnel's scope of work included construction of a Tier One Station, a Tier Three Station Shell, an inner Spur Track using the cut-and-cover method, and a tunnel portal at the Vehicle Maintenance Facility. Further, Clark performed the MEP fit-out of the tunnel stations as well as the installation of the Emergency Radio Supplemental Radiating System for handling emergency communications with the Metropolitan Washington Airport Authority, Loudoun County, and Fairfax County.
Two 21-foot diameter Robbins Single Shield Tunnel Boring Machines (TBM) were utilized for excavation of over 3,600 linear feet of the tunnels. The machines were fitted with 15-inch disc cutters for excavation through relatively soft rock (UCS < 7,000 psi). Precast concrete segments were erected, concurrent with the excavation, within the tail shield to form the final tunnel lining. Both TBMs were moved through a 600-foot-long station, as well as negotiated radii as shallow as 410 feet to successfully complete each run.
On the northern end of the East Automated People Mover project the joint venture team excavated two 400-foot-long, 22-foot diameter tunnels utilizing NATM techniques. This system utilized roadheaders for excavation, robotic shotcrete application machines, and a very complex system of convergence and settlement monitors to determine the state of relaxation of the ground around the tunnel. NATM tunnels were excavated in one meter advances of the 14-foot-high topheading, followed by immediate and occasionally concurrent application of up to 12-inch thick shotcrete. In the event surface subsidence was detected, additional shotcrete was applied, and the tunnel bench was removed to complete the excavation and form a complete circular structure - thereby limiting the potential for surface subsidence.
On the West Automated People Mover of the airport, two 1,200-foot-long tunnels and two 210-foot-long tail tunnels also were excavated using NATM techniques. Surface subsidence was of great concern to the team throughout the project given all of the tunnels were excavated at shallow depths (less than two tunnel diameters) below active taxiways with settlement criteria of .25-inch at the taxiway surface. Throughout the excavation sequence, the shotcrete delivery system was kept continually active in order to be able to apply shotcrete at any time during the excavation cycle.