Boston's Silverline Phase II Opens!

MBTA of Boston, MA opens first new transit tunnel line since 1918.  The opening marked the completion of the Silverline Phase II segment of the Bus Rapid Transit system in the downtown business district.  The new tunnel route is constructed beneath the historic Russia Wharf waterfront complex and crosses the Fort Point channel to the Court House Station.  To complete this challenging engineering project several innovative methods of tunnel construction were used, including NATM and immersed tube tunnels.

The Dr. G. Sauer Corporation played a primary role in the design of the innovative tunnels constructed beneath the Russia Wharf complex.  The curved "binocular" tunnels were mined using NATM construction in conjunction with ground freezing to stabilize the soil beneath the building foundations; the crown of the tunnel was only 18 feet (5.4m) beneath the structures basement level. While excavating the tunnel headings, a systematic approach was implemented to cut, isolate, reinforce and integrate the overlying timber foundation support piles into the tunnel lining.

Excavation of the mined segment beneath the building took about a year to complete.  The tunnels were excavated in two-passes, one for each side of the binocular tunnel.  Once the initial lining was placed, a PVC membrane waterproofing system was used to seal out water inflows, creating a dry, clean environment for the transit way buses to operate.

Overall the project was a resounding success, validating the flexibility of NATM/SEM tunneling in densely populated urban environments, a credit to the design, construction and forward thinking of MBTA the project's owner.

An overview of services DSC provided for this innovative project included:

• Detailed Tunnel Design Service
• 3D FEM Calculations for NATM tunnels
• Waterproofing System Design for Tunnels and Cut-&-Cover structures
• Detailed Instrumentation & Monitoring Plan
• On-Site Construction Support Services
• Analysis of Monitoring Data & Waterproofing System Installation
• VECP for Fast Ring Closure (Excavation & Support Sequences)
• VECP for Steel Fiber Reinforced Shotcrete for the Tunnel Final Lining.