Gloucester Drawbridge Replacement
Project Overview – Existing Structure
The Gloucester Drawbridge (Bridge No. G-05-028) is located on the MBTA Newburyport/Rockport commuter line, approximately 2600 feet west of Gloucester station. The bridge carries the commuter rail passenger service on the Rockport line. To the east of the bridge, service is provided to Gloucester and Rockport stations, where the rail line ends. During a typical weekdays, an average of 26 trains cross the bridge.
The existing bridge was built in 1911, reconstructed in 1932 and structurally rehabilitated in 1984. The span drive operating machinery and electrical drive were replaced in 1985. The bridge includes a fixed steel stringer span and Strauss trunnion bascule span that incorporates a fixed steel stringer span. The fixed span is 27 feet long, and the bascule leaf is 46 feet long. When fully opened, the bridge provides a 40’ wide navigational channel (between the timber fenders), without vertical restriction.
Figure 1: Existing Bridge System
To the west of the bridge is approximately 2200 feet causeway composed of large granite blocks and fill. To the east of the bridge is an approximately 140 feet long timber trestle consisting of eleven span timber trestle supported on timber piles. The eastern approach repairs were made after a fire severely damaged the existing timber trestle. The bridge carries two tracks. The tracks switch to single track about a quarter mile east of the bascule, before Gloucester commuter rail station.
The scope of the project includes full design and replacement of the bridge, including the mechanical and the electrical components. The existing double track will be replaced, and the existing east approach trestle will be replaced with precast/prestressed box beams atop steel pipe piles. The control tower will also be relocated to the Gloucester Station side of the bridge. The new design will reverse the direction of opening of the bascules to facilitate maintenance.
Figure 2: Proposed Bridge System
In order to reduce impact to existing rail traffic during construction, the bridge will be built in two stages. One half of the bridge will be demolished and reconstructed at a time, while rail traffic is maintained on the other half. To accommodate this process, a temporary right hand turnout would be installed on the west approach in advance of work on the northern and the southern tracks. Railroad operations interruption will be at a minimum during construction, however there will be limited times that may require construction over nights or weekends with no rail traffic on the bridge.
The project is currently at the 90% design phase. Due to the additional environmental permitting required for the project, the 100% design submission is anticipated in September 2015, and advertising in February, 2016. These dates are still subject to the US Coast Guard (USCG) issuing the required bridge permit. The anticipated construction duration is 4 years.