Dumbarton Rail Corridor Improvement
CTLGroup was retained by HNTB Corporation, on behalf of the Peninsula Corridor Joint Powers Board, to perform an evaluation of the concrete caissons of the Dumbarton and Newark Slough rail bridges located in the south end of the San Francisco Bay. The structures were evaluated as part of the Dumbarton Rail corridor improvement program, a proposed 21 mile long commuter rail corridor that links Alameda and St. Mateo Counties across the south end of San Francisco Bay on the existing Dumbarton Rail and Newark Slough bridges.
The Dumbarton Rail Bridge is composed of various structure types, including: approximately 2,400 feet of 1910 vintage timber trestle, a portion of which was heavily damaged by a fire; 3,300 feet of cast-in place and precast prestressed concrete spans supported on concrete piles constructed between 1967 and 1975; and 1,500 feet of steel truss spans supported on deep concrete caissons, including a swing span truss over the main navigation channel.
The Newark Slough Bridge comprises 600 feet of timber trestle approach spans, 1910 vintage, and a 217 foot swing span steel truss on concrete piers and timber piles, similar in construction to the Dumbarton Bridge steel spans.
CTLGroup’s efforts were focused on the caissons supporting the steel truss spans of the Dumbarton Bridge and the center caisson of the Newark Slough Swing span structure.
The evaluation included a field investigation to obtain core samples from selected caissons, documentation of the existing condition in the field and laboratory testing to asses the overall quality and characteristics of the concrete. Cores were taken vertically, along the length of the caissons to depths up to 80 feet. To complete the coring, CTLGroup coordinated access to the bridge, hiring the support of a Marine contractor and a specialty coring contractor.
During the field investigation, concrete was observed to vary in quality along the length of the caissons, with three general regions. The upper 12 to 14 feet of the caissons consisted of good quality, uniform concrete in generally good condition. At the water line, concrete appeared to be a combination of good quality concrete with abrupt changes to poor quality concrete. The region at the water line and below, approximately 12 to 15 feet below the top of the caisson, was observed be of poor quality. Below the water line, concrete appeared to be of generally poor quality, with the majority of samples extracted as rubble from this region.
Material testing was performed to evaluate compressive strength, static modulus of elasticity and chloride ion concentration. Additionally, petrographic examination was performed to evaluate the overall quality of the concrete.
The condition observed at the water line and below indicated that bay water was intermixed with the concrete during placement. This condition appears to be a result of the original construction procedures. In spite of this poor quality concrete, the confinement provided by the steel jacket, along with the presence of timber piles in the caissons and the steel frames utilized to align the steel jacket during its erection has allowed the structure provide adequate serviceability.
The results of the material sampling and testing regime implemented by CTLGroup will be used by HNTB to evaluate the potential reuse of the caissons and develop rehabilitation plans for the bridge, in support of corridor upgrades necessary to reopen the rail corridor for desired commuter train traffic.