Precast triple

Nov. 9, 2007

To support the opening of a new Loudoun County School, Toll Brothers was in need of a bridge that would span Broad Run Stream. Rosa Lee Carter Elementary was due to open in the fall, and the goal was to have a finished bridge installed in less than five months.

To support the opening of a new Loudoun County School, Toll Brothers was in need of a bridge that would span Broad Run Stream. Rosa Lee Carter Elementary was due to open in the fall, and the goal was to have a finished bridge installed in less than five months.

The solution was a triple-span BEBO arch bridge, a project that involved Virginia Department of Transportation approval, a complex design and fast-tracked production and construction. The BEBO E-Series arch bridge includes a center 72-ft span cell and two outer 60-ft spans. The different spans were combined to accommodate a major floodplain and hydraulic requirements.

Production of the precast BEBO arch units began immediately in order to meet an aggressive delivery goal. At the start of installation, Westlind Construction and William A. Hazel crews encountered some unforeseen delays in placing the first units on day one. On days two and three, however, crews were able to install the remaining rings of the center 72-ft span cell with no problems, making up for lost time and keeping the project on track.

The two outer 60-ft arches also were installed successfully over the following weeks.

During footing construction, Westlind Construction provided some unique dewatering solutions in the midst of heavy rainfall conditions to save the excavation and prevent delays.

"A very tight schedule made this project a challenge," said Mike Carroll of William A. Hazel. "If not for [the supplier] and our erection subcontractor, Westlind, we would not have been able to perform to our owner's expectations."

Two potential problems that posed a threat to the project were tall headwall heights and installation challenges due to precast counterforts extending over precast joints. A hybrid headwall system was therefore developed in lieu of using precast counterfort walls. This system, consisting of precast wall panels, cast-in-place counterfort walls and the traditional precast wingwall system, eliminated these potential challenges.

Varying non-erodible rock elevations required the engineering team to design both shallow and deep foundation systems. Shallow spread footings beared at the higher rock elevations, while 48-ft-diam. caissons to rock were used at the lower rock elevations. The foundation design also needed to avoid an existing 60-in. sanitary sewer line under the east cell and a proposed 60-in. sanitary sewer realignment under the west cell.

"Responsiveness and cooperation were an integral part of a team effort in making the project a success, completing three weeks ahead of schedule," said Jon Cannizzo, P.E., land development manager for Toll Brothers.

The success of the project took coordination and dedication from the teams from Westlind Construction, William A. Hazel, CONTECH Bridge Solutions , Toll Brothers and Bowman Consulting. The Loudoun County school was able to open in August without concern over road construction or traffic delays and with an impressive new bridge to cross.

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