1999 WACA Awards - Special Applications/Technical

Precast concrete girder supports arrive by barge.

Building a juvenile fish bypass in the Columbia River proved to be a challenging project and also an award winner in the Special Applications/Technical category. The bypass was designed to transport salmon smolt safely around the Bonneville Dam.

The project's outfall structure had to be cast in the winter to avoid fall salmon migrations and high spring water flows. The outfall structure looks like two hollow concrete pipes that jut out into the river. Invisible beneath the surface are massive concrete support casings, needed to hold the outfall against the powerful current of the Columbia.

The first two months of the project were spent building an access trestle to reach the spot were the casings would be installed. The trestle had to be strong enough to support big cranes and heavy loads of concrete.

A casing under construction.

The supports for the bypass were created by first driving steel casings into the river. They are 10 feet in diameter and range between 135 and 207 feet in length. Rebar cages were then lowered into the steel casings. The cages were equipped with strain gauges, thermocouples and a plumbing system for heated water to circulate to protect the concrete from cold river water as it set. Concrete was pumped into the casings from a boom truck and tremie. A total of six casings were created in the river and two on the beach.

In order to support the outfall's girder sections, crossheads were cast upon the casings. Since high water flows in the spring destroyed the access trestle, that work had to be performed from a barge-mounted crane platform. A total of 12 concrete girder supports were precast and brought to the site by barge.

The project team included: U.S. Army Corps of Engineers, owner/developer, structural engineer and architect; General Construction, general contractor; and Aphis Ready Mix & Concrete Pumping, ready mixed supplier.

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