Cofferdam Construction: I-205 Columbia River Bridge

The foundations for I-205 North Channel crossing of the Columbia River are two types, large spread footings with seals bearing directly on dense conglomerate and double bell shaped footings supported on bearing pile driven into the conglomerate. The methods used to construct these two types of foundations consist of two unique and separate cofferdam systems. The first system utilizes 100 feet long interlocking H-pile sections to form a cofferdam 145 feet by 59 feet by 100 feet deep. The use of these interlocking sections allows a designed dewatered depth of 62 feet with only two support points for the entire wall. The second system utilizes two bell shaped steel forms, stiffened to withstand a hydrostatic head of 54 feet. The larger of these two forms is 83 feet by 66 feet in plan by 62 feet high and weighs 450 tons. These forms, except for the top 14 feet, are one solid welded unit which was set and stripped in one piece.