Innovative Cofferdams used on the I-205 Columbia River Bridge-PDF

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The I-205 Columbia River Bridge used two innovative cofferdam systems to build 28 marine pier foundations on Troutdale sand and gravel, which lay beneath deep river deposits of unconsolidated sand.<br /><br />For Piers 12 and 13, the foundations had to reach the Troutdale formation at depths of up to 100 feet. Conventional sheet piling was unsuitable, so engineers used interlocking H-piles to form a cofferdam about 145 by 58 feet. The piles’ high strength limited the need for bracing. Sand-filled pockets between the piles improved watertightness and added weight, reducing the volume of concrete needed for the underwater seal. Installation required a large floating bracing frame and, where dense sand impeded driving, air-and-water jetting.<br /><br />For the remaining 26 piers, the contractor developed a system using two reusable, bell-shaped steel forms. The largest was 83 by 66 feet, 62 feet high, and weighed 450 tons. After piles were driven, each form and its prefabricated reinforcing cage were lowered into place. A tremie-concrete seal was poured at the base; the form was then dewatered so the remaining reinforcement and structural concrete could be installed. Special design features—including sloping walls, lubricated surfaces, and hydraulic ports—enabled the forms to be stripped and reused. All 26 were removed successfully, with the first requiring less than 10 tons of additional lifting force.<br /><br />The bell-pier method averaged 37 calendar days per pier, aided by prefabricated reinforcement and fewer construction steps. Both systems proved safe and effective, and all 28 piers were completed within the 1,000-day schedule and the owner’s original estimate.

Keywords

I-205 Columbia River Bridge

cofferdam systems

marine pier foundations

Troutdale sand and gravel

interlocking H-piles

bell-shaped steel forms

tremie concrete seal

prefabricated reinforcing cage

underwater construction

bridge pier construction