October 30, 2018
The new Peter Courtney Minto Island Bridge is a steel arch bridge spanning the Willamette Slough, connecting the downtown Riverfront Park to the park on Minto-Brown Island. The new bridge provides access to over 1,300 acres of parks on both sides of the Willamette River for the community while, also linking more than 30 miles of off-street trails.
The main span of the bridge is a 304.5-foot tied-arch span with four total approach spans, three at 50 feet and one at 35 feet. Combined, the bridge is a five-span bridge, totaling 489.5 feet long. The deck consists of cast-in-place components as well as precast panels for the main span. The supports are made of cast-in-place tapered columns that used a form liner for aesthetics.
The foundation for the bridge consists of a combination of driven pile and drilled shafts. There are 12 driven piles for the end bent; eight for the Observation Platform and four for the East Approach. The Observation Platform piles were PP 12.75 x 0.500 x 35-feet in length, closed ended, and the East Approach piles were PP 12.75 x 0.375 x 40-feet in length, open ended. The Observation Platform piles were driven to 350 kips and the East Approach pile were driven to 134 kips. There are six drilled shafts for Bents 1-6.
Legacy Contracting, Inc. was required to install a temporary work bridge as well as temporary support towers that supported the arches in place prior to installing the precast concrete panels. The temporary work bridge consisted of 46 driven piles and the temporary support towers consisted of 12 driven piles, all of which had to be removed once the construction of the bridge was complete. The work bridge piles were PP 22 x 0.375 x 60 feet in length, open ended, and the support tower pile were PP 18 x 0.500 x 100 feet in length, open ended. The work bridge piles were driven to 518K kips and the support towers piles were driven to 395K kips.
For the cast-in-place deck portions of the bridge, Legacy Contracting also drove piling for the falsework. Sixty 12-foot wood piles, two 22 x 0.375-foot and six 18 x 0.500-foot piles were driven for the falsework. Legacy Contracting had to pull all the steel piling as well as the wood piling they could reach after concrete was poured.
All of this work was completed in an environmentally sensitive area and done during stringent in-water work timeframes. The existing material that the piles were driven into had a rock shelf that required Legacy Contracting to drive the piling in a matter that didn't cause the piling to cave in on itself.
The Minto Island Bridge is a one-of-a-kind structure from the foundation up. Although there are only 12 permanent piles in the finished product, many more were required in order to build this beautiful bridge. Driving the work bridge, support tower and falsework piling in a very sensitive area and completing the work within the in-water work window was challenging. Legacy Contracting was able to use a vibratory hammer and impact hammer to drive the pile through the difficult subsurface conditions.