In the next two months, we'll reach another major milestone on the new Willamette River Bridge: the closing pours will complete the first set of deck arches.
The arches for the new southbound I-5 bridge will sail gracefully upward from their foundations and may appear decorative to the casual observer, but far from it. They are actually highly technical, carefully engineered supports based on a modern update of construction principles used by engineers to build bridges and aqueducts during the Roman Empire, 2,000 years ago. The process to build the arches requires a series of complex steps executed perfectly in concert.
To give you a close-up look at each step in the process, we'll post photographs over the next few weeks showing the various stages of the arch construction. This week, we'll start by taking you 50 feet underground for the first stage of the arch ribs process.
Most structures - from the smallest garage to the tallest skyscraper - start with a sturdy foundation. Before ODOT's team could start building the arches they had to make sure the whole structure would connect with solid ground, or what project engineers call "competent rock." To reach that ground, they had to drill down about 50 feet and install rebar-reinforced, poured-concrete drilled shafts. The southbound structure requires 20 drilled shafts in total, which crews were able to build in only 30 days. These shafts will support and distribute the weight of the bridge and the traffic it carries for the coming decades.
Once the holes are drilled, crews installed a rebar cage that provides critical strength for the concrete that fills the shaft. Crew members were lowered into the shaft to inspect the work, ensure everything was correctly placed and make last-minute changes before the contractor poured the nearly 100 cubic yards of concrete that filled each shaft.
Next time: shaft caps.
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