Few cofferdams remain

From ODOT-

In order to construct the new Willamette River Bridge, crews need to reach the riverbed to build the bridge foundations. It would be impossible to work safely to drill shafts and install the bridges piers fighting the river’s current. But by building cofferdams, temporary steel or wooden enclosures that keep water and soil out of the work area, the crews can safely and quickly complete their work – all with the water whizzing by.



On the WRB, crews built nine cofferdams for column construction. Once the columns are complete, crews remove the cofferdams, letting rock and dirt fill in around the base of the columns.

Workers build a column inside a cofferdam on the Willamette River's north bank.

Now that the foundations are complete and the arches are taking shape, we’re approaching another milestone: by the end of summer, crews will remove all but one the project’s cofferdams.



The cofferdam located in the center of the river at the base of the two arches will remain until the bridge is complete.

This cofferdam protects workers as they install rebar for bridge arches in the center of the river.

Crews dish up a menu of specialized concrete mixes

From ODOT-

The concrete suppliers and specialists at the Willamette River Bridge are mixing up the standard components of concrete to deal with the complexity of the project. They designed 17 mixes to build the recently completed southbound structure. The arches, the drilled shafts, the columns and the deck all needed concrete specific to each function.

The mix for the bridge deck is made of high-performance concrete, which includes a special fiber additive. This fiber helps the concrete resist stresses placed on it, significantly reducing the risk of cracking. The fibers are evenly distributed throughout the concrete to form a secondary matrix of support.

Even though the drilled shafts forming the piers supporting the bridge were surrounded by rock, there was a chance that the concrete could leak into the river before it hardened. To ensure this would’t happen, our team created a mix for the piers that includes a special anti-washout additive to keep the concrete contained within the rock socket.

The two footings on the banks of the river require a lot of concrete — nearly 500 cubic yards each. The more massive the concrete structure, the more heat is generated while it cures. The more heat, the more the concrete expands and then shrinks as it sets, which can create cracks. So we designed a special mix for the riverbank piers to minimize heating and reduce cracking.

The point where the arches meet in the river and the first 23 feet of the arch ribs themselves required a concrete mix capable of flowing almost like water, then self-consolidating without vibration, filling every nook and cranny within the form.

The special concrete designed, mixed and placed resulted in a strong, beautiful southbound bridge. We’ll repeat our success as we build the new northbound bridge.

Supporting the bridge columns, Part 1

From ODOT-

In addition to two graceful arches supporting the bridge, the new northbound Willamette River Bridge will have nine columns, supported by 22 drilled shafts filled with concrete, which create the foundation for the columns.



The team surveyed and staked column locations before clearing and constructing cofferdams at each site. Next, crews built platforms to accommodate a large drill rig near each cofferdam.



The drill rig can drill to a depth of 200 feet, though for this project, the average depth is 35 feet. The bit or auger attached to the drill rig varies in size, but is typically 8 feet in diameter and 6 feet long. Once the drill is positioned, the drilling takes one to two days per shaft.

The drill rig positioned beside the cofferdam is ready to drill one fo the two shafts at this site.

Initial drilling is usually through soft material, which is removed from the hole. To prevent soil from falling back into the hole, workers insert a temporary steel casing (below) before pouring the shaft.