Wednesday, October 2, 2019

Bridge building in Tacoma

By Cara Mitchell

It has been a busy summer for construction crews building a new southbound Interstate 5 bridge across the Puyallup River.

In June, crews moved three lanes of southbound I-5 near the Port of Tacoma Road on to a recently completed new bridge that also carries northbound I-5 traffic across the Puyallup River. This traffic shift set the stage for design-builder Guy F. Atkinson Construction to begin building the foundations of the new southbound I-5 bridge. This is an example of how we work very hard to keep travelers and the project moving at the same time.

Bridge building starts at bedrock

Drivers likely have seen the large cranes towering next to the highway – a sign that work activity is occurring under the bridge decks. The cranes are there to help build bridge piers, which support the entire bridge and are the foundations that carry vehicles across the river. Piers are made up of multiple columns connected by a crossbeam.

If you think these piers look tall above ground, you might be surprised to learn the columns that support them go more than 100 feet underground into bedrock! This allows our crews to build a new bridge that meets modern earthquake standards.

Our construction teams build new piers using an in-depth five-step process:
  1. Drill a 10-foot wide, 100-foot deep hole in the ground and insert a temporary casing.
  2. Break up and remove dirt from the hole using a crane-mounted excavating tool.
  3. Insert a cylindrical cage made of steel rebar deep into the earth. These cagws are used as the center of the pier structure to reinforce the concrete. 
  4. Fill the hole with concrete from the bottom up. This causes any debris in the hole to rise to the top as the concrete fills the hole. 
  5. While the concrete fills the hole, the team removes the temporary casings. With one column done, we repeat the process until the pier is complete.
Rebar shaft cages go over 100 feet underground and form the initial structure for support on the new I-5 bridge.

The new southbound I-5 bridge will have 10 piers total, with fewer piers in the river than its predecessor. Just as we did on the northbound I-5 Puyallup River Bridge, a temporary work platform is being built on both sides of the river. This platform will give crews access to the in-water shafts for the new bridge as it spans the Puyallup River.
A temporary structure made from steel beams and wood decking gives us the ability to work in water.

A bridge over clean water

One of our top concerns during any construction project is to be good stewards of the environment. In this case, it is critical that our crews help ensure the work does not degrade water quality. Crews placed three water monitors upstream from, in the middle of, and downstream from our construction site.
We have water quality monitoring at three different points of our construction site.

When we drill down with our temporary casing, we use water to balance the earth’s pressure at the bottom of the hole. After we install the casing, we pump the water out and store it in four 20,000-gallon tanks under the bridge. From there, we test it for turbidity (water cleanliness) and pH levels, treat it if necessary, and then send it to the sanitary sewer system. As work progresses, crews are careful to not allow any silty water to enter the river.

Removing the old bridge structures

In order to make way for new bridge piers, crews have started removing sections of the 57-year-old northbound I-5 bridge. Not only do we need to remove the old bridges to make way for the new southbound I-5 Puyallup River Bridge, we are removing them because they are part of an aging infrastructure that no longer meets current seismic standards.  As crews advance this work, there will be overnight closures of East Bay Street between East 27th Street and East 28th Street. Signed detours will be in place during this work.  Removing the old bridge piers will greatly reduce obstructions within the river channel as the new bridges have significantly fewer in-water piers.

Widening northbound I-5 near East L Street

One item of work that is very visible to travelers is the widening of northbound I-5 between the I-705 onramp and the Portland Avenue exit. Once finished, the widened pavement will be used for a new auxiliary lane which will provide much needed capacity to a section of northbound I-5 that sees on average 115,000 vehicles a day.

Looking ahead

The contractor anticipates the project will be complete in fall 2021. When that happens:
  • Southbound lanes across the Puyallup River will increase from four to five, which includes four general purpose lanes and one HOV lane.
  • Northbound I-5 will include an additional lane at Pacific Avenue, an extra lane between the I-705 on-ramp and the Portland Avenue exit, and an HOV lane that connects HOV traffic from State Route 16 into Seattle and destinations beyond.
The completion of this project will mark the end of the last funded piece of HOV construction on I-5 in Tacoma as part of the Tacoma/Pierce County HOV project. In short, this means highway construction between the Puyallup River and SR 16 in Tacoma has been completed.

For up-to-date closure information during construction, visit www.TacomaTraffic.com.