A Multiple Winner

Edited by Carl Molesworth | September 28, 2010

It is somewhat rare when a project nominated for a local award attains national notoriety as well, but so it was for the Snohomish South Slough Bridge No. 91, an American Public Works Association local and national award winner. Additionally the project was a national winner of the 2008 Portland Cement Association's Bridge Design Award of Excellence.

A Caterpillar excavator performs demolition on the deck of South Slough Bridge No. 91

In 2005, the 90-year-old Luten arch beauty, which carries Smokey Point Boulevard over South Slough, was deemed functionally obsolete per current standards. However, Project Manager Larry Brewer and his team were not ready to embrace a full replacement strategy until they investigated all options.

"She was cutting-edge in her day... carrying Model-Ts across the slough," Brewer said. "She's a lovely structure that we made a conscious effort to rehab. The county would have been worse off without her, in terms of history, aesthetics and budget."

Although the 2005 inspection revealed several major deficiencies of the bridge, the project team felt the Luten arch and its foundation were structurally sound. The possibility of keeping it and replacing only the upper portions of the bridge interested the engineers. In order to determine the feasibility of this option, engineers conducted an in-depth review of the existing structure and the loads it carries. Two essential requirements had to be met:

  1. The new design had to maintain approximately the same dead load distribution on the original arches, which was necessary to keep them in compression.
  2. The load increase on the substructure and foundations had to be minimized to reduce the risk of excessive settlement.

At 9 feet 8 inches wide, the bridge's lanes were just right for Ford Model T's and other vehicles of the day, but were no longer adequate for today's wider loads. And since there were no shoulders, it was clear that a wider structure was needed to meet current safety and design standards. So pre-cast concrete slabs were chosen.

However, the pre-cast concrete slabs that would form the wider deck would also add a substantial amount of weight to the bridge, and in order to counterbalance the additional dead load, engineers needed to reduce weight in another area. Several lightweight materials were studied to replace the existing heavier soil fill atop the arches, and Elastizell, a lightweight cellular concrete that has significantly less density than soil fill (approximately 30 pcf for lightweight concrete versus soil's 125 pcf) was selected. The concrete could be formed in place to create an even contact on top of the existing structure.

Innovative Construction Techniques

The new bridge deck is wider without being heavier

The correct load distribution and a well-planned construction sequence were essential for the success of this widening project. To maintain the integrity of the arches and ensure safety during construction, the process of removing existing soil fill from the arches and replacing it with the lighter cellular concrete had to be conducted incrementally in 2- to 4-foot lifts, alternating between the arches.

The project cost $929,000. By using the existing concrete arches and cellular concrete as a replacement fill, the project team eliminated the need for a complete replacement project; shortened the project duration by approximately 4.5 years; saved approximately $5 million; and increased the deck's square footage by 62 percent without adding significant weight to the arches.

In addition to achieving such enviable metrics, the project team also succeeded in communicating detour information to the 50,000 people who were expected to attend the 39th Annual Arlington Fly-in, the second-largest aviation convention in the nation. This thorough understanding of the public's vested interest in public works projects is what drives this engineering services team on each project they undertake.

"It would have been easier to demolish and replace the bridge, but we feel a great sense of responsibility as stewards of public funds and were willing to make the case that modernization was the right choice, both dollar-wise and for aesthetic reasons," commented Brewer.

This is the second year running that Snohomish County, WA, has been a winner of a national APWA award.