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By Dick Upton, Major Projects Unit manager, Oregon Department of Transportation

One of the less glamorous aspects of a heavy highway construction project is the constant checking and rechecking for quality assurances and quality controls. On the Oregon Department of Transportation’s largest bridge replacement to date, the $187 million Interstate 5 Willamette River Bridge project, such continual comparison of as-built specifications with actual measurements and capacities paid off, big time.

The project’s demolition plan called for excavators to peel away the bridge deck from above, essentially working backward to remove the bridge structure from beneath them as they went. With excavators crunching away at the bridge surface, inspectors from demolition subcontractor McGee Engineering conducted one of many planned reviews of a box girder span that provides the majority of the structure’s strength—and found exactly what they didn’t want to find. The foundation of the box girder, which supports its own weight as well as that of multi-ton excavators, was not as strong as originally estimated. This discovery changed the game not only for the excavator operators but for the entire demolition team.

Long before deconstruction began, our project team had been hard at work, dividing and conquering the numerous tasks needed to prepare the site. In August 2009, it was time to get to work building the demolition containment structure, the platform that would not only help to protect the Willamette River below but also provide a stable surface for the people and machinery doing the work of demolishing the decommissioned structure and building a new one in its place.

Our CM/GC partner, Hamilton Construction, built the containment structure below the decommissioned bridge. It consists of 24-foot-long, one-foot-square timber decking on a steel frame standing 10 feet above the high-water mark. On top of the steel pilings and massive timbers, the team installed an impermeable plastic layer and complete drainage system, then covered the surface of the deck with plywood sheeting and a one-foot thick layer of sand to absorb the impact of the chunks of steel, rock and concrete the bridge is reduced to as the demolition experts disassemble it.

Once we’d prepared the banks and adjacent areas, demolition experts Staton Cos. got the Manitowoc 3900W crane—equipped with a canola oil-powered Delmag D25/42 50,000-foot-pound hammer—to work driving the piles that create the foundation for the platform. In some sections with limited vertical clearance, a vegetable oil-powered APE 9.5 hydraulic hammer allowed the team to generate sufficient force despite overhead restrictions. Choosing a natural oil is not only a cleaner environmental choice, it also reduces the risk of inadvertently contaminating the river below.

One of the benefits of the deck arch design used for this project is that each structure—northbound and southbound—will touch the river at only one point. Compared to the five piers required to support the decommissioned structure, this is a significant improvement. To see that improvement constructed, though, the team had to drive 263 temporary pilings into the riverbed to provide vertical support for the work bridge.

As with all aspects of this project, our CM/GC team members worked hard to ensure they achieved the dual goals of efficient work and minimal impacts, a careful balancing act that occasionally requires tackling problems from a different angle. In this case, the team’s initial goal was to protect fish and other aquatic creatures from disturbances generated by pile driving. Sound waves travel exceptionally well in water and may disturb fish and other critters nearby. Building on previous construction project success, the team decided to employ a bubble curtain, a ring that releases compressed air, causing bubbles to rise continually to the surface. These bubbles act as a damper around the pilings, which helps isolate the surrounding water from vibrations and noise.

On a project of this scale, everything is bigger, and Hamilton custom-built a large steel enclosure in line with that trend. It not only provides support for the bubble curtain but also serves as a safe work platform and a secure frame to help guide pile driving. Continuing the theme of creativity, the team even named the brightly colored device the Bubbleator.

While Hamilton’s construction team was completing the demolition containment structure, Staton—a local company that brings almost 40 years of demolition expertise to the team—moved its equipment into place and set up additional containment systems, including hanging nets from cranes to protect against flying debris, installing geotextile fabric over railroad tracks and roadways, and diapering the underside of its equipment to prevent leaks.

Properly attired, the massive excavators rolled onto the bridge and the work platform to disassemble the bridge. The machines, Link Belt 210s weighing 46,000 pounds each, can be equipped with a variety of tools such as demolition hammers or hydraulic shears to make short work of massive sections of steel-reinforced concrete.

The project’s extensive demolition plan called for the excavators to pull the bridge apart from both the top and bottom. Predemolition engineering had determined the steel stirrups that give the box beams their strength would be sufficient to support the excavators working from the bridge’s surface.

Unfortunately, as the first girders were cut away, the QA/QC inspection revealed an unwelcome surprise. Instead of being built from a single piece of steel forming a strong U-shape, the stirrups were actually built from two separate pieces of steel; each stirrup was composed of two inward-facing L-shape pieces. Although the design was strong enough for daily highway traffic, it would not support the excavators’ weight as they weakened the structure during demolition.

To address this weight restriction problem, all demolition had to occur from below, requiring the machines to stretch 50 or more feet into the air and disassemble the bridge overhead. For this challenging and specialized task, Staton brought in a pair of 400-class, 50-ton Komatsu high-reach excavators with custom-designed and -built long-reach booms. This modification allowed the machines to reach 70 feet into the air and work with any of the company’s standard attachments to reduce the former interstate bridge to a pile of rubble and rebar. The Komatsu machines, equipped with their Staton-customized booms, proved to be a great solution to a potentially significant problem.

Once demolition began, the excavators worked nearly constantly, devouring large sections of bridge at a time. The carefully coordinated process was designed to allow the operating engineers to safely pull the bridge onto the deck, where they could break down large pieces into smaller rubble and separate materials for reuse and recycling.

The steel was stripped of almost all concrete, then grouped into long bundles or compact balls. The machines sorted all other material for easy removal. ODOT directly reuses products whenever possible by identifying opportunities to give construction byproducts a new life without any additional processing. This careful forethought helps reduce the energy required to get new life out of existing materials. Some of the steel and concrete went to recyclers to be crushed and made into gravel, while the rest was used as fill by local gravel mines. All told, ODOT removed about 30 million pounds of demolished material from the project site.

The team demolished the decommissioned bridge over the river to the level of the work bridge in less than two months. However, a big part of the deconstruction remained, both literally and figuratively. Five piers weighing more than 200,000 pounds each remained below the level of the work bridge, mostly submerged in the river.

To remove the piers, the demolition crew used a diamond-impregnated wire saw—a thin, ultra- sharp blade that wraps around the base of the piers—to cut them off flush with the riverbed. Heavy-duty cranes then attached to the piers, via bolts inserted through three-inch diameter holes, and lifted all 1 million pounds of concrete and steel onto the bridge deck to be broken down by an excavator equipped with a demolition hammer.

This process is a vast improvement over earlier demolition methods, which required building an extensive cofferdam to reroute the river, then demolishing the structure on the riverbed’s mud.

Even as this initial process draws to a close, project team members are already hard at work preparing for the next step of the project. In a couple of years, after constructing and opening the new bridge, they will disassemble the work bridge and then reuse most of the steel and wood to build another work bridge before repeating the demolition process on the current detour structure.

In the near future, the team will start building the foundation for the arch ribs at the point where the new bridge’s gracefully curving arches will touch down in the river. These massive concrete arches will serve as the strength-providing frame for the entire bridge, which is about 800 feet long where it crosses the river. The ribs will rise from eight-foot-diameter shafts drilled into the riverbed and be constructed using cast-in-place continuous pours.