Microtunneling Bridges Bascule Towers in Seattle

Northwest Boring is an experienced contractor that has seen microtunneling projects come in all shapes and sizes. The Woodinville, Wash.-based contractor recently completed a microtunneling project that wasn’t large in scope but critical to the progress of a $97 million bridge replacement project going on in Seattle.

King County, Wash., in the process of constructing a new bascule bridge over the Duwamish Waterway, replacing its existing 80-plus year-old bascule bridge and bringing the new structure a modern design and state-of-the-art technology to operate it. Microtunneling played a key role in making that state-of-the-art technology operational.

“It was a unique project for us, requiring special accommodations in order to bring the microtunneling work to its successful completion,” said Northwest Boring vice president Dennis Molvik.

The original South Park Bridge was built between 1929 and 1931 and spans the navigable channel of the Duwamish Waterway, which is used for industrial commercial and recreational purposes. The south side of the bridge is home to the South Park community and the bridge serves as a connector to the north side of the waterway, which includes King County International Airport — also known as Boeing Field — and substantial Boeing property.

The existing bridge was closed in June 2010 due to its severe deterioration and vulnerability to earthquakes. The original foundation piles were not driven deep enough, making the bridge susceptible to settlement and resulting in the tilting and cracking of the main piers. The mechanical and electrical systems that operated the bridge were unreliable and required frequent repairs. With lanes narrower than standard lanes used on modern bridges today, the bridge carried up to 20,000 vehicles daily, with 14 percent being truck traffic.

“The bridge is a very high trucking route that connects the industrial core [of the area],” said Jim Markus, King County managing engineer of bridges and structures.

The replacement bridge, whose construction began in May 2011, is located downstream and parallel from the existing bridge between 14th and 16th Avenues S. The new bridge will meet current structural, seismic and traffic standards and includes four lanes and 8-ft sidewalks. New bicycle lanes will be built on the roadway shoulders and the sidewalks will be separated from the roadway by a traffic rail.

State-of-the-art mechanical and electrical drive systems will substantially improve the bridge’s operation. To this end, state-of-the-art technology was needed to connect the two bascule towers on either side of the bridge for proper operations — i.e. raising and lowering the leaves.

Microtunneling’s Role
When the bridge was originally built, crews didn’t have a way to electronically connect the two bascule towers other than simply dropping the cable in the water. “In the old days before [microtunneling technology], they used to just drop a cable down from one side of the bridge to the other, just laying it on the riverbed,” Markus said. “There were problems associated with that and it was vulnerable to damage if a ship came through at low tide. At times you would have to replace the cable. This is a more modern way to protect the cable so the telecom and power from one tower to the other are better protected.”

Northwest Boring, a leading trenchless contractor based in the Pacific Northwest of the United States, was subcontracted by general contractor Kiewit-Massman, a joint venture, to microtunnel 26-in. OD steel casing between the two bascule tower’s caissons, located on the north and south sides of the Duwamish Waterway. The tunnel that needed to be microtunneled was 194 ft in length and about 45 ft below the water table of the river.

Critical Role for Small Tunnel
This part of the project was relatively small in scope but critical to moving the project forward, said Markus. The caissons, which are 60-ft by 60-ft concrete box foundation structures that support each of the bascule towers, were constructed in 20-ft sections, the inside of each dredged and then lowered into the riverbed. The launch and retrieval shafts for the microtunneling work would be built  from inside these  caissons, referred to as Pier 3 (south side) and Pier 4 (north side). Once the casing and later the power and communications conduit are installed, each caisson is capped and the bridge project can move to the next phase.

According to Kiewit-Massman project engineer Enrique Nunez, microtunneling was the only technology that could complete this vital portion of the project, noting other trenchless methods such as auger boring of horizontal directional drilling couldn’t be used given the logistics and location.

Molvik said this project presented a slew of challenges for his crew in planning and execution. “We have never microtunneled from caisson to another in the water like this before,” Molvik said. “Typically you go under a river or under a waterway from one shore to another. Here, we were actually out in the water for not only the launch but the retrieval. This is the first time we had done a project like this.”

Northwest Boring set up its Herrenknecht AVN 500XC MTBM system on Pier 3; its Brandt separation plant, bentonite mixer, generator and tool containers were set up in a shore staging area. The launch seal was approximately 50 ft down in the launching shaft. Setup took 10 days prior to the launch on Oct. 31, 2012.
Northwest Boring had several challenges in this project, most notably making sure the microtunnel was on target as there was just one location that the crew could tunnel in on the caissons. VMT GmbH provided surveying for the project and proved to be invaluable partners. “Because of VMT’s extremely accurate survey, we were able to hit our target right on,” Molvik said.

There were also environmental concerns that had to be considered. The Duwamish Waterway is part of a U.S. EPA SuperFund cleanup site, requiring that all involved take measures to minimize the disturbance of contaminated sediment during in-water pile work and use containment systems to keep all construction debris and runoff out of the river. In Northwest Boring’s case, that kept its soil separation plant on shore and encasements were built around its slurry lines to protect from any leakage. The microtunneling was done below the contaminated sediments, avoiding any issues there.

Molvik said once the microtunneling began, everything was routine and took nine days to tunnel out of Pier 4. One of the challenges crews had was controlling the limit the MTBM could push against the caisson. “We couldn’t push more than 100 tons against the wall due to its design,” he said. “however, we were able to keep our jacking pressures relatively low and therefore that didn’t come into play as an issue.”

“Once we started microtunneling, everything was pretty routine,” Molvik said. “It was a ready, aim and fire challenge for us — the ‘ready’ was handling the site logistics, the ‘aim’ was the accurate survey of our target and ‘fire’ was the launch.”

The new South Park Bridge is expected to be partially opened to traffic during the first quarter of 2014, with total project completion by May 2014.

Sharon M. Bueno is managing editor of Trenchless Technology.