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Originally Posted by maccoinnich
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An underwater construction site
By: Inka Bajandas
West Coast Contractors crews pump concrete under groundwater at the LOCA mixed-use development site in Southeast Portland (Sam Tenney/DJC).
Before a once-vacant Southeast Portland lot served as pasture for a herd of goats and became the future site of a mixed-use superblock development, it was a pond.
In the late 1800s and early 1900s, water from the dammed-up Asylum Creek covered a large portion of the site, bounded by Southeast Belmont and Taylor streets and 10th and 11th avenues. The pond and creek were then buried in a series of fill projects intended to promote more development on the east side of the Willamette River.
Now the site is under water again. Andersen Construction crews excavating the 385-spot below-grade parking garage for Killian Pacific’s LOCA project over the past month have exposed and are working in and around millions of gallons of groundwater formerly hidden beneath the former pasture.
“It’s a filled-in lake, so that’s a problem,” said Stuart Albright, geotechnical engineer for the project and principal with environmental engineering firm Apex. “Really we’re just undoing what they did after the turn of the previous century when the city filled it in.”
Planning around the large amount of groundwater has been one of the biggest project challenges, he said. Anticipating how the water would impact construction involved identifying which portions of the site were once covered in water by examining old land plat maps. Wells drilled on the site also helped paint a picture of the location and depth of groundwater.
“We basically knew there was a pond here before we even started working on the site,” Albright said. “We relied heavily on the existing information. All of that data factored into us making an informed decision for where we thought the water would be.”
He learned that the pond once covered the Goat Blocks’ eastern side. It provided drinking water to a nearby mental hospital and the city of East Portland. That use was eventually abandoned after East Portland became part of Portland in the late 1800s and the cities combined drinking water systems.
“At the time it was filled, nobody was using it and people were dumping garbage into it,” Albright said. “It was probably not very pleasant, which is probably why it was filled in.”
Draining site not an option
Normally, a construction site where groundwater is exposed is drained first so that crews can work on a dry surface, but the city sewer system surrounding the Goat Blocks site wouldn’t be able to handle the large amount of water seeped up during excavation, said Chris Copeland, a senior project manager with Andersen Construction.
“The project was kind of conceived by Killian with the knowledge that there was no place close to dump the water to do this the standard way,” he said. “We didn’t have any choice except to get creative for how we were going to do this … We had to come up with a scenario to contain that water.”
Draining the whole site would also be expensive because the city of Portland charges a fee for dumping groundwater into its sewer system, Albright said.
“There was a time when we would drain all this water, but the city charges for it so everybody wants to figure out where the water is and get around it,” he said. “It changes how we deal with (ground)water in the city of Portland. Very few clients want to pay for draining.”
Underwater construction
The Goat Blocks project team has devised a way to work around and in all that water. As excavation of the parking garage continues, construction crews are containing the groundwater in pools, ranging from 15 to 18 feet deep, using a steel sheet piling system. The metal walls are typically used to retain either water or soil.
“We’re essentially building underwater,” Killian Pacific Vice President Noel Johnson said. “Once you build it, you pump the water out.”
Project subcontractor West Coast Contractors, a Coos Bay-based firm that specializes in bridges and marine facilities, has assisted with this underwater work. West Coast crews used a boat to traverse the groundwater and check the level of the terrain underneath using grade rods. This helped a long-range excavator operator digging the flooded parking garage space, Copeland said.
A lengthy concrete pour that started Monday is taking place with the water still in place using a type of concrete that cures in wet conditions. Crews will pump concrete using hoses under the water to form a 6-foot-deep plug that after curing in about a week should prevent more groundwater from seeping into the project site, Copeland said.
“It’s the same technology that you would use to pour a bridge pier in a river,” he said.
Once the concrete has set, crews will pump out the remaining water and the construction site should be essentially dry within the next couple of weeks, Copeland said. Crews have pumped out about 5 million gallons of water already, he said.
Although the project team was expecting a lot of groundwater to come up during the excavation process, Copeland said that in some cases it was deeper than anticipated.
“When you expose this water, the most interesting thing to me is it’s not all at the same level,” he said. “It’s very dynamic. It’s very fluid. (You have to) change your methodology as you go.”
A watertight parking garage
The LOCA below-grade parking garage is designed to sit just slightly above the typical groundwater level, Albright said. If the water were to rise above that level, it would flow into and be drained out of the garage, he said.
Johnson said Killian Pacific has owned the Goat Blocks site for a long time, and the Vancouver, Wash.-based developer learned of the underground pond about 10 years ago.
“We did a lot of testing and drilling test pits even when the goats were still there,” he said.
The amount of groundwater on the site didn’t come as a surprise because that’s typical for projects involving excavation in the surrounding Buckman neighborhood, Johnson said.
“That’s the reason there aren’t a ton of basements in that whole area of town,” he said.
But building below ground level was necessary for the superblock development, which will include 247 apartments and grocer Market of Choice and Orchard Supply Hardware as anchor retail tenants, he said. The development is expected to be finished by early 2017.
“The entire foundation is built like a ship, where it keeps the water out,” Johnson said. “We spent a lot of good engineering money figuring out how to build it given the amount of water.”
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