While Keller is renowned throughout the world for its wide range of geotechnical solutions, perhaps less well-known is its expertise in groundwater behaviour and decontamination. A good example of this is on a multibillion-dollar regeneration project in Washington DC…
Earlier this year, Keller completed a wide scope of foundation works on the second phase of a redevelopment project in the southwest waterfront area of Washington DC.
The Wharf, as it’s known, is a 110,000m2 development that will include a marina, offices, residences and public spaces. The challenging project included jet grouting, tiebacks and a wide-range of piling techniques – but it also highlighted Keller’s significant expertise in site dewatering and treatment of contaminated groundwater. Dewatering of the site and treatment of the contaminated groundwater and high pH construction water were relatively low profile, behind-the-scenes services, but very complex and imperative to keep the job moving.
Keller first became involved with the project during the preconstruction phase when our expertise was sought for a tricky dewatering issue which also required addressing contaminated groundwater.
“With metro subway tunnels running through the north half of the site, the owner and design team asked us to design a dewatering system which would protect the exposed tunnels from movement caused by uneven groundwater pressure,” explains George King, Area Manager. They also wanted us to dewater the deep excavation within the perimeter groundwater cut off wall on the south side of the tunnels.
“Initially, we requested permission to discharge all of the site water into the combined sewer system which would ultimately treat the water at the municipal wastewater treatment plant, but this was denied by the water authorities. So everything had to be discharged into the storm-water system that empties into the Washington Channel, an estuary of the Potomac river.”
To meet strict surface-water discharge standards, this required treatment beforehand to remove contaminants such as iron, arsenic and pesticides.
Cost-saving sand filtration
Having worked on phase one of the project, George knew that levels of naturally occurring iron, which is severely high in particular areas of Washington, DC, would be the biggest challenge. The iron tends to precipitate out of the groundwater and build up rapidly in pipes, pumps, drainage structures, etc.
The solution that Moretrench (prior to Keller acquisition) developed about 10 years ago to address this was to inject a chemical iron sequestering agent into the pumping system. This keeps the iron in solution, allowing it to pass through the carbon and resin filters rather than plugging the filter beds of the treatment system.
But Keller water treatment expert Ed Carter had the idea of adding automatic back-flushing sand filters to the treatment train. This reduced the amount of expensive iron sequestering agent and bag filters used, saving the client over $20,000 a week.
“While bag filters are fairly standard, they’re pretty labour intensive and have to be replaced regularly,” he says. “The sand filtration system is self-cleaning and requires less sequestering agent, which at $28 per gallon is quite expensive.”
Typically, wherever you find iron you also find manganese, and this was above the authorities’ permissible levels. The team added a 12% sodium hypochlorite solution to the system to precipitate out the manganese so it could be easily filtered and removed. With levels fluctuating as the project progressed, the team had to carefully monitor and adjust the solution, a tough balancing act.
The team also had to be wary of iron biofouling. As Ed explains: “The iron can be incredibly pervasive and will over time dramatically reduce the efficiency of the dewatering system. If there are specific naturally occurring bacteria in the groundwater, they will feed off of the iron, grow rapidly and plug up a dewatering pump system completely in a matter of weeks or months. So, we submitted remediation plans to the authorities that would allow us to treat the dewatering wells in the event these conditions occurred. To ensure this didn’t impact the water discharge, we kept an eye on the water quality with increased sampling.
"As Keller geotechnical teams carried out jet grouting and installed tiebacks, this increased the pH level of the site water to 14 – well above the permitted discharge levels of six to nine. To overcome this problem, we injected sulfuric acid to bring the levels down. This pH neutralisation is a very common approach to treating site construction water, particularly where cement is introduced into the groundwater."
“Keller’s dewatering and treatment system has been in operation for almost a year now and we’re coming to the end,” adds George. “It’s been a large, complex project, but the feedback we’ve had from Balfour Beatty has been very positive. If we had any hiccups with the discharge water quality, it would have brought the job to a screeching halt. As it stands, thanks to our experience, the project never waited on water treatment, and it was all done very cost effectively.”