April 16, 2010

TERRATECHNIK

A special truss system had to be installed to allow excavation of contaminated soil inside the building.

FEATURE | Demolition/Environmental Engineering

Terratechnik Environmental shares details of soil remediation project in Mississauga, Ontario

PETER KENTER

correspondent

If it was baseball, Terratechnik Environmental would be the equivalent of the bull pen, called in to save the win.

The analogy isn’t a stretch: An environmental assessment can make or break a real estate deal, particularly on industrial land with soil contamination.

Making the save is critical for all parties and that’s just what environmental remediation contractor, Terratechnik of Mississauga did recently.

“It was an industrial facility in Mississauga,” says Jean-Pierre François, operations manager. “The client had a buyer, but the original assessment ordered by the seller indicated no soil contamination. Bore holes drilled during a second assessment ordered by the purchaser found a problem.”

The soil underneath the building was impacted with chlorinated solvents (trichloroethylene and cis-1,2-dichloroethylene) as well as chemicals associated with the breakdown of those materials.

“During industrial activity several decades ago, the facility had used an old processing area with sump pits and approximately 150 metres of associated sub-floor trenches,” says François. “The consultant’s report identified this former processing area as the source of contamination.”

A further environmental assessment indicated that the site contained highly impacted soil, rock and groundwater. The contamination was located across a 1,025-square-metre footprint and ranged in depth from four metres to eight metres below ground surface.

Terratechnik was contracted to remediate the soil beneath the building in a project budgeted at $1.2-million. The project design called for removal of a significant portion of the floor.

“Often, we can approach this type of project by underpinning the columns, but at this depth it would be rather cumbersome and require lateral support of the columns as well,” says Kyle Dacey, manager technical services. “We knew we would be going down seven or eight metres at least, but what we didn’t want is to start the work for that depth, then find later that we needed to go down to 10 or 11 metres.”

After a review of options, Terratechnik and its structural engineer developed a truss system that would support eight interior columns, beams and the roof, transferring that load to three interior columns and another three along the wall. The design allowed almost unrestricted excavation of soil and fractured rock below the footings, at whatever depth necessary.

The approach proved to be prescient. As impacted soil was uncovered, the contractor found plumes of chemical extending to 11 metres below the surface.

“One of the challenges of such a project is that when you’re chasing chemicals into the ground, you don’t necessarily find them going where you think they’re going to go,” says Dacey.

Once the contractor removed about two-thirds of the floor, the building owner requested the remaining floor also be removed to allow for an entirely new concrete floor to be added in a single pour.

The soil was excavated using two excavators; a 20-tonne and 30-tonne model. Clean soil was segregated from impacted soil, which was moved to an on-site treatment pad for processing.

Sample soil was collected for bench tests to determine the optimum treatment strategy.

TERRATECHNIK

Some 6,000 tonnes of soil were treated and certified for reuse.

To break down contaminants, a customized blend of oxidants was mixed into the soil using a mobile soil processing plant. The treated soils were covered to prevent soil migration and left for one month. All told, 6,000 tonnes of soil were treated.

“At that point, a third-party lab verified the soil had been treated to meet Ontario Ministry of Environment criteria so we could use it as backfill,” says François.

On completion of the excavation the backfill was seeded with strong oxidizers to reduce the potential for rebound contamination that may occur in groundwater. Injection systems were also utilized to target areas inaccessible to excavation with additional oxidizing agents.

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