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Impact rammed aggregate piers make for a solid foundation

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by Jessica Krippendorf

Impact rammed aggregate piers will make sure a third digester at Greater Nanaimo Pollution Control Centre (GNPCC) is on solid ground, despite less than ideal soil conditions.
rendering of pollution control centre facility in Nanaimo
rendering of pollution control centre facility in Nanaimo

Impact rammed aggregate piers will make sure a third digester at Greater Nanaimo Pollution Control Centre (GNPCC) is on solid ground, despite less than ideal soil conditions.

The 17-meter-high reinforced cast-in-place concrete digester will be about 20 meters in diameter.

It uses heat to break down solid waste into an inert form before a dewatering centrifuge separates the solids and water.

“It is basically a sealed, reinforced concrete cylinder tank that breaks down the sludge,” said Sean DePol, manager of wastewater services for the Regional District of Nanaimo (RDN).

The project will include the installation of a gas collection unit on top and a linear motion mixer – a shaft with a disk-shaped apparatus on the end of it that uses the floor of the digester to create a rebound motion for mixing.

Soils on the site consist of deposits of sandier soils over fine-grained silty soils.

Because of the variability and no real set bedrock level, the project will use impact rammed aggregate piers to densify the soil, said David Lycon, senior wastewater engineer with Aecom and project manager for the upgrade.

The impact rammed aggregate pier system was developed by Geopier Foundation Company in North Carolina.

Cavities for the piers are created to full depth – seven to 35 feet – using a large static force augmented by vertical impact energy to drive a mandrel and tamper foot into the soil.

The tamper foot varies from 12 to 16 inches in diameter.

Aggregate is placed inside the mandrel and it is lifted about three feet and then driven back down two feet with the bevelled tamper, forming a one-foot-thick compacted lift.

The tamper foot displaces aggregate laterally into the sidewalls of the cavity, while the hammer densifies the material vertically, creating a column of gravel and displacing the soil below.

“Literature has documented that impact rammed aggregate piers are better suited in more fine-grained soils, such as fine grained sands, silts and clay deposits by creating a stiff soil column and increased drainage to mitigate soil liquefaction,” said Don Kaluza, senior geotechnical engineer and associate for Levelton Consultants Ltd..

The technique also causes smaller surface vibrations compared with other ground improvement methods, minimizing the effect on existing foundations.

The number of piers needed to support the 1,110-square-meter addition depends on the spacing, which is determined at the beginning of construction through test sections, said Kaluza.

“Typical spacing is in the order of 1.5 to two meters,” he added.

The diameter of the piers will be in the order of 750 mm and installed 12 to 14 metres below grade.

Work on the project includes auxiliary equipment, mainly related to the methane gas used in the adjacent co-generation plant.

The GNPCC uses waste methane from its digesters to power a 2.9 annual megawatt hour electricity generation system on site.

The $7.8 million general construction contract is currently out to tender and work is expected to start this summer.

The timeline for construction is 12 to 18 months, including ground improvements and a conveyance system and pipe chase.

Work on Digester 3 precedes the plant’s transition into a secondary treatment facility.

Detailed design work for that job should wrap up in 2012 and construction is set to begin in 2013.

The budget for the secondary treatment upgrade is around $50 million.

In primary treatment, sedimentation tanks slow the velocity of the waste water and the solids that settle out.

The secondary treatment will use another biological process and will require similar circular tankage used as secondary clarifiers, said DePol.

“The upgrade will require four additional tanks and four bio-reactors,” he said.

“Two additional circular tanks approximately 10 meters in diameter will be used as gravity thickeners to take the sludge from the bottom of the sedimentation tanks prior to going to the digesters.”

The preliminary design of the secondary treatment facility will be sole-sourced to Aecom, which has a three- to five-year contract with the regional district for all wastewater developments.

The detailed design will go to competitive bid.

The GNPC will also receive a new pump station and electrical upgrade, and some replacement pumps over the next two years with a $2 million budget.

The pumps and electrical for the pump station were implemented in1973.

A two-kilometre-long marine section outfall replacement is also scheduled for 2012, after the RDN made its second repair in two years to an effluent discharge line that runs from the plant to the Strait of Georgia.

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