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Ground freezing technology builds in unshakeable stability

By Heather Hudson

Question: What ingenious construction technique has been around since the 1800s but is reserved for only the deepest, darkest jobs? 

Answer: Ground freezing.

Primarily used to provide ground support, groundwater control or structural underpinning during construction, the technology is mainly used for mine shafts. But its simple, virtually fail-proof design has made it a more popular option for civil projects in recent years.

“Ground freezing is still used a lot in mining, but we’re seeing it used for civil work like water tunnels,” said Joe Sopko, director of ground freezing at Moretrench, a nationally renowned geotechnical contractor based in Rockaway, N.J.

What is ground freezing?
Ground freezing converts pore water to ice by circulating chilled liquid via a system of small-diameter piles in drilled holes. The ice fuses the soil or rock particles together, creating a frozen mass of improved compressive strength and impermeability. This is done in difficult, disturbed or sensitive ground.

The ground freezing process uses a series of drilled freeze pipes and large refrigeration plants to convert existing pore water in the soil to ice, creating a strong, watertight frozen earth material similar to rock or concrete. The frozen ground acts as an excavation support system requiring no bracing, tiebacks or additional shoring. It has been used extensively for shafts extending over a thousand feet deep.

Its impermeable nature eliminates the need for dewatering, making the technique practical for large groundwater barriers. These barriers can be used to reduce or eliminate flow into excavations and contain contaminants or restrict the flow of contaminated plumes.

The water that goes in is comprised of 32 per cent calcium chloride to ensure it will freeze effectively. While there are other coolants that could be used, saltwater has been found to be the most cost-effective.

“There are some rare applications in emergencies where we use liquid nitrogen, but those are small jobs and we get down to about -200 degrees Celsius,” said Sopko.

Depending on how warm the ground is at the outset, how thick the wall has to be and how deep the excavation is, freezing can take between five to 10 weeks and take up to a year to thaw out.

Moretrench makes it work
Moretrench has been the North American industry leader in this technology since 1970. And while their techniques have evolved over the years, so has the equipment.

“The advent of the screw compressor was a big deal because it allowed you to get the brine folder,” said Sopko.

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Piling Canada is the premier national voice for the Canadian deep foundation construction industry. Each issue is dedicated to providing readers with current and informative editorial, including project updates, company profiles, technological advancements, safety news, environmental information, HR advice, pertinent legal issues and more.