Citizen columnist Kelly Egan
Photograph by: The Ottawa Citizen , The Ottawa Citizen
OTTAWA — We learn from Saturday’s paper that carbon dioxide, the main greenhouse gas, is at its highest concentration in the atmosphere in three million years.
Over shouts of “We’re No. 1!”, it might be opportune to mention an unexplored consequence of climate change: our houses are cracking up.
Ottawa is built, with some exceptions, on a vast stretch of what the experts call “sensitive clay soil”, usually referred locally as leda, the old basin of the Champlain Sea.
During a prolonged drought in an urban area, here is the shorthand version of what happens: the band of clay dries out and shrinks, thirsty trees go deeper and further looking for water, buried voids begin to form, the footings of houses begin to slip and sag, and the hapless homeowner arrives home one day asking: “That’s funny, wonder why the front door is sticking?”
Here is a more scientific description from Canada Mortgage and Housing Corp.
“Sensitive clays have a higher proportion of water among their small particles. These tiny plate-like particles are arranged like a house of cards, holding each other up when the spaces between them are filled with water.
“When there is a severe loss of water, they collapse, leading to a reduction of soil volume and soil shrinkage.”
With our summers getting hotter and drier, where is this all headed?
Michael O’Neill is general manager of Ottawa Structural Residential Services Ltd., a company formed in 1979. Stories of suddenly-appearing cracks in walls, foundations and brick work are becoming old hat.
“It’s a huge problem in the Ottawa region, there’s no question about it,” he says. “Last year, we saw an alarming increase of inquiries into the office, starting around mid-July.”
No one is sure of precise numbers (thousands, surely?), but very dry years have brought on spikes in complaints from areas like Orléans, which has an abundance of clay, or Greenboro, which experienced a major problem with soil shifting about 12 years ago.
“From 1999 to 2004, it was just non-stop, inquiry after inquiry. It was unbelievable and it was widespread across the city.”
In conventional construction, a house foundation sits on a concrete footing (hopefully poured on solid ground) that distributes the weight load from the walls. When pockets of air or weak soil develop beneath the footing, it may sink slightly, putting stress on the basement wall and possibly causing “step-cracking,” which thereupon stresses the upper walls.
“The funny thing is, it can happen instantly,” said O’Neill. “People will say I was gone for weekend to the cottage, I came home and had to hip-check my door open.”
Generally, he added, mature trees are almost always involved. This creates a public policy dilemma for the City of Ottawa, as the city has tens of thousands of trees, deliberately planted on front lawns, that now potentially damage private property.
This was at play in Greenboro, where a geotechnical firm, Sarafinchin Associates, was hired to examine why so many foundations and even roads were cracking in 2001. It found that year was the sixth-driest since 1960, with annual precipitation 118 millimetres below average.
“The contributing factors to soil shrinkage settlement were found to be climate change, urbanization and deep rooted vegetation,” reads a summary on the firm’s website.
News reports from the day said the city was exposed to millions in liability (due to its tree-planting and site approval process), but the Citizen was unable last week to find out the final price tag.
O’Neill, meanwhile, said a good part of their business is repairing cracked foundations, which is a messy, costly operation. It involves digging down to the footing, then reinforcing with poured concrete.
There is a way to prevent this, of course: frequent watering of the lawn, especially if a large tree is present. And don’t wait until August, says O’Neill, when cracks begin to appear.
He himself once had a water bill in the $300 range, he offered, but this is better than a $40,000 repair bill to the foundation or ripping out a lawn full of dead grass.
The species of tree also matters to the so-called “zone of influence” created by the root system. The CMHC document says the lateral reach of roots is usually two to four times the height of the tree and most of it occurs in the top 30 centimetres of soil.
Among thirstier trees are red, silver and Manitoba maples, poplars and burr oak, while those requiring less water include pine, spruce, fir and honey locust.
And despite the weekend rain and what felt like a snowy winter, precipitation to date in 2013 is still almost 100 millimetres below average.
All of which gives us this forecast: hot, dry and cracked.
To contact Kelly Egan, please call 613-726-5896, or email firstname.lastname@example.org“>email@example.com
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