Building on Tewin's clay soil won't be a problem, engineer vows

Many areas of Ottawa have been built on sensitive clay, and the same can happen at Tewin, according to an engineer hired by the group that wants to build the city's fourth suburb.

Comprehensive studies in the Tewin area to come after official plan approval

The future suburb of Tewin will be built in Ottawa's rural southeast, and while existing residents have expressed concerns about the clay soil, one senior geotechnical engineer involved with the project doesn't share their worries. (Francis Ferland/CBC)

Many areas of Ottawa have been built on sensitive clay, and the same can happen at Tewin, according to an engineer hired by the group that wants to build the city's fourth suburb.

The new official plan, to be voted on at city council this month, adds hundreds of hectares for a dense urban community in the rural southeast, where soil conditions have been talked about for decades

Residents on Piperville Road have questioned the cost and feasibility of building in the countryside, far from transit and pipes and on poor soils.

They know well that neighbours have paid for costly fixes after foundations shifted, while new homes need to be surrounded by a light material akin to Styrofoam, rather than heavier fill. 

The City of Ottawa hasn't studied the soils in the Tewin area in the eight months since February's controversial council vote.

But a senior geotechnical engineer with Paterson Group, which was hired by Tewin landowners Taggart Investments and the Algonquins of Ontario (AOO), says he's not overly concerned.

"Yes, development over clay deposits is something we have to be cautious of," said David Gilbert, director of Paterson's geotechnical division, a company that has worked with Ottawa's biggest developers.

"But I'll say this: the designers in Ottawa are very aware of clay deposits, and Tewin doesn't pose any significant concern for me."

This map in the proposed new official plan captures the areas that will be added within Ottawa's urban boundary. The area in orange forms the new Tewin suburb. (City of Ottawa)

Proper technique already used in Ottawa

Gilbert pointed to neighbourhoods in south Orléans, Riverside South and Kanata that have the same clay from the Champlain Sea that covered the Ottawa area thousands of years ago.

Highrises can be built if hundreds of steel tubes are driven through the ground to bedrock and filled with concrete. That technique is often used in the city, Gilbert said, pointing to newer buildings at Petrie's Landing in Orleans. 

Midrise buildings, meanwhile, can be built on "raft foundations" where solid concrete extends from one side of the building to the other, he said.

So far, Paterson Group has not produced a full-blown report on Tewin, just a much shorter geotechnical summary for Taggart Investments.

"Our knowledge is not light. We do have a significant number of bore holes out here, so this is something we're very confident with," Gilbert said, adding more study and analysis is to come.

Tewin land owners also provided CBC News with a different four-page preliminary report done for Taggart Investments by Golder Associates Ltd. in November 2020. In the report, Golder reviewed its own data from investigations of the area between 1973 and 1975, and described how taller buildings would need deeper foundations.

At the Feb. 10 council meeting, councillors voted down a move to give staff years to study the Tewin area. Mayor Jim Watson said it would be an "abdication of leadership" to delay the urban boundary expansion, and instead described the "extra diligence" that would take place at Tewin before the official plan was approved. 

City staff have not resolved the soil question themselves, however, saying there wasn't the time for such comprehensive studies.

They have focused on mapping the area, developed a long list of future studies to be done at Tewin, and come up with a financial arrangement so that the suburb pays for itself.


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