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

Many areas of Ottawa have been built onsensitiveclay, and the same can happen atTewin, 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 councilthis month, adds hundreds of hectares for a denseurban 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 thatneighbours have paid for costly fixes afterfoundations shifted, while new homes need to be surrounded by a light material akin to Styrofoam, rather than heavier fill.

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

But a senior geotechnical engineerwith Paterson Group, which was hired byTewin 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'sgeotechnical 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."

Proper technique already used in Ottawa

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

Highrises can be built ifhundreds ofsteel tubesare driven through the ground to bedrock and filled with concrete. Thattechnique is often used in the city, Gilbert said,pointing to newer buildingsat 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 shortergeotechnical 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 studyand analysis is to come.

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Tewin land owners also provided CBC News witha differentfour-page preliminary report done for Taggart Investments by GolderAssociates Ltd. in November 2020. In the report, Golder reviewed its own data from investigations of the areabetween 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 notresolved the soil question themselves, however, saying there wasn't the time for such comprehensive studies.

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