Our Sensitive Clay

A recent article in the Ottawa Citizen reminds us of the hazards of building on what was once the bottom of the Champlain Sea. "Leda" clay, which covers much of the city's core, can liquify almost instantly in conditions of moisture and vibration.

"Ottawa’s notoriously unstable soil conditions have long challenged architects and builders and they may again be behind setbacks for at least one south-end construction project [this one?]. While digging into Ottawa’s soil has become a lot more predictable as emerging research maps where exactly the soft soil is and how it may respond to building weight, architects and engineers say it still presents challenges ..."

The image above is a detail from a 1982 Geological Survey map of soil types in the Ottawa area. Pink indicates sedimentary rock (shale, limestone etc.) at or near the surface. Green shows "ice age" glacial till while yellow and orange denote sand and sandy soil. Blue areas are covered in clay.

We can see that almost all of the downtown core between Bank and Elgin has been built on Leda clay, likewise much of the Golden Triangle, Sandy Hill, Old Ottawa East, the Glebe and Old Ottawa South.

I took this photo of 121 MacDonald (built c. 1900, Golden Triangle) in April of 2006. The 2½-storey wooden house sat atop a limestone foundation which, over the decades, sank unevenly into the clay soil, like a ship listing sadly to port. It was demolished shortly thereafter and replaced by a house with a lower centre of gravity.