Quebec City's geological foundation rests heavily on the legacy of the Champlain Sea, which left behind thick deposits of sensitive marine clays across much of the Saint Lawrence lowlands. Designing shallow foundations here means confronting low undrained shear strengths — often below 30 kPa — and a frost penetration depth that routinely exceeds 1.5 meters. A standard prescriptive footing will not perform reliably in these conditions without careful geotechnical interpretation. The site investigation must quantify both the intact sensitivity and the remolded strength, because a disturbance during excavation can reduce bearing capacity by an order of magnitude. At depths where refusal is not reached, we often combine in-situ CPT testing with laboratory consolidation tests to calibrate settlement predictions, and when shallow bedrock is suspected within the project footprint, seismic refraction surveys help delineate the rock profile before the excavator ever arrives.
In the Champlain Sea clay, a 25% overestimate of the undrained shear strength will produce a settlement error of more than 40% — precision in the lab is not optional.
