The most expensive mistake we see on Quebec City road projects is designing a granular base thickness for Montreal conditions. That 600 mm section that works fine in the St. Lawrence Lowlands fails spectacularly here after two freeze-thaw cycles. The Champlain Sea clays underlying much of the region don't just heave—they lose 60% of their bearing capacity during spring thaw when the ice lenses melt. We've pulled pavement cores on boulevard Laurier that showed 80 mm of rutting after three winters because the CBR road testing was done in August instead of April. A proper flexible pavement design for Quebec City has to account for the full -30°C to +30°C temperature swing, the frost penetration depth that reaches 1.8 m in exposed areas, and the fact that your subgrade moisture content in Sainte-Foy is completely different from what you get in the Limoilou basin. Our approach integrates the AASHTO 1993 method with MTQ's empirical adjustments for northern climates, then validates everything with plate load testing on the prepared subgrade before we place the first lift of asphalt.
A pavement section that ignores the Champlain Sea clay subgrade will show terminal rutting before the first maintenance cycle—usually by year three.
