How deep should the footing be for a retaining wall in NB?

In New Brunswick, retaining wall footings must extend below the frost line — a minimum of 4 feet (1.2 metres) below finished grade in southern NB and 4.5-5 feet in northern regions like Bathurst, Campbellton, and Edmundston. This is the single most important structural requirement for any retaining wall in the province, and it is non-negotiable.

NB's frost depth is among the most significant in Eastern Canada. When the ground freezes to depths of 4-5 feet each winter, any footing above that depth will experience frost heave — the upward displacement of the soil as the water within it expands by approximately 9% during freezing. A retaining wall footing that sits above the frost line will heave upward in winter and settle back in spring, repeating this cycle every year. Over 3-5 cycles, this movement tilts, cracks, and eventually topples the wall. The most common cause of retaining wall failure in NB is inadequate footing depth — walls that were built to save on excavation costs.

The footing must bear on undisturbed, native soil below the frost line, not on fill, topsoil, or organics. In areas of NB with significant organic soil layers — common in low-lying areas near Miramichi, the coastal communities of Northumberland County, and river floodplains near Fredericton — contractors may need to excavate deeper than the minimum frost depth to reach stable bearing soil.

For footing dimensions, standard practice for a residential poured concrete retaining wall is a footing that is twice the wall thickness in width and at least as thick as the wall thickness. For a typical 8-inch wall retaining 3-4 feet of soil, that means a 16-inch wide footing at least 8-10 inches thick. For taller walls (4-6 feet), footing dimensions should be designed by an engineer who accounts for the specific soil bearing capacity and lateral loads at your site.

In practical terms for NB contractors, this means most retaining wall projects require excavating 5-6 feet below the existing grade to reach frost-safe bearing depth and have room to form and pour the footing. This is significant excavation that usually requires a small excavator, which adds to project cost but is absolutely necessary for a wall that will last 30+ years.

One consideration for properties in Fredericton, Moncton, and Saint John with high water tables: the footing excavation will often encounter groundwater in spring. Proper drainage — a perforated weeping tile at footing level draining to daylight or a sump — should be planned alongside the footing depth. A footing sitting in standing water is at risk of erosion and freeze-thaw cycling even at frost depth.

Get matched with an experienced retaining wall contractor through New Brunswick Concrete — they will assess your specific site conditions and confirm the appropriate footing depth for your project.