What gravel base is needed under a concrete driveway in New Brunswick?

A concrete driveway in New Brunswick requires a minimum 6-inch (150 mm) compacted granular base of crushed stone or bank run gravel — and 8 inches is a better target, particularly on clay subsoils or sites with drainage challenges. The base is arguably as important as the concrete itself: a properly prepared base provides uniform support, drains water away from the slab, and prevents the differential movement that causes cracking and settlement.

Why the base matters so much in NB: New Brunswick's freeze-thaw climate puts extraordinary stress on concrete driveways. When the ground freezes, soil moisture expands and can push sections of the slab upward (frost heave). When it thaws, sections can settle back unevenly. A well-draining granular base resists frost heave because water drains away rather than accumulating under the slab where it can freeze and expand. Clay soils — common in parts of Moncton, Dieppe, Fredericton, and river valley communities — retain moisture and are particularly prone to frost heave; a deep granular base over clay is essential.

Granular base material specifications: The base material should be crushed gravel or pit-run gravel with particles ranging from fine sand up to 19-25 mm (3/4 inch) maximum particle size. This gradation compacts well and drains effectively. Avoid using dense clay-like fill, topsoil, or uniformly fine sand as base material — these either do not drain or do not compact adequately. Crushed limestone is excellent where available in NB; bank run gravel sourced from local pits is the most common and economical option.

Compaction is mandatory. Loose gravel is not a base — compacted gravel is. The granular material must be compacted in lifts (layers) of 4-6 inches maximum using a plate compactor or roller. For a typical driveway, this means compacting each 4-inch layer before adding the next, until the full 6-8 inch depth is achieved. On top of the finished compacted base, the surface should be relatively smooth, free of voids, and firm under foot with no soft spots. A proper compaction test (Proctor test) is used on commercial projects; for residential work, an experienced contractor can judge compaction by walking the base and checking for give.

Geotextile fabric between the clay subgrade and the granular base is an excellent addition on NB sites with clay or silt subsoil. The fabric prevents clay particles from migrating up into the gravel over time (a process called pumping that gradually degrades the base), extending the effective life of the base significantly. Fabric costs $0.30-$0.80 per square foot and is a worthwhile investment on challenging soil sites.

Base thickness on different NB soils: On good granular subsoil (sand and gravel), 6 inches of compacted base is adequate. On clay subsoil (common in Moncton, Dieppe, and along the Saint John River valley), 8-10 inches of compacted gravel base with geotextile fabric is recommended. Your driveway contractor should assess the existing subgrade during the excavation phase and adjust accordingly.