What is the best gravel base depth for a concrete driveway on clay soil in Cranston? My contractor said 6 inches, but I heard 12 inches is better?

Your contractor's recommendation of 6 inches is likely adequate for most Calgary residential driveways, but 12 inches would provide superior long-term performance on Calgary's challenging clay soils. Given that Cranston sits on expansive clay that's notorious for freeze-thaw movement, the extra investment in base depth is often worth it.

The standard approach in Calgary calls for 6 inches of compacted granular base (typically 3/4" minus crushed gravel) under a 4-6 inch concrete slab. This meets Alberta Building Code requirements and works well for most residential applications. However, Calgary's expansive bentonite clay soils present unique challenges that make a deeper base beneficial. These clays swell dramatically when wet and shrink when dry, creating significant ground movement that can crack concrete over time.

A 12-inch granular base provides several advantages on clay soils. The deeper aggregate layer creates better drainage, reducing water infiltration to the clay subgrade. It also provides a more stable platform that better distributes loads and minimizes differential settlement. The extra 6 inches acts as a buffer zone between the concrete and the problematic clay below. Many experienced Calgary contractors recommend 8-12 inches of base on clay soils, especially for driveways that will see heavy vehicle traffic.

Cranston's specific conditions make this even more relevant. The community was built on former agricultural land with significant clay content, and many homeowners have experienced foundation and concrete issues related to soil movement. The area's relatively new development means the clay hasn't fully stabilized from construction disturbance. Additionally, Cranston's location means exposure to Calgary's extreme chinook temperature swings, which accelerate freeze-thaw cycling in saturated clay soils.

Cost considerations for the upgrade are reasonable. The additional 6 inches of gravel typically adds $3-6 per square foot to the project cost - so roughly $600-1,200 extra for a typical two-car driveway. Compare this to the $8,000-15,000 cost of replacing a failed driveway in 5-10 years, and the investment makes financial sense.

Professional guidance suggests discussing soil conditions with your contractor. A geotechnical assessment isn't typically required for residential driveways, but if you're seeing clay soil that gets sticky when wet and cracks when dry, that's expansive clay. Proper drainage around the driveway perimeter is equally important - ensure positive drainage away from the concrete and consider installing weeping tile if water tends to pond near the driveway area.

Your next steps should include asking your contractor about upgrading to 8-12 inches of base, ensuring proper compaction in lifts (don't dump 12 inches at once), and discussing drainage details. A reputable contractor will understand Calgary's soil challenges and be willing to explain their reasoning for the 6-inch recommendation versus the benefits of going deeper.