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

Your contractor's 6-inch recommendation is likely insufficient for Calgary's expansive clay soils, especially in newer communities like Cranston. For concrete driveways on clay in the Calgary area, 8-12 inches of properly compacted granular base is typically recommended to prevent cracking and settling.

Clay soil challenges in Calgary are particularly severe due to our bentonite clay that expands dramatically when wet and shrinks when dry. Cranston, being a newer southeast community, sits on these challenging soils that require extra attention for any concrete work. The freeze-thaw cycles from our chinook weather patterns make proper base preparation even more critical — temperatures can swing 20-30°C in a single day, creating extreme stress on inadequately supported concrete.

The ideal base system for your concrete driveway should include 8-12 inches of Class II crushed gravel (3/4" minus) compacted in 4-inch lifts to 95% standard proctor density. This material provides excellent drainage and load distribution. Some contractors prefer a two-layer system: 6 inches of Class II topped with 2-3 inches of Class I (road crush) for the final grade. The key is proper compaction — each lift must be compacted with a plate tamper or vibratory roller before adding the next layer.

Drainage considerations are crucial on clay soils. Your base should extend 6 inches beyond the concrete edges and slope away from structures. Consider installing weeping tile along the edges if drainage is poor, as standing water will cause the clay to expand and heave your driveway. The concrete itself should be 5-6 inches thick with 30MPa strength and air entrainment for freeze-thaw resistance.

Professional guidance is essential here because inadequate base preparation on clay soils almost guarantees future problems. A structural engineer or experienced concrete contractor familiar with Calgary's soil conditions should assess your specific site. The extra cost of proper base preparation (typically $8-15 per square foot) is far less than replacing a failed driveway in 3-5 years.

Next steps: Get a second opinion from a contractor experienced with clay soils, request soil compaction testing, and don't compromise on base thickness. The Alberta Building Code and good engineering practice support the 8-12 inch recommendation for your soil conditions.