Most of the Umpqua Valley sits on clay-heavy soil. Clay expands when wet and shrinks when dry, and that movement is what cracks concrete poured directly on top of it. The base layer is what isolates the slab from the clay and lets the concrete do its job.
What the soil does under your driveway
Through a wet Oregon winter, Umpqua Valley clay holds water and swells. Through a dry summer, it dries out and shrinks. The difference in volume is significant: a clay subgrade can move a quarter-inch or more vertically across the seasons. Concrete cannot flex with that movement. It cracks.
What proper base prep looks like
A proper base does three things: drains water away from the subgrade, distributes the slab load evenly, and provides a compacted, stable surface to pour on.
- Excavate to a uniform depth, removing organics and soft spots
- Install woven geotextile fabric on high-clay subgrade
- Lay four to six inches of compacted three-quarter-minus aggregate base
- Compact in lifts, not in one pass
- Final grade to slope, screeded flat before pouring
How to spot a bid that skips it
Cheap bids cut base prep first because it is hidden and easy to skip. Red flags:
- No aggregate base specified, or less than four inches
- No mention of compaction or compaction equipment
- Pour directly on native soil with no geotextile fabric
- No mention of drainage or slope away from structures
- Slab thickness only, with no base spec in the line items
What it costs
Proper base prep adds $1 to $3 per square foot to a concrete pour, depending on the soil and how much excavation is required. On a 600-square-foot driveway that is $600 to $1,800. On a slab that should last forty years, it is the cheapest insurance you will ever buy.