Why concrete actually cracks.

1. Expansive clay soil

Large parts of San Diego County sit on clay soil that swells when it gets wet and shrinks when it dries out. That constant movement underneath a driveway puts stress on the slab that plain concrete strength can't fully resist over time, especially if the base wasn't compacted properly before the original pour.

2. Tree root intrusion

Roots from mature trees near a driveway or walkway can grow underneath the slab and physically lift or crack it as they thicken. This is one of the most common causes of cracking and heaving in older San Diego neighborhoods with mature street trees.

3. Poor original base preparation

If the aggregate base underneath a driveway wasn't compacted correctly when it was originally built, the slab has essentially been sitting on an unstable foundation from day one. This kind of cracking often shows up as sinking or uneven panels rather than a clean, straight-line crack.

4. Missing or incorrectly spaced control joints

Control joints are cut into concrete on purpose, to give the slab a planned, clean place to crack instead of cracking randomly. If a driveway was poured without enough control joints, or with them spaced too far apart, random cracking is almost guaranteed as the concrete naturally shrinks while curing and continues to move slightly over the years.

5. Heavy loads it wasn't designed for

A driveway poured to a standard residential thickness can crack under the repeated weight of an RV, box truck, or heavy equipment it wasn't originally designed to carry. If cracking is concentrated where a heavier vehicle regularly parks, this is often the cause.

So which one is it?

Random spiderweb cracking across a large area often points to soil movement or base failure. A crack that follows a straight line near a large tree usually means roots. Cracks that only appear in one specific spot where a heavy vehicle sits usually mean a load issue. The real answer requires a look at the specific slab, since more than one of these can be happening at once.

Related reading

If you've decided it's time for a full replacement, How Much Does a Concrete Driveway Cost in San Diego? walks through what actually drives the price of a new one.

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