Why Your Fence Posts Are Failing: Expansive Clay Soil in the San Fernando Valley

If you've had a fence post lean, heave, or crack its concrete footing within a few years of installation, the fence crew isn't necessarily to blame. A lot of the San Fernando Valley — including much of Woodland Hills — sits on expansive clay soil, and that soil moves in ways that catch a lot of fence installations off guard.

What Expansive Clay Actually Does

Clay-heavy soil absorbs water and swells, then dries out and shrinks, cycling through the seasons. In the Valley, that means wet winters swell the soil around a post footing, and the long dry summer shrinks it back down — sometimes unevenly, since one side of a footing may stay shaded and moist longer than the other. This swell-shrink cycle exerts real lateral force on anything embedded in it, including fence post footings. Over a few years of cycling, a footing that isn't set deep enough or wide enough can start to rotate, heave upward, or crack.

The USGS and California Geological Survey both map expansive soil distribution across LA County — and much of the Valley falls squarely inside those zones.

Why Standard-Depth Footings Sometimes Aren't Enough

A lot of fence posts get set using rule-of-thumb depths that work fine in stable, sandy, or well-draining soil. In expansive clay, that same depth can leave the footing sitting entirely within the "active zone" — the upper layer of soil that swells and shrinks the most. The footing ends up riding the movement instead of anchoring below it. Deeper embedment, a properly sized footing diameter, and in some cases a belled or underreamed footing base can get below the active zone and anchor into more stable soil.

Signs Your Soil Is Driving the Problem

• •Posts that lean gradually over a year or two, especially after a wet winter
• •Concrete footings that show hairline cracks radiating outward
• •Gate posts that shift out of alignment seasonally, then partially self-correct
• •Multiple posts along a run showing the same lean direction, suggesting a soil-wide issue rather than one bad install

What Actually Helps

• •Deeper post footings sized for the specific soil conditions on your lot, not a generic depth
• •Good drainage away from post locations, since standing or pooling water accelerates the swell cycle
• •Avoiding footing placement right at grade transitions or drainage low points, where moisture concentrates
• •For larger structures like gate posts or retaining elements, a soils-informed approach rather than a one-size answer

Expansive clay isn't unique to any one neighborhood in the Valley — it shows up across a wide swath of Woodland Hills, Tarzana, and the surrounding foothill communities. If your fence has failed this way before, the fix isn't just "set it again the same way." It's setting it to match what the soil is actually doing underground.

Ready to Start Your Fence Project?

Contact Infinity Fence Company for a free estimate today.