Footings in Reactive Clay
The Soil Does the Lifting
Expansive clay changes volume with moisture content. Wet it and it swells, dry it out and it shrinks, and both happen every year on a predictable seasonal cycle. A concrete footing sitting in that material has forces applied to its sides as the clay presses in, and forces applied upward along its shaft as the swelling ground tries to lift it. A gate post is a tall lever attached to that footing, so a small rotation at the base becomes a large movement at the top of the leaf.
Depth Beats Diameter
The instinct is to pour a wider footing. What actually helps is going deeper, past the zone where moisture content varies seasonally, so that the bottom of the footing sits in ground that stays at roughly constant moisture year round. A shallow wide pad is simply a larger surface for the swelling clay to push against. Local practice on depth exists for good reason and is worth asking about specifically.
The Shape of the Hole
A footing that is wider at the top than the bottom gives the swelling soil a shoulder to lift against, which is the worst possible geometry. A straight sided or slightly belled shaft, belled at the bottom rather than the top, resists uplift instead of inviting it. This detail costs nothing at the digging stage and cannot be corrected afterwards.
Reinforcement and Post Embedment
Uplift and lateral load both put the concrete in tension, and concrete has very little tensile strength on its own. A reinforcement cage carries that. The post itself should be embedded well into the footing rather than bolted to a plate on its surface, because a base plate concentrates every force from a heavy leaf into a handful of anchor bolts and their pull out capacity.
Drainage Around the Base
Most of the movement is driven by moisture change, so anything that stabilises moisture at the footing helps. Grading so surface water runs away from the post rather than pooling at it, and keeping irrigation from discharging directly at the base, both reduce the annual swing. It is unglamorous and it is the cheapest intervention available.
Curing Takes Time
Concrete reaches usable strength in days and full strength over weeks. Hanging a heavy leaf on a footing poured yesterday puts a permanent load on material that has not developed its capacity, and the resulting lean does not correct itself. An installer who wants to come back to hang the gate is doing it properly, and the wait is worth it.