Why Your Fence Posts Lift in Reactive Clay (And Why It’s Not the Concrete)

The Reason Why

What’s actually going on

Large parts of Australia sit on reactive clay soils. When these clays get wet, whether that’s from a decent rain event, a change in drainage, or just a wet winter, they swell. Sometimes significantly. And when the ground expands, it pushes upward on anything sitting in it. That includes fence posts set in concrete.

Here’s the thing: a fence post and its concrete footing don’t carry much weight. They’re not like a house slab with tonnes of structure holding them down. So when the clay swells with enough force, it can literally jack the post and its footing upward out of the ground. In a full-scale field study, da Silva Burke et al. (2022) measured up to 355 kN of tension on piles in expansive clay, with the ground surface swelling an average of 59mm around the test site. That’s serious force acting on a footing that weighs next to nothing by comparison.

When the soil dries out again, it shrinks back. But the post doesn’t always settle back to where it was. Over repeated wet and dry cycles, the post works its way up a little more each time. Contractors and engineers call this a “ratcheting” effect. Lab testing by Ferreira et al. (2020) put numbers to it: in one test, uplift displacement went from 8.1mm in the first cycle to 9.1mm by the third. Each cycle also reduced the pile’s ability to resist uplift by roughly 6 to 15 percent. So the problem gets worse, not better, with every season that passes.

The Real Driver

It’s the soil, not the install

This is important to understand: post uplift in reactive clay areas is a natural soil behaviour. It’s not a sign that the concrete was mixed wrong, the hole wasn’t deep enough, or that someone did a dodgy job. The same swelling pressure that lifts fence posts can crack house slabs and shift retaining walls. A lightly loaded fence post doesn’t stand a chance against it.

If a post moves upward in reactive clay country, look at the soil conditions first. Chances are, the ground is doing exactly what reactive clay does when it gets wet.

Soil Types

What drives it

A few factors make the problem worse. Clay type matters. The more reactive the clay, the more it swells. Moisture changes are the trigger, so areas with seasonal wet-dry cycles or changes to drainage and irrigation near the fence line are more prone. The shape and size of the footing plays a role too. A round concrete footing gives the swelling clay more surface to grip and push against. Research by Soundara & Robinson (2015) found that every reactive soil has an “active zone” where moisture fluctuation causes swelling. In their study, that active zone extended to around 8.5 metres depth, with swelling pressures measured between 115 and 195 kPa depending on the test method. Below the active zone, the weight of the soil above holds everything in place and uplift pressure drops to zero.

And it’s cumulative. Each wet season lifts the post a fraction, and each dry season doesn’t quite bring it back. Ferreira et al. (2020) confirmed this in lab testing: with repeated moisture cycles, uplift displacement increased every cycle and the pile’s capacity to resist it dropped by 6 to 15 percent over just three cycles. The load distribution along the pile also shifted, with the deepest resistance weakening first. Over a few years on your property, you can end up with posts sitting noticeably higher than where they started, and they’ll keep going.

The Surprising Answer

Why concrete makes it worse

This is the bit that surprises most people. A post set in concrete is actually more prone to uplift than a plain driven post with no concrete at all.

A concrete footing creates a big, flat-bottomed mass sitting in the clay. That flat base gives the swelling soil a larger surface to push against. The rough texture of the concrete also gives the clay more grip along the sides of the footing, so as the soil expands it really latches on and pulls the whole thing upward. da Silva Burke et al. (2022) measured shaft friction of 56.2 kPa along their test piles. That’s the grip strength between the swelling clay and the pile surface. More surface area means more total grip, which means more uplift force.

A plain steel post driven straight into the ground has a much smaller cross-section and a smoother finish. There’s simply less for the swelling clay to grab onto. Ferreira et al. (2020) confirmed this statistically: pile diameter was the single biggest factor affecting how much uplift occurred. The larger the diameter, the more the soil could grip it. That’s why you’ll often see concreted posts lifting while a slim driven post right next to them stays put.

The Solution

What can you do about it

Knowing it’s a soil issue rather than a bad install is the first step. From there, there are a few proven approaches that help.

If you’re fencing in reactive clay country, it’s worth having a conversation with your contractor or our team about the local soil conditions and what approach makes sense for the site. Every property is different, and locals who’ve been fencing in the area will know what the ground does better than anyone.

1.

Go deeper. This is one of the most effective things you can do. The idea is to get the base of the post or footing below the active zone, the layer where the soil is expanding and contracting with moisture changes. Below that depth, the soil stays stable and the uplift forces become negligible. Soundara & Robinson (2015) showed that uplift pressure drops to zero once you get past the active zone boundary. da Silva Burke et al. (2022) found the transition from uplift to resistance at around 4 metres depth in their test site. Indian engineering standards recommend a minimum founding depth of 3.5 metres in deep expansive soils. In highly reactive areas, the extra depth makes a real difference.

2.

Bell out the hole. Some contractors dig the hole wider at the base, creating a bell shape. The wider footing at the bottom acts as an anchor, making it harder for the swelling soil to push the whole thing upward. It’s extra effort at install time, but it pays off in reactive ground.

3.

Pin the post with a star picket and LokDown Lugs. Another approach is to drive a star picket deep into the ground alongside your main post and lock the two together using LokDown Lugs. The star picket acts as an anchor, pinning the main post in place even as the soil moves around it. It’s a practical solution that uses the depth and grip of the star picket to resist uplift on the main post.

4.

Consider a plain driven post. Where conditions suit, a steel post driven straight into the ground without concrete can be less prone to uplift altogether. Less surface area, smoother finish, less for the clay to grab. It’s not the right answer for every situation, but it’s worth considering in areas where uplift is a known issue.

5.

Manage drainage. Good drainage around fence lines reduces the moisture swings that trigger the cycle in the first place. It won’t eliminate the problem in seriously reactive ground, but it helps.

More Information

References

da Silva Burke et al. (2022) “Measurement of pile uplift forces due to soil heave in expansive clays.” Published in the Canadian Geotechnical Journal. Full-scale field study in South Africa using 450mm diameter bored concrete piles instrumented with strain gauges, installed in highly expansive clay (plasticity index 37 to 40). The site was flooded over several months to induce swelling. Measured a maximum tension of 355 kN, average surface swell of 59mm, and an active zone extending to around 7 metres. Shaft friction transitioned from negative (pulling the pile upward) to positive (holding it down) at roughly 4 metres. The best prediction model used a limiting shaft friction of 56.2 kPa and matched measured results within 10%. Durham University Repository

Soundara & Robinson (2015) “Swelling Pressure and Uplift of Piles in Expansive Soils.” Lab study using model cement mortar piles in expansive soil from Tamil Nadu, India. Measured swelling pressures of 115 to 195 kPa across three standard test methods. Found the active zone extended to approximately 8.5 metres depth, defined as where the overburden pressure equals the soil’s swelling pressure. Below this boundary, measured uplift pressure dropped to zero. References Indian standard IS:2911, which recommends a minimum founding depth of 3.5 metres for piles in deep expansive deposits. Academia.edu

Ferreira et al. (2020) “Behavior of pile foundations in expansive soils.” Published in the International Journal of Civil Engineering & Construction. Lab study using model steel piles in three diameters (20mm, 30mm, 40mm) and two lengths (400mm, 600mm), installed in black cotton soil (plasticity index 45%) and subjected to three full wetting-drying cycles. Uplift displacement increased each cycle (e.g. 8.11mm to 9.06mm over three cycles for a 30mm/400mm pile). Pile capacity dropped roughly 6 to 15% by the third cycle, with shaft load redistributing from the toe to upper segments. Statistical analysis (ANOVA) confirmed pile diameter was the dominant factor affecting uplift. Int. Journal of Civil Engineering (PDF)

Anchor Foundation Repair “What Is Soil Heave?” Plain-language summary of soil heave for property owners. Covers the mechanics of how expansive clay swells when wet and shrinks when dry, common moisture triggers, and effects on structures including house slabs, retaining walls, and other lightly loaded elements. Notes that once heave occurs, there is no prescribed repair to push a structure back to its original position, and that prevention through moisture management is the most effective approach. Anchor Foundation Repair blog