If you have watched a crack creep up a wall, felt a door start to stick, or read one too many headlines about subsidence in a dry summer, the question almost asks itself. You are weighing up screw piles for an extension or a new build, and one worry keeps surfacing: what happens to them if my ground moves?
It is a fair question, and it deserves a straight answer rather than a sales pitch. Here is the honest version. Across most of the South East, the ground will move. Shrink-swell clay does it every year. The job of a foundation is not to stop the ground moving. It is to keep your building still while it does.
The ground will move. A correctly designed screw pile foundation does not have to move with it.
First, separate two kinds of movement
When clay gains or loses moisture, two things can happen. In a wet winter it swells and the ground lifts, which engineers call heave. In a dry summer it shrinks and the ground drops, which is the settlement most people recognise as subsidence. Both will move a shallow foundation, and both can do it unevenly across a single building.
That last word is the one that matters. Uniform movement is rarely the problem. Differential movement, where one part of the structure shifts relative to another, is what actually cracks walls and racks door frames.
Why traditional footings ride the seasonal movement
Most strip and trench-fill foundations sit within, or only just below, what is known as the active zone. This is the upper layer of soil that responds to the seasons, drying and swelling as the weather changes. In London Clay and the other shrinkable clays common across the South East, that zone is typically the upper metre or two, and it reaches deeper still where mature trees are drawing moisture out of the ground.
A footing planted in that zone is, in effect, tied to the weather. When the clay shrinks back in a drought, the footing has nothing stable beneath it and the ground gives way. This is why NHBC guidance on building near trees in shrinkable soils exists, and why so many subsidence claims trace back to a thirsty tree and a hot summer.
How screw piles work with variable ground movement
A screw pile foundation is designed to do the opposite. Rather than rest in the seasonal zone, its load-bearing helix is driven down into competent strata that does not change with the weather. Reach ground that stays put, and the foundation stays put with it.
There is a structural bonus to going deeper, too. Research into axially loaded screw pile design is consistent on one point: the further the helix sits below the surface relative to its size, the higher the capacity it develops, in both clay and granular soils.
So driving the helix well below the active zone does two jobs at once. It clears the moving ground, and it builds a stronger foundation in the process. In granular soils, the helix also end-bears directly on firm strata, which is a large part of why a screw pile resists downward load so reliably.
What installation torque does, and doesn’t, tell you
One of the genuine advantages of screw piling is feedback. As the pile turns into the ground, the installation rig reads the torque, which gives a live measure of how hard the ground is fighting back. Torque correlates with capacity, so it is a real, useful check that the ground is behaving as the design assumed.
It is worth being precise about its limits, though, because some of the industry oversells it. The research is clear that installation torque most closely reflects the soil conditions a pile would meet under uplift. Under compression, the downward load is carried by undisturbed soil below the lowest helix, which the torque reading does not directly capture. In short, torque confirms what the design predicted. It does not replace the design.
How ground beams share the load
A single pile resists movement at a single point. A foundation, though, is a system, and this is where differential settlement is actually defeated.
The pile heads are tied together by a reinforced concrete ground beam, which spreads the structure’s load across every pile and locks them into one rigid platform. Even if conditions vary slightly from one pile to the next, the beam shares the load and resists the uneven movement that cracks buildings. The piles reach stable ground. The beam makes them work as one.
What this means for your screw pile foundation project
If you take one thing from this, make it a short checklist to run past whoever is designing your foundation:
- Commission a ground investigation. It establishes your soil type and how deep the seasonal active zone runs, especially if there are trees nearby.
- Check the helix sits below the active zone. Ask your structural engineer to confirm the design embedment relative to that zone, not just a standard depth.
- Ask to see the installation torque records. They are your evidence that the ground matched the design on the day.
- Confirm a ground beam is designed for your loads. This is what holds the piles together and resists differential movement.
- If your property is already moving, screw piles are also a low-disruption way to stabilise it through underpinning.
Screw Piles Are a Reliable Option for Unpredictable Ground Conditions
So, what happens to screw piles if the ground moves? In a foundation designed and installed correctly, the answer is reassuringly dull: very little. The seasonal movement plays out in the upper soil, while your foundation bears quietly on stable ground below it, tied together and indifferent to the weather above.
That dull, dependable outcome is not luck. It comes from a proper ground investigation, an engineered design, and verified installation. Get those three right and the moving ground stops being your problem.
If you are planning a project on clay, near trees, or anywhere movement is a concern, get in touch with Preformed Substructures. We will talk you through a foundation design built to stay still.
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