The ground decides
A pile is only as strong as the soil it bears on. The same pile can carry far more in stiff clay than in soft silt, so there's no single number for "a screw pile".
What matters is the pile's size, how many helix plates it has, how deep they go, and what the ground is like at that depth.
Three directions of load
A foundation has to resist more than weight:
- Compression: the weight of the building, its contents and any snow, pushing down.
- Tension (uplift): wind or an earthquake trying to lift the structure. Helix plates are good at this, which is why screw piles suit decks, carports, signs and solar arrays.
- Lateral: sideways forces, resisted by the stiffness of the shaft and the soil near the surface.
Torque as a check
As a pile is screwed in, the torque needed to turn it rises with the strength of the soil around the helix. Engineers estimate capacity by multiplying the final installation torque by a factor that depends mainly on the shaft size.
For consented work, the engineer sets a target torque for each pile. The installation record then shows that every pile reached it, at the depth it was installed to.
Load testing
On larger or unusual jobs, a pile can be load tested on site. It's pushed or pulled with a jack beyond its design load while its movement is measured, which confirms the design for that ground. We can arrange load testing wherever the engineer wants it.
Who sizes the piles
For consented work, a chartered engineer designs the pile size, layout, depth and target torque for your building and site, and issues a PS1. For smaller jobs that don't need consent, like many decks and sheds, we size the piles to suit the structure and the ground, and tell you if we think it needs an engineer.