Clay
Much of Auckland and Northland sits on clay that swells when it's wet and shrinks as it dries. Shallow footings can move with it. Screw piles take the load down below the layer that moves most, and the helix plates hold them in place.
Soft ground, silt and peat
Parts of the Waikato and the Hauraki Plains have peat and soft silt several metres deep. Digging there is slow, and the holes collapse or fill with water. Screw piles go straight through the soft layers, with extensions added until they reach firm ground and their target torque.
Volcanic ash and pumice
The Bay of Plenty and much of the Waikato have volcanic ash and pumice soils, some of which lose strength when they're dug up and disturbed. A screw pile displaces very little soil as it goes in, which suits ground that's best left undisturbed.
Sand
Helix plates bear well in dense sand. Loose sand needs the piles to go deeper, and near the coast the engineer also allows for corrosion.
Fill
Many sections have fill from earlier earthworks, and its quality can vary a lot. Screw piles pass through it to the natural ground below, and the torque shows when they've got there.
Rock and boulders
Solid rock close to the surface, or big boulders, can stop a screw pile short of where it needs to be. Sometimes the engineer can shift a pile a little; sometimes concrete suits the site better. We check before we start, so it doesn't come as a surprise on the day.
Finding out what you have
For small jobs, a site visit and a look at the ground are often enough. For houses and consented work, the engineer may want a geotechnical report, usually with hand augers and Scala penetrometer tests, a simple test of how firm the ground is with depth. On the day, the installation torque adds another check at every pile.
