Screw piles
All pile types and their advantages.
Three kinds of force
- Compression: the weight of the structure, snow and whatever stands on it
- Tension (uplift): the wind blowing under a roof or panels and trying to lift them
- Lateral force: the wind pushing against a screen, sign or façade
How a screw pile holds in tension
In tension the helix is pulled upwards. The soil above holds it down, like an anchor. So the top helix has to be deep enough: for tension piles at least 5 helix diameters. Any shallower and the soil breaks out in a cone up to the surface.
Manufacturers use a larger safety margin in tension than in compression. The Finnish method of our manufacturer Paalupiste, for example, uses an overall factor of 2.2 in compression and 2.5 to 3.0 in tension.
Ground screws and lateral force
With a ground screw with a continuous thread, the friction along the whole screw holds it. For signs, lighting and noise barriers, where the wind pushes sideways, we use winged piles or a concrete block around the pile head: they spread the force over a larger volume of soil.
What this means for your project
- Solar panels: helical piles as tension anchors, as in our project with 1.2 m piles and a 15 cm helix
- Carports and closed canopies: the uplift often decides the choice of screw
- Signs and screens: winged or block piles against tipping
- The wind load itself follows from the Eurocode (EN 1991-1-4) and the location: higher in open country or on the coast
Soiltek is a specialist contractor, not an engineering firm. We provide the manufacturer's product data and align the pile proposal with your structural engineer.
Sources: H.A. Perko, Helical Piles, Wiley, 2009; NBN EN 1997-1; NBN EN 1991-1-4; Paalupiste.