Screw Pile Capacity Calculator

Screw piles and helical piles are the same thing

Screw pile is the everyday name in Australia. The research literature calls the same foundation a helical pile: a steel shaft with one or more helix plates, screwed into the ground. This calculator covers both names and applies the CPT-based design method of Bittar et al. (2023), developed at UWA and published in the Canadian Geotechnical Journal. You can read more on the helical pile method page.

What the calculator does

It computes the axial capacity of a screw pile in sand, in both compression and tension (uplift), directly from a cone penetration test profile. Capacity is reported at the helix depth you enter, together with the load-displacement response. To find the embedment your design load needs, run trial helix depths; the pile depth guide covers the general approach.

It calculates geotechnical capacity. It does not estimate pile quantities or installation cost for a deck or slab; for that you need a layout, not a capacity method.

What you need

A CPT trace from your site investigation: depth, cone resistance (qc) and sleeve friction (fs). Most Australian geotechnical reports include CPT soundings. The importer reads CSV and Excel files, AGS4, and raw contractor files, and sorts out the units automatically. No CPT file handy? Use the sample data button above to see how it works.

Who it is for

Geotechnical and structural engineers running capacity checks for screw pile designs, and screw piling contractors who want a method-based capacity estimate when quoting rather than a rule of thumb. In Australia, pile design generally falls under AS 2159, with residential site classification to AS 2870. The calculator provides ultimate geotechnical capacity from the CPT; the choice of load factors and geotechnical strength reduction factors remains with the design engineer.

Frequently asked questions

Is a screw pile the same as a helical pile?

Yes. Screw pile is the common term in Australia and the UK, while North America and the research literature usually say helical pile or helical pier. Same pile, same calculation.

How is this different from torque correlation?

Torque correlation (Q_ult = K_t × T) estimates capacity from the torque measured while installing the pile, so it can only verify capacity during or after installation. The CPT-based method predicts capacity beforehand from soil data, so you can size the pile at the design stage. Many projects use both: CPT-based design first, torque as an installation check.

Does the calculator work in clay?

The Bittar et al. (2023) method was developed and validated for screw piles in sand. For clay or mixed profiles, treat results with caution and seek specific geotechnical advice.

Does it calculate uplift (tension) capacity?

Yes. Capacity is reported in both compression and tension, so it covers uplift cases such as light structures.

How do I choose the pile depth?

Enter a trial helix depth and compare the reported capacity with your required load, allowing for your safety or reduction factors. Adjust the depth and rerun until it works. There is a step-by-step pile depth guide.

Do I need my own CPT data?

For a real design, yes: the calculation is driven by the CPT profile at your site. To explore the tool first, the sample data works with every module.