Screw piles
All pile types and their advantages.
What is installation torque?
It is the turning force the drive head needs to advance the pile, in kNm. Our hydraulic drive heads deliver up to 132 kNm. We record the torque of every pile, down to the final depth.
From torque to capacity
In the international literature (Hoyt and Clemence, Perko) the relationship is written as a simple multiplication:
Qu = Kt × T
Q is the ultimate capacity in kN, T the installation torque in kNm and Kt an empirical factor that depends on the shaft diameter. For round shafts Perko gives approximately Kt = 1433 / d^0.92 (d in mm, Kt in 1/m).
| Shaft diameter | Kt (approx.) |
|---|---|
| Ø 76 mm | 26.7 m⁻¹ |
| Ø 89 mm | 23.1 m⁻¹ |
| Ø 114 mm | 18.4 m⁻¹ |
| Ø 140 mm | 15.2 m⁻¹ |
A worked example
A Ø 76 mm pile that finishes at 6 kNm gives approximately Qu = 26.7 × 6 ≈ 160 kN. International practice uses an overall safety factor of 2.0 when the capacity is confirmed on site by torque, so about 80 kN usable. Perko shows that 90 to 99% of measured capacities then lie above that value. This is an illustration, not a design.
When is torque less reliable?
- When the helix is too shallow: for tension piles the top helix must be at least 5 helix diameters deep
- In ground with stones, rubble or gravel, where the torque shows peaks
- For large projects or unusual ground: there we start from CPT soundings and do a load test where needed
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: A Practical Guide to Design and Installation, Wiley, 2009; ICC-ES AC358; NBN EN 1997-1. More in our free e-book (in Dutch).