About

Screw piles (helical piers) are a rapid, low-impact foundation solution used for everything from decks to industrial masts. Unlike concrete, their capacity is often verified empirically during installation by measuring the torque required to advance the pile into the ground.

This calculator utilizes the standard torque-correlation method ($Q_{ult} = K_t \times T$) to estimate the Ultimate Capacity. It is essential for installers and engineers to verify that the pile has reached a soil layer capable of supporting the design load with an adequate Factor of Safety (FOS), typically 2.0 to 3.0 for permanent structures.

Formulas

The relationship between installation torque and ultimate capacity is linear:

Q_ult = K_t × T

To find the Safe Working Load (SWL), we apply a Factor of Safety:

Q_safe = Q_ultFOS

Reference Data

Shaft Type / Geometry	Kt Factor (ft-lb units)	Kt Factor (SI units m&sup-1;)
1.5" Square Bar (SS150)	10 - 11	33 - 36
1.75" Square Bar (SS175)	10 - 11	33 - 36
2.875" Round Shaft (RS2875)	8 - 9	26 - 30
3.5" Round Shaft (RS3500)	7 - 8	23 - 26
4.5" Round Shaft (RS4500)	6 - 7	20 - 23

Frequently Asked Questions

Kt is the empirical torque correlation factor. It represents the relationship between the twisting force (torque) needed to install the pile and the vertical load it can support. Slender shafts (square bars) generally have higher Kt values than large diameter round shafts because they have less shaft surface area creating friction.

For temporary structures, an FOS of 1.5 might be acceptable. For permanent residential or commercial structures, ICC-ES AC358 standards typically recommend an FOS of 2.0 to 3.0 to account for soil creep, corrosion, and measurement uncertainties.

Torque correlation is a widely accepted method, but for critical infrastructure, it should be validated with a static load test. Soil conditions like highly plastic clays can sometimes give false high torque readings without providing long-term settlement resistance.