About

Helical screw piles rely on the empirical relationship between installation torque and ultimate capacity. Geotechnical engineers and foundation contractors use this calculation to verify that a pile has reached sufficient depth and soil resistance to support the design load. Accuracy is critical here; underestimating capacity leads to foundation settlement, while overestimation risks structural failure. This tool utilizes the standard Torque Correlation Factor (K_t) method, widely accepted in the deep foundation industry for verifying field capacity during installation.

The relationship assumes that the resistance to torque is directly proportional to the soil's bearing capacity. However, this linearity only holds when specific shaft geometries match their corresponding K_t values. This calculator integrates standard factors for common shaft sizes (Square and Round) and applies a customizable Factor of Safety (FOS) to determine the Allowable Working Load.

Formulas

The empirical equation for Ultimate Capacity (Q_ult) is:

Q_ult = T × K_t

To determine the Allowable Capacity (Q_all), a Factor of Safety (FOS) is applied:

Q_all = Q_ultFOS

Where T is the final installation torque averaged over the last 3 diameters of penetration.

Reference Data

Shaft Type	Shaft Size (OD)	Default K_t (ft&inverse;⋅lb)	Default K_t (m&inverse;⋅kg)
Square Bar	1.50″	10	33
Square Bar	1.75″	10	33
Square Bar	2.00″	10	33
Round Shaft	2.875″	9	30
Round Shaft	3.00″	8	26
Round Shaft	3.50″	7	23
Round Shaft	4.50″	6	20
Round Shaft	6.00″	5	16
Round Shaft	8.625″	4	13

Frequently Asked Questions

A Factor of Safety (FOS) of 2.0 is the industry standard for permanent structures where soil data is known. For temporary structures or rigorously load-tested piles, an FOS of 1.5 may be permitted. Unknown soil conditions often require an FOS of 2.5 or 3.0.

Technically, yes. While the standard factors listed are used for most soils, highly plastic clays or very dense sands may require site-specific load testing to determine a calibrated Kt value. This tool uses the standard ICC-ES AC358 default values.

Larger shafts have a larger surface area, which generates more torque from friction along the shaft rather than from the helical plate's bearing capacity. To correct for this "useless" frictional torque, the correlation factor (Kt) is reduced for larger diameters.

Yes, helical piles generally provide equal capacity in tension (uplift) and compression. However, the connection detail at the pile cap usually governs the maximum allowable tension load, not the soil capacity.