What is the Relationship between Installation Torque and Soil Bearing Capacity?

Installation torque is used as a real-time indicator of soil bearing capacity because the rotational resistance encountered by the helix plates reflects the soil’s shear strength. As the helical pier advances into competent strata, the torque required to turn it increases proportionally to the load it can support. This linear relationship allows us to confirm in the field that each pier has reached a competent bearing stratum capable of supporting the structural load.

To translate these field measurements into capacity, we use the torque factor method, which follows the equation:

Qult = Kt × T

1. Qult: The ultimate pile capacity (measured in pounds or kips).
2. Kt: The empirical torque factor (measured in ft⁻¹), which varies based on soil type and helix geometry.
3. T: The measured installation torque (measured in ft-lb).

Typical Kt ranges vary significantly by soil type. For example, soft clay typically has a Kt range of 3–5, while gravel can range from 10–15. In general, stronger soils such as dense sand and gravel require more torque per unit of ultimate capacity than softer soils like clay.

While torque correlation is a widely accepted and efficient method for real-time verification, its accuracy depends on using a calibrated Kt factor. For critical applications, we recommend site-specific load testing to validate the torque-capacity relationship for your local geology.

Disclaimer: Consult a structural engineer or our engineering team for project-specific design and installation guidance. Our products are certified to ICC/ISO where indicated — installations must comply with applicable building codes and ICC-ES acceptance criteria (AC358).

• How to Calculate Soil Bearing Capacity for Helical Piers