The load capacity of helical piles can be determined 3 ways. The method(s) used are dictated by the information available. The capacity of a helical pile must equal or exceed the factored load that the pile is required to support.
Today, it is generally accepted that installation torque can be used to verify the axial capacity of helical piles. The International Building Code (IBC) 2012, 2015 & 2018 Section 1810.3.3.1.9 states there are three ways to determine axial capacity. Method 2 states the ultimate capacity can be determined from well documented correlations with installation torque. The installation torque-to-capacity relationship is an empirical method originally developed by the A. B. Chance Company in the late 1950’s and early 1960’s. For over 60 years, Hubbell Power Systems, Inc. has promoted the concept that the torsion energy required to install a helical anchor/pile can be related to its ultimate capacity. Precise definition of the relationship for all possible variables is the subject of on-going research. However, simple empirical relationships, originally derived for tension applications but also valid for compression capacity; continue to be used as part of project specifications to verify capacity. The principle is that as a helical anchor/pile is installed (screwed) into increasingly denser/harder soil, the resistance to installation (called installation energy or torque) will increase. Likewise, the higher the installation torque, the higher the axial capacity of the installed pile/anchor. The CHANCE® torque correlation equation is:
In response to a demand for predictable high capacity foundation solutions, a fully grouted screw displacement pile was developed by CHANCE® engineers. Comprised of a centralized steel shaft and a patented displacement assemblies, the pile, known as the Drivecast™ screw displacement pile, is designed to create a cylindrical annulus around the central shaft that is continuously filled with grout from a gravity-fed reservoir at the surface.
There is no equivalent for measuring torque directly in the tooling string. Using differential pressure (or pressure drop across the motor) can work if it is properly set up and calibrated, but keep in mind that pressure is being measured, not torque.
Capacity-to-torque relationships for helical piles are used frequently to determine termination criteria for helical piles. Empirical relationships between installation torque and capacity have been established for several helical pile shaft sizes, including square shaft and pipe shaft piles.