The shaft type/size of a helical pile is critical to both the axial and lateral capacity – especially for compression in soft/loose overburden soils where lateral stability of the shaft must be considered. The following is a brief summary of the 4 different shaft types commonly used for helical piles and their relative advantages and disadvantages based on site conditions and application. It is very important to understand that helical pile installation must be considered in the design process.
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:
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.