Helical piles are an established solution for foundation underpinning, but their applications are as far-reaching as the creativity of an engineer. Helical piles are typically associated with gravity load - force pushing down on the pile. In the project below, helical piles are utilized to support lateral loads, including wind and seismic loads.
Helical piles have become a common piling system in the North American construction market. They are used for many applications with low to moderate tension and compression loads. Slender helical piles, less than 4.5 inches in diameter, have historically been used for light shear loads of 1 to 5 kips in vertical applications. This method derives the lateral capacity as a function of the pile’s allowable structural bending moment and the allowable passive earth pressure of the soil. It is common to analyze the capacity of the piles by theoretical slender pile equations, P-Y curves, or software programs that model P-Y response. An adequate safety factor or a service limit on the allowable deflection is applied to the allowable loads to determine an applicable pipe size. In recent years larger piles, with diameter greater than 5.5 inches, have been used for larger lateral loads. Large diameter piles have been designed for loads in excess of 20 kips.
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.
When building a new home, short-term decisions usually include what floor tile to use in the kitchen or which coverings to put on the living room bay windows. Few homeowners think about which type of foundation offers the best long-term stability.
With residential remedial foundation repair becoming a greater concern, the CHANCE® helical pile was developed in the mid-1980s and was issued a helical underpinning methods patent in 1992. This all sounds well and good, but the question remains: Why are helical piles so effective for remedial foundation repair?
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.