Helical pile installation, South Beach, FL (Courtesy of Brownie Companies) Helical Pile Deflection and Reliability Helical piles are a useful tool in the deep foundation engineering tool belt. Invented nearly 180 years ago, helical piles were a primary method for support of marine structures including lighthouses, ship docks, moorings and bridges in the 19th century. Some of the early helical piles consisted of a 13 cm (5 in) diameter shaft with 1.2 m (4 ft) diameter helix and were rotary driven into soft marine soils using a large wooden capstan with teams of men and/or draft animals. Developments in hydraulics caused a renewed interest in helical piles. Their applications have expanded, aided by advances in the engineering principals that govern their capacity and performance. As with any deep foundation, it is important to use sound engineer- ing amidst growing applications. Here, we provide a brief summary of some of the modern engineering behind the helical piles. Capacity and Sizing. The inventor of the helical pile, Alexander Mitchell, described as early as 1848 that the capacity of the helical pile is a function of the area of the helical bear- ing plates, the consistency of the bearing stratum and the depth of instal- lation. Today, engineers use similar traditional soil mechanics approaches to size helical piles (e.g., to determine the number and diameter of required helical bearing plates). The two most common traditional soil mechanics approaches are individual bearing and cylindrical shear methods, the least of these being the limit state capacity for a AUTHORS FEATURE ARTICLE particular geometry and ground condition. Individual bearing consists of taking the sum total area of the helices times a soil bearing pressure. Cylindrical shear assumes the bearing elements behave as a group causing shear along the cylinder of soil encompassed by the helix arrangement and end bearing of the lead helix. Testing information is necessary to perform the requisite “to support a superincumbent weight...its holding power entirely depends upon the area of its disc, the nature of the ground into which it is inserted, and the depth to which it is forced beneath the surface.” On Submarine Foundations; Particularly Screw-Pile and Moorings, by Alexander Mitchell, Civil Engineer and Architects Journal, Vol. 12, 1848 traditional soil mechanics calculations to size helical piles, soil borings and soil shear strength or standard penetration. A factor of safety of 2.0 is commonly used in these calculations, and generally provides 90% reliability when compared to load tests. However, the 90% reliability is insufficient for final helical pile design. The boost in reliability for a helical pile occurs when installation torque is also used to verify capacity during installation. If installation torque is not used in combination with proper helical pile sizing through individual bearing and cylindrical shear, a factor of safety of 2.5 to 3.0 is more appropriate, depending on re l i abi l i ty of geotechnical data and use of static load testing. Torque Verification. Since the 1970s, engineers have known that as a helix penetrates soil, a spiral-type of plate bearing test is being conducted and that the installation torque generated is a rough measure of the consistency of the bearing stratum adjacent to the helices. Installation torque should be used as a termination criterion for helical piles James A. Cherry, P.E. and Howard A. Perko, Ph.D., P.E., Helical Pile Design Engineers, Magnum Geo-Solutions, LLC. Perko is the author of “Helical Piles: A Practical Guide to Design and Installation,” John Wiley & Sons, N.Y. DEEP FOUNDATIONS • JAN/FEB 2013 • 69
