Copyright 2011 – Magnum Piering, Inc. Reliability and Theory of Helical Pile Sizing Howard A. Perko, Ph.D., P.E. Magnum Geo-Solutions, LLC Copyright 2011 – Magnum Piering, Inc., All Rights reserved. May be distributed and printed by express written permission of the author only. MAGNUM Informational Bulletin ISO-MHP3-11A

Copyright 2011 – Magnum Piering, Inc. Axial Capacity and Helix Spacing Theoretical Bearing Capacity Theoretical Pullout Capacity Helix Sizing Charts Reliability of Charts Capacity to Torque Ratio Background Variation in Capacity:Torque Ratio Reliability of Torque Method Combined Reliability Contact Information Outline

Copyright 2011 – Magnum Piering, Inc. Axial Capacity Limit States: Cylindrical Shear & Individual Bearing There are two ways to calculate the theoretical capacity of a helical pile. If the helices are far apart, they behave independently and one would use the individual bearing method. If the helices are close together, they would behave as a group and one would use the cylindrical shear method.

Copyright 2011 – Magnum Piering, Inc. Helix Spacing Most efficient spacing where: Cylindrical Shear = Individual Bearing Generally occurs at 2 to 4 Helix Diameters

Copyright 2011 – Magnum Piering, Inc. Bearing Capacity Traditional Soil Mechanics: Individual Bearing Q = R 2 (1.3cN c + N q ) Cylindrical Shear Q = 2 RL(c+K tan ) + R 2 (1.3cN c + N q ) Limit State: Calculate both, and report whichever is least. From Perko (2009) Helical Piles

Copyright 2011 – Magnum Piering, Inc. Pullout Capacity Pullout Capacity is calculated the same way as Bearling Capacity. However, generally in practice, Pullout Capacity = 82% Bearing Capacity From Perko (2009) Helical Piles

Copyright 2011 – Magnum Piering, Inc. Magnum Helix Sizing Charts From Magnum 2011 Product Catalog Magnum Piering, Inc. sizing charts were created using individual bearing and cylindrical shear methods. Charts are provided for cohesive soils, non-cohesive soils, and weak bedrock. Based on the SPT Blow Count of the bearing stratum, they yield required ultimate capacity. In helical piles, engineers typically use a factor of safety of 2.0.

Copyright 2011 – Magnum Piering, Inc. Reliability of Sizing Charts From Perko (2009) Helical Piles Test data indicates sizing charts are 90% reliable when used with a F.S. = 2.0.

Copyright 2011 – Magnum Piering, Inc. Hoyt and Clemence (1989) -91 Full-scale tension load tests -Torque is shown to be statistically a strong verification of capacity Capacity:Torque Background

Copyright 2011 – Magnum Piering, Inc. Perko (2000) -Torque is shown to be theoretically related to capacity Capacity:Torque Background

Copyright 2011 – Magnum Piering, Inc. Variation of Torque Correlation Capacity to Torque Ratio Varies with Helical Pile Shaft Diameter. From Perko (2009) Helical Piles

Copyright 2011 – Magnum Piering, Inc. Capacity-to-Torque Ratios Magnums Product Catalog Contains Suggested Capacity:Torque Ratios. From Magnum 2011 Product Catalog

Copyright 2011 – Magnum Piering, Inc. Reliability of Torque Method From Perko (2009) Helical Piles Test data indicates torque is 98% reliable when used with a F.S. = 2.0.

Copyright 2011 – Magnum Piering, Inc. 0.3% with FS=2.0 Statistically Independent Variables Combined Reliability From Perko (2009) Helical Piles Using Sizng Charts in Combination with Torque Verification yields 99.7% Reliability.

Copyright 2011 – Magnum Piering, Inc. Contact Information Magnum Piering, Inc PIER (7437) 6082 Schumacher Park Drive West Chester, OH 45069