Design of Helical Pile Groups for Support of Electric Power Structures-Paper

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The paper presents a design approach for helical pile groups supporting electric power transmission structures, particularly those exposed to large overturning moments, such as monopoles and H-frame structures. Although these foundations are increasingly used, current industry guidance offers limited detail on pile-group design, and there is no consistent standard for resistance factors or performance criteria.<br /><br />The authors explain how transmission-line foundation design differs from building design. Failure consequences and serviceability requirements may differ, while allowable settlement, deflection, and rotation limits vary among utilities. The National Electrical Safety Code (NESC) uses load-factor design, but its resistance factor of 1.0 does not explicitly address geotechnical uncertainty. The paper describes current practice using a resistance factor of 0.7 with factored NESC loads, or 0.5 with service loads.<br /><br />The proposed workflow combines preliminary hand calculations with computer modeling. Hand calculations estimate individual pile axial and shear reactions from the structure’s ground-line loads and pile-group geometry. The designer then models the group in Ensoft Group, accounting for pile properties, soil layers, pile-head conditions, loads, and grillage weight. The analysis provides pile reactions, group deflection and rotation, and reduction factors for closely spaced piles.<br /><br />A case study of an eight-pile foundation for a transmission-line dead-end structure illustrates the process. Hand calculations produced preliminary compression, tension, and shear demands; group analysis refined these demands and identified group-efficiency effects. The modeled foundation’s predicted displacement and rotation were small, and the selected piles were adequate for the resulting reactions.<br /><br />The paper concludes that helical pile groups can provide an economical and effective solution for power structures, but standardized safety factors, performance limits, and comprehensive design guidance—including practical examples—are needed.

Keywords

helical pile groups

transmission line foundations

electric power structures

overturning moments

monopoles

H-frame structures

pile group design

NESC load-factor design

geotechnical uncertainty

group analysis