Publication Abstract

SuperPile 2017 Online Proceedings, (DFI)

Design of Grillage and Helical Pile Foundation for Limited Access Transmission Line
Jason Herron, P.E. and Benjamin Jennings, E.I.T.

In the summer of 2016 Xcel Energy experienced a storm event that resulted in the collapse of approximately 10 miles of a 345 KV transmission line near Rogers, Minnesota. In design of the new transmission line, Xcel Energy determined that ten tower locations would be replaced with steel H-frame structures supported by helical piles instead of wood H-frame structures supported by driven wooden piles. A steel foundation was proposed for the towers due to time constraints and site conditions unfavorable to large pile driving equipment. Xcel Energy contacted Hubbell Power Systems, Inc. (HPS) for aid in the design of a complete steel foundation for the structure legs. With the shear, moment, and axial loads provided, HPS used pile group modeling and axial capacity software to design a four helical pile group; and used 3D modeling and finite element software to design a moment resistant grillage to connect to the tower base. The steel grillage utilizes a new moment resistant connection and clamp system to transfer loads from the piles to the grillage. This connection provides a true fixed head condition and reduces lateral deflection issues typical to smaller diameter piles in pinned head connections. The clamp is designed to deform the helical pipe pile shaft and transfer the full axial load of the structure through the mechanical and friction connection. HPS manufactured the helical piles and the moment resistant grillage with shorter than average lead times and teamed up with Atlas Foundation Co. (Atlas) to perform the installation. An expedient plan of install was developed by Atlas that resulted in an average installation rate of one complete set of tower foundations per day. The design of the moment resistant grillage, clamping system, and excellent foundation construction by Atlas provided an expedient and efficient foundation system to be discussed in this paper.

 article #2675; publication #1031 (SP-2017)