Design of Grillage and Helical Pile Foundation for Limited Access Transmission Line-Paper

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Following a 2016 storm that damaged a 345-kV transmission line near Rogers, Minnesota, Xcel Energy replaced ten tower locations with steel H-frame structures supported by helical piles. Wet, soft ground, high water levels, and restricted access made conventional pile-driving and concrete foundations impractical.<br /><br />Hubbell Power Systems designed a steel grillage connected to four RS3500.300 helical pipe piles per structure. Piles were modeled using GROUP and HeliCAP software, with the helices bearing in stiff clay beneath soft surface soils. The grillage was designed to transfer axial, shear, and moment loads while providing a fixed-head connection to limit lateral movement. A new clamp-and-grout system connected piles to the grillage without field welding: clamps mechanically engaged the pile shafts, while grout in the connection tubes provided fixity and transferred moment and shear. Where greater above-ground pile exposure was needed, 4-inch Schedule 80 sleeves increased bending and buckling resistance.<br /><br />The contractor developed an installation method suited to saturated sites and small equipment. The grillages helped position and align the piles, which were installed to specified torque requirements before being cut, clamped, and grouted. Installation averaged two grillages—one complete tower foundation—per day. The project demonstrated that a prefabricated, moment-resistant grillage and helical-pile system could provide an efficient foundation solution where access, groundwater, and soil conditions limit conventional construction.

Keywords

Xcel Energy

Rogers Minnesota

345-kV transmission line

helical piles

steel H-frame structures

steel grillage

clamp-and-grout system

stiff clay

soft wet soils

tower foundation