Axial Tension and Lateral Capacity of Step-Tapered Helical Piles for Transmission Pole Structures

The foundations for most transmission line structures constructed in North America today are drilled shafts. However, drilled shafts are not always a suitable foundation choice. This paper details one example where helical piles were used as the foundation at select tower locations on the XCEL Energy Stinson-Bayfront Line 3315 32.1 Mile Re-Build 115 kV transmission line project. The structures were steel H-Frames, made by Thomas & Betts Meyer. The tangent structures were specified as direct embedded, and the running angle and dead-end structures were either helical piles or drilled shafts. The line is located in a remote area of northwest Wisconsin near Duluth, MN. Helical piles were used at tower locations with poor road access, and in areas considered environmentally sensitive where large vehicles would destroy the landscape and vegetation. Xcel, Hubbell / CHANCE, and Thomas & Betts teamed together to design this project. Xcel Energy produced the load and design drawings, Thomas & Betts designed the H-Frame Pole structures and the pile transition connections, and Hubbell / CHANCE designed the helical pile foundations. Prior to line construction, full-scale load tests of the helical pile / connection were performed at the Thomas and Betts Structure Test Facility located in Hager City, Wisconsin. The running angle and dead-end pile and transition systems were successfully loaded and tested to values that meet or exceed the ultimate design structure ground- line reactions. These results are presented. The piles, tested together in a multi-pile group, were subjected to a combination of axial tension, compression, and lateral shear. In addition, four full-scale load tests were conducted on the line right-of-way at the actual tower locations. The tests consisted of pile penetration tests, axial tension, and lateral shear tests. These test results are compared to the theoretical capacity of the piles based on site soil type and strength. The results of the tests allowed the helical pile design to be adjusted prior to start of construction to provide the contractor with minimum depth and installation torque requirements.