Lateral Capacity of Helical Piles–Actual vs. Theoretical Foundations for Solar Power Plants

Abstract

The construction of new solar power plants is occurring all over North America. Helical piles offer a cost effective solution for ground mount solar power plants due to their low material and production costs, fast installation in most soils, and their ability to develop sufficient uplift and lateral capacity with relatively short lengths. Structures that support solar panels are typically racks with panel mount systems supported by two to four piles. The typical size range for helical piles used for solar power plants is 3.5 to 8 inch (89 to 203 mm) diameter pipe shaft, and 7 to 15 foot (2.1 to 4.6 m) embedment depth. The helical piles are designed to resist compression, uplift, and lateral load. The axial forces are relatively small, except in areas of significant depth seasonally frozen ground or expansive soils. However, the lateral load can be significant and is typically the limiting condition for the pile design. This paper will examine the theoretical lateral capacity of helical piles based on the finite difference method used in the software program LPILE PLUS by ENSOFT, Inc. LPILE is typically used for larger diameter driven piles and drilled shafts; it's validity for use with smaller pile diameters is not clear. The results of full-scale in-situ lateral load tests will be compared to the results obtained theoretically with LPILE by use of the soil parameters provided in geotechnical reports. Data from three test sites will be examined, with each soil condition being different. The results show that the actual lateral capacity based on in-situ tests is greater than what theoretical models predict, both in terms of load capacity and stiffness; regardless of the soil type. The LPILE model appears to predict better in clay and when the pile diameter increases. The possible reasons why the actual capacities are greater than the theoretical will be discussed, and what can be done to correct the theoretical models used. More research is needed to adjust the lateral capacity model so that it will provide more accurate results with respect to lateral capacity and deflection.