June 13, 2017
As Solar Racking Evolves, So Do Pile Load Tests
Solar arrays must produce energy day after day and remain unaffected by the forces of nature. Situated outside, solar projects are subject to just about anything nature can throw at them, such as gale-force winds, hurricanes, floods, earthquakes, and snowstorms. Natural occurrences such as these put owners and developers at risk of losing their investment, so it is important that solar projects are structurally sound and able to handle whatever blows their way.
Solar modules are supported by a structural system composed of racking and pile (post) foundations - and depending on the size of the project, the number of piles can be in the thousands. Project owners, builders, and operators rely on this structural system to buffer the modules from the elements so they can continue to produce energy.
Pile load tests optimize foundation design
The first key step in designing a structural system that is built to last is the pile load test. Consisting of applied loads and measurements of the subsequent pile deformations, these tests are used by engineers to assess the structural integrity of the piles through the application of physical loads to the pile, and then measurement of the subsequent deformations. The test measures pile performance in both the horizontal and vertical directions. Standardized by the American Society for Testing and Materials (ASTM) International, pile load testing is used throughout modern engineering practices as a method to confirm pile capacity and optimize pile foundation design.
In the solar industry, three types of load tests have become common: lateral, axial tensile, and axial compressive (depicted below). Lateral tests are used to simulate a combination of the horizontal wind, seismic, and motor torque forces, axial tensile tests simulate wind uplift forces, and axial compressive tests simulate a combination of the downward wind, snow, and equipment loads. Combined, these tests help to minimize material costs and ensure structural performance for the thousands of piles supporting a solar project.
Solar Pile Load Tests – Axial Tensile (left), Lateral (upper right), and Axial Compressive (lower right)
Pile design advances with the solar market
Starting in the mid-2000’s, multi-megawatt, utility-scale solar began to take-off, and these ground-mounted projects utilized fixed-tilt racks to support PV modules and hold them in place. The module-rack systems, however, would act like kites or wind sails sticking up in the air at an angle. The racks supporting them would impose significant horizontal and uplift wind forces on the piles. To help engineers better understand these forces, axial tensile and lateral load tests quickly became standard tests for solar pile design.
Today, single-axis trackers are quickly gaining popularity for use on ground-mount solar projects. Thanks to cost reductions and their additional energy output, worldwide tracker installation accounted for approximately 30% of all ground mounted solar in 2016 and current projections forecast trackers to reach 50% by 2021.
As solar racking continues to evolve, so do the forces applied to the piles. Single-axis trackers tend to have more components (e.g. combiners, motors, gears) and therefore heavier loads compared to fixed-tilt systems. Add snow load and the axial compressive loads become significantly larger than the tracker’s axial tensile loads.
Conservative designs can result in significant costs
Axial compressive loads may control design of single-axis tracker piles, but field-test methods have lagged behind. Axial tensile tests remain the dominant vertical load test in the solar industry. Pile designers, such as Westwood, can develop designs with axial tensile data; however, the International Building Code states that end bearing resistance developed at the bottom of the pile shall be ignored unless specifically allowed in the project geotechnical report. Ignoring the additional axial compressive capacity gained from end bearing can result in conservative designs (up to twice the embedment depth) and significant cost.
Westwood’s pile design services are optimized to include axial compressive load testing across the country, improving design and reducing risk for developers and contractors, and reducing costs to the project.
Axial compressive load testing methods
Using calibrated hydraulic jacks and mobile reaction counterweights, such as an excavator or backhoe, compressive loads are applied to the pile and vertical displacement is recorded simultaneously. The below photos display compression tests performed by Westwood, using portable equipment and operated by experienced technicians.
Axial Compressive Pile Load Tests – Backhoe Reaction Force (left), Excavator Bucket Reaction Force (center), and Excavator Frame Reaction Force (right)
Projects that benefit from axial compressive load tests
Solar projects utilizing single-axis trackers and/or situated on sites having poor soils benefit from axial compressive load tests by moving away from an overly conservative and costly approach. Westwood geotechnical engineers understand the most complex subsurface conditions and develop site-specific optimized solutions. Contact us to learn more.