With the large number of photovoltaic (PV) solar power facilities being developed across the country, one of the major expenses is the PV panel support system and foundation. These costs can vary based on site conditions and other factors, often more significantly than critical outlays such as electrical cable and the PV panels themselves. Over-built, conservative, expensive foundation solutions can render a project unviable, emphasizing the importance of engaging geotechnical engineers early in the process and having them maintain an active role throughout project development and even construction. In the absence of well-established industry practices engineers and consultants developed a variety of methods and procedures to design foundations for solar panel arrays. Olathe, Kansas-based engineering consulting company Terracon has worked to increase the quality, safety, and efficiency of industry practices while delivering success to clients on several recent solar projects.
Solar panel arrays must be anchored to resist pull-out and lateral loads. The two main foundation options for supporting PV panel arrays are a mass of large concrete blocks (ballast foundations) or short steel posts driven into the ground (pile foundations). Driven pile foundations are typically more economical, especially on large sites.
Full-scale load testing – the most precise and the only direct method to determine foundation capacity – has been widely adopted in the industry as a key design component. In testing, the movement of a prototype foundation is measured under simulated design loading conditions. Empirical test results can help economically optimize the design. Other theoretical design methods rely heavily on correlation and probability. Solar farms can contain tens of thousands of pile foundations, so if full-scale testing demonstrates sufficient performance slight reduction, in pile lengths or section thicknesses will likely result in significant savings in steel and installation labor.
Terracon’s standardized pile load testing approach, in use successfully across the country, includes a custom tripod system that exceeds American Society for Testing and Materials (ASTM) specifications for deep foundation load testing allowing test methods to withstand independent engineering reviews that are often required by financing partners. The portable frame can access locations in rough terrain. Terracon has developed alternatives and improvements to the load application portion of pile testing. In the Southeastern U.S., the company developed and fabricated a more portable and efficient load frame system to increase load-test productivity. In the Southwest, a portable, trailer-mounted load frame has been successful. Laboratory-precision data acquisition equipment has been adapted to eliminate errors in manual transcription of results, improving the accuracy and reliability of testing results. The system can simultaneously record data from multiple instruments, several times a second. The versatility of data acquisition equipment has enabled reliable data collection from the reaction pile used in lateral load testing – effectively doubling the number of tests for a given amount of effort. Specifications for load testing using data acquisition have become increasingly common on industry request for proposals (RFPs).
In early 2015 Terracon assisted Strata Solar in meeting an aggressive schedule for a large number of 5MW (AC) solar farms in central and eastern North Carolina. Hundreds of full-scale load tests were performed at project sites during the first half of the year, allowing seamless transition from the project design to construction phase. The improved efficiencies permitted rapid turnaround of testing results and real-time collaboration, supports solar projects that generate electricity to meet the projected needs of more than 20,000 homes per year.
Testing improvements also helped another solar company avoid costly construction delays and potential demolition and reconstruction on a project in eastern North Carolina. Months into construction of a utility-scale solar project, pile-installation crews reported poor driving resistance for production piles. Terracon engineers in Raleigh, Savannah, and Charleston completed hundreds of production pile load tests within a two-week period, leading to a project redesign. Terracon’s testing procedures, and data acquisition methods allowed rapid turnaround of testing results. The data collected showed that installing a limited number of extra production piles at the perimeter of the completed arrays would allow construction to resume quickly. A redesign of foundations for future phases of the project allowed for use of piles already purchased, cut to design length, and stockpiled on site.
Terracon’s geotechnical service line has used specialty subsurface investigation techniques such as cone penetration testing (CPT) and dilatometer modulus testing (DMT) to provide better value to clients on projects, especially in the coastal plain geology of the Southeast. Near-continuous, in-situ soil data that can be better correlated to soil engineering properties such as strength and density. Testing at locations adjacent to full-scale pile load tests has allowed more accurate pile capacity predictions as compared to traditional soil test boring approaches and indirect design methods. Using results of specialty testing, engineers have been able to design site-specific load test programs, resulting in better value per load test with less iteration and testing to further optimize foundation design. With continued research and work with specialty testing, the number of full-scale load tests per site can likely be reduced in the future as the body of knowledge improves and industry methods become more standardized, reliable, and repeatable.
Collaboration between Terracon geotechnical and structural engineers typically results in a more holistic and intimate understanding of the loads and failure modes impacting foundations. Using a collaborative team to centrally evaluate project risks can eliminate the safety factors layering effect that often occurs when separate firms – one specializing in geotechnical engineering and the other in structural engineering – complete a project working together can often lead to decreases in uncertainty or safety factors that can unnecessarily increase construction costs.
Understanding geotechnical site characteristics is essential for establishing a cost-effective foundation for any project. Terracon’s site-evaluation procedure – report of expected geotechnical conditions (REGC) – assists developers in the critical site selection process. A dedicated GIS resource group mines and aggregates public domain data from numerous national and local sources to include geologic maps, soil surveys, glacial geology, topography, corrosivity, wetlands, karst features, water well records, historical aerial photography, and many other sources of information pertinent to the subject site and proposed development. While Terracon performs this service for many different project types, it has been especially well-received by the solar industry, where large sites are typical and unforeseen geotechnical issues can be disastrous.
About the author: Thomas Bartlett, a professional engineer in Terracon’s Raleigh, North Carolina geotechnical department, has managed projects involving geotechnical investigation and/or design on more than 250 proposed solar power facilities in North Carolina, Virginia, and surrounding areas. He can be reached at 919.218.9422 or firstname.lastname@example.org.