Geotechnical Analysis and PV Foundation Design

Inadequate site assessments can lead to overengineered and unnecessarily expensive foundations. Worse, they can lead to costly foundation failures.

Ground-mounted PV power plants require two basic foundation design components: geotechnical engineering and structural engineering. Geotechnical engineering focuses on evaluating soil mechanics so that the foundation design can incorporate these characteristics. Structural engineering focuses on modeling the foundation as a supported beam to ensure that it can successfully support the design loads.

Of the factors that determine optimal foundation design, geotechnical site characterization is arguably the most challenging. This is partially due to the fact that feedback from the field about long-term foundation performance invariably lags behind project deployment. Given the risk associated with foundation problems, which can impact both short-term and long-term project profitability, geotechnical investigation is one of the solar industry’s most overlooked site-selection criteria.

Here we briefly consider the unique nature of PV system foundations. We detail the challenges and basic components of a geotechnical site assessment. We explain why analyzing load-test data is essential to a site-optimized foundation design. Finally, we review why designing from the ground up is essential to your bottom line, in terms of both up-front costs and long-term profits.

Solar-Specific Foundation Design

Given that the utility sector has driven much of the US solar growth in recent years, it is easy to forget that large-scale ground-mounted PV power plants are a relatively recent phenomenon. Veteran project developers might have a decade of experience in designing and deploying solar farms. Further, the market has changed dramatically, in terms of both typical project capacity and average installed costs. As a result, solar-specific geotechnical engineering is in its infancy compared to geotechnical engineering for more conventional applications such as vertical construction, buildings, bridges or dams.

AquaSoli CEO Jürgen Schmid has specialized in solar-specific geotechnical analysis and foundation design since 2004. He notes that solar foundations present unique design challenges: “PV power plants have a very high number of relatively small piles. People tend to underestimate the skills required to use small piles effectively, because the design loads are very low compared to those for a high-rise building or a bridge. However, there is a considerable need for pile optimization in terms of economic material utilization and embedment depth. Further, climatic effects that influence the first six feet of soil can lead to plastic deformation of soils and structural fatigue of the piles.”

In other words, a well-designed solar foundation needs to be cost-effective without sacrificing reliability. While the design loads associated with ground-mounted PV systems may be small compared to those for other structures, the foundation still needs to support considerable dynamic loads. In the Boston area, for example, design wind loads approach 120 miles per hour and static snow loads are roughly 60 pounds per square foot. Some mounting systems have almost 70 square feet of rigid sail per foundation. Depending on rack design and static and dynamic loads, this can translate to as much as 5 tons of force per foundation. Any foundation system can fail over time when subjected to these forces, and foundation system failures are expensive to mitigate.

Quality geotechnical data are key to designing a reliable and cost-effective foundation. “Without the proper geotechnical information, we have to make conservative foundation design assumptions,” notes Daniel Stark, PE, CEO of Stark Foundations. “While design conservatism is not necessarily a bad thing, being overly conservative can cost our clients money. This could make the difference between a project moving forward or not, between winning a project or not. The minimal expense to conduct a proper geotechnical analysis at the beginning of a project far outweighs the cost of an overdesigned foundation system on the back end of the project.”

Given the considerable price pressures that factor into the development of large-scale PV plants, foundation design must be based on adequate site characterization. The better you understand these conditions, the more effectively you can work with your engineer to optimize the foundation. “Geotechnical engineering is the first step to a well-engineered project,” explains Adam Tschida, PE, a principal engineer at Kleinfelder. “Proper geotechnical engineering requires a good understanding of what you will be building and how the development will interact with the earth and the environment. This is especially true for PV project development.”

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