The short answer is: it depends.
Ground-mounted arrays penetrate the ground-surface to stabilize the rack structure and have a variety of foundation types. Soil composition, local climate conditions, module size, array tilt and other features of the proposed site and array influence what makes a ground-mount foundation the right fit for an individual solar project.
“Arrays may be mounted on driven beams, anchor systems, ballasts or hybrid racking systems,” said Bill Taylor, CEO of DCE Solar.
Driven beams are support beams, usually made of steel, that are driven into the ground at a pre-determined depth. The superstructure of the rack and panels is then attached to those beams. The size and the length of the beam are determined by site conditions and array configurations, according to John Klinkman, Vice President of Engineering at AET. Driven beams are inexpensive, and may be constructed more quickly and simply than other foundation methods. However, unpredictable subterranean conditions can cause issues and impact performance.
In locations where soil is not well-suited for driven beams, an anchor system may be used instead. Examples include helical piles, expanding anchors and ground screws. “This type of system allows for an improvement in the ground mount’s ability to handle vertical loads at relatively shallow depth,” Klinkman said. “However, anchor systems are more expensive and take longer to build.”
A ballast system uses a man-made foundation to hold the rack and panel in place. Ballasts are most often used in commercial installations where ground penetration is not advised or permitted. A ballasted system usually has two vertical posts connected to a single concrete block approximately 2 ft. x 2 ft. x 8 ft, whereas a driven system would only require a single post. While completely controllable, ballasts can be very expensive and are not typically suitable for smaller installations. Precast pavers are less expensive, can save on labor costs and install more quickly, according to Taylor.
A hybrid system is an absolute “must” when dealing with multiple support methods, variable site conditions or unknown soil issues, said Taylor. These systems are comprised of a mix of ground-mount types, including driven beams, anchor systems and ballasts. Hybrid systems are used to address the specific needs of an installation with more than one type of terrain.
Ground-mounted systems can be installed on almost any type of land or soil, Klinkman said. North American examples include bedrock, clay, sand and even cobblestone. The most popular land types for installations are farm fields, empty lots, commercial parking lots, landfills and simple open spaces with minimal shading.
“The best way for a contractor to determine the ideal foundation for a given soil type would be to conduct a proper subsurface investigation,” said Klinkman.
“A qualified geotechnical firm is a good method to evaluate the existing soil types on any site,” said Taylor. “A detailed report will include prevailing soil types present on the property, as well as any obstructions, rock formations, drainage considerations and other challenges present on the site.”
Knowing the results of a proper geotechnical investigation will allow contractors and installers to select the most appropriate mounting approach, as well as the type of racking necessary to work with the mounting system.
Other considerations a contractor should be aware of are installation costs and time, rack maintenance requirements, quality of the racking system, site accessibility, weight restrictions and classification and history of the property.
“The goal is to spend the correct amount of resources at the beginning of the project to properly understand the site conditions and constraints. The foresight produces the most effective foundation design,” said Klinkman.
“Like all construction projects, the right answer is usually a mix of considerations that vary depending on the individual client and project,” Taylor added.
I agree completely with all the different ground mounts discussed. But, having encountered some really nightmarish situations, I must say that doing your pre-planning work with a really experienced geotechnical firm will save you a ton of headaches. As with most types, if not all, of construction a poor foundation will result in a poor build and future problems.