How Soil Types Can Affect Solar Installations


From kilowatt backyard installations to megawatt projects, soils will have an impact on the type and cost of the foundation. A typical saying in geotechnical engineering is, “pay me now or pay me later”. By taking the time and effort to know your site before construction, you can optimize your foundations to save labor, time, material costs, and avoid headaches.

In this article, we will show you the basics of the soils you may encounter.

First, a designer or engineer should know that soils can consist of shallow bedrock, cobbles, and gravel, sand, or clay—each of which can have an impact on the foundations. Shallow bedrock or cobbles and gravel can cause issues with installing pneumatic hammer-driven piles (the upright posts that underpin the arrays) resulting in pile refusal and the need for concrete cast-in-place, ballasted designs, or groundscrew foundations. Sand soils can have low vertical or “pull-out” capacity, requiring deeper foundations. Soft clay can have a reduced lateral capacity, meaning piles can be pushed over, which can result in a need for deeper foundations and larger steel sections to remain stable.

Soils can also cause additional problems if there is shallow groundwater. Soils can be prone to liquefaction, and some soils are in susceptible environments that can cause frost heave or clay heave. Even if the soil tests indicate these issues do not exist, corrosion can occur in all soil types and can have a big impact on cost of foundations.

Determining Soil Type

Because of these issues, it is important to know the soils that comprise your site. This is usually determined from geotechnical investigations that include on-site testing. The testing will determine what kind of soils are on site and identify potential problems that will need to be considered during the design phase.

People often refer to the Natural Resources Conservation Service web soil survey as a substitute for an onsite geotechnical investigation, but the data in the web soil survey is vague, has not been verified, and does not provide the detailed soil properties needed to determine any potential soil issues or calculate foundation load capacities. It is best used as a reference point to determine areas of interest and focus for a geotechnical investigation. It is similar to viewing a house for sale on a website—you would not buy the house based off just a website and an aerial photo. You would want to see pictures of the inside taken by someone who made the site visit, or you would want to visit it and see it yourself if possible. Home inspections by qualified people will determine if the house has issues, much as a geotechnical engineer will determine if your site will have soil issues, and if issues are identified will be able to provide a design that neutralizes those issues.

Geotechnical investigations can identify potential issues such as:

  • Liquefaction, in which the soil can turn into quicksand during seismic events
  • Organic soils that will provide zero capacity and be extremely corrosive
  • Karst formations that can contain soft clay and intermixed bedrock
  • Potential frost heave or clay heave that can cause piles to progressively be jacked out of the ground
  • Bedrock or dense gravelly soil that will require special equipment
  • Soft soil that will require deeper larger foundations
  • Corrosive soils that will corrode the galvanization and steel in a few years

Using Data to Plan

Being able to identify these potential issues allows the engineer to design for them and mitigate the cost impact to the foundations. A total understanding of the soil composition, before construction begins, will help the project encounter fewer delays and costs. Imagine if, after installing all the steel foundations, you were to find out the soils are corrosive and the piles will be too weak to withstand design loads in five years. The piles would need to be removed, new bigger foundations ordered (which can take up to 12 weeks) and the piles would then need to be reinstalled, all resulting in significant cost and time impacts.

For kilowatt-size backyard solar projects, it is typically more cost effective to over-design the foundations using concrete than to have a geotechnical investigation done. International Building Code allows for this if cast-in-place concrete foundations are used, by allowing assumed soil properties. This approach can be a little risky because it does not account for any soil issues identified earlier. If the soils do not have the issues mentioned, the soil assumptions stated in IBC usually make the design very conservative but still practical. Some installers choose to use smaller foundations, but these cannot be justified using IBC and the installer may not get a passing inspection by a building official since no engineer or code was used to design the foundations.

For large kilowatt to megawatt projects, it is usually most effective to use driven piles however even IBC states to determine capacity, an approved geotechnical method or pile testing is required. Pile testing is the best way to determine capacity. It is testing the actual soils on-site and is not based on empirical formulas that can be conservative. Pile tests often prove the piles can resist higher loads at shallower depths that result in shorter piles and significant cost savings. Geotechnical investigations are still needed to rule out potential issues because pile testing cannot determine how corrosive the soils are or if other issues will be a concern.

Unirac’s Commercial Services team can always help with further questions about soils. Please contact us at or call us at (505) 248-2701.

To learn more make sure to sign up for our webinar on The Importance of Soils for Solar Ground Mounts.

About the Author

Jon Schermerhorn is a Professional Senior Civil Engineer at Unirac with more than seven years of experience. His career has spanned designing specific foundations for more than 600 solar groundmount projects that have ranged from a few kilowatts in size to more than 400 megawatts in size.