Solar Array Foundation Options for Flood-Prone Sites

January 24, 2018

With the continued growth of solar in the U.S. and the uptick in utility-scale solar projects across the country, there is a continual need for sites that are suitable for solar development. Placing a solar plant on land that floods periodically can sometimes be a solution that makes both practical and economic sense. Such sites might be otherwise ideal for solar because they’re level, and close to transmission lines and power users, but yet unsuitable for other types of development. The cost of the land can therefore be more economical than other options, but must be weighed against any increased costs that go toward ensuring that the plant will perform successfully when flooded. One key aspect of this is the design of the foundation for module mounting systems. Let’s take a quick look at some things to be considered when designing foundations for these flood-prone areas.

Foundation design for a flood-prone site starts with knowing the design flood elevation. In areas subject to flooding caused by storms, the latest and best available flood maps should be consulted. FEMA is the main source of these maps in the U.S., but it should always be confirmed whether other mapping exists at the state, county or local government level, for a particular site. Sometimes all or part of a site may be used as a retention basin that is designed to flood to a maximum depth, in which case the design water level can be quickly determined. Raising the height of the solar array, so that modules and key components are above the design water level, is a main reason why foundation costs can increase on such sites. SunLink’s TechTrack, for example, is designed to automatically adjust and reduce its tracking range by going into flood mode in response to rising water levels. This feature reduces the impact of flood-prone sites on foundation cost.

As part of any foundation design it is important to understand the soil that will support it. When designing a foundation for a flood-prone area, it becomes particularly important to understand the capacity of the soil when it is submerged. Underwater, it is the buoyant weight of the soil that influences the soil’s capacity, which explains why design soil capacity will typically be diminished in flood-prone areas. This effect is important for any site where the below-grade water table may fluctuate seasonally in the zone of the foundations, but it becomes more pronounced where flooding occurs.  Geotechnical investigation data, such as soil type and SPT results from boring logs, should be be used, together with standard soil mechanics tools and software, to determine a suitable foundation design. Driven piles can often be the most cost effective solution, but in some cases such as very sandy soils, helical piles or a concrete ballast footing may be preferable.

There are a number of other potential issues with flood-prone sites that the foundation designer should be on the lookout for:

  • If scour can occur, this should be accounted for in the design. Scour is the lost of ground around the foundations caused by the movement of water.
  • Swelling and contraction of the ground due to expansive clay may need to be considered.
  • Pile testing results, if obtained, must be evaluated considering the effect of the ground water level, as discussed above.
  • In marine areas, corrosion of steel foundation elements is always a key concern, and pile coatings are often required. However, corrosive soils, brackish water, and effluents with corrosive potential in retention basins, can exist in many places far removed from marine areas.

With a proper understanding of the challenges, solar plants can be successfully deployed in flood-prone sites. Over the past decade SunLink has provided successful solutions for solar plants of all sizes in water retention areas and in area where storm surge and storm-caused flooding can occur.

Rob Ward is vice president of Engineering at SunLink. He is a registered structural engineer in California with over ten years of experience in the solar industry. He is a member of the Structural Engineers of California PV Systems Committee.

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