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A Terrasmart Innovation Engineer looks at ways of overcoming weather challenges particular to the installing solar in Midwest states.
Frost heave is one of the most common concerns in the Midwest and especially in Illinois, which is prone to frost heave due to the combination of bitter winters and one-of-a-kind soil composition.
Because frost heave is the process of soil expanding after it absorbs water and then freezes, Illinois and other states with periods of sub-zero temperatures for prolonged periods of time are especially vulnerable. It’s not possible to completely prevent the physics of frost heave, but it is possible to prevent these physics from wreaking havoc on solar sites even in the depths of winter.
Nearly all soils can heave, but certain soils are more susceptible than others, such as varied clays and other fine grained branded sediments common in Illinois. It’s also important to understand the rate at which soil can heave, dictated by its grain size, structure, permeability, and capillary flow. Illinois sites frequently have more than one type of soil on the same site, and design and installation need to be adapted accordingly.
Installations in Illinois can be tricky. The Midwest tends to have loose topsoil, but it can still be hiding boulders, bedrock, and cobble, even if it’s been used for agriculture for years. The depth of frost also significantly impacts foundation design, and Illinois winters can generate frost depth of more than five feet. Developers are faced with all of these considerations on top of variations in slope and undulating terrain.
A common mistake in weather mitigation is focusing on one piece of a timeline. Sites can be vulnerable from the planning stages to years post-operation.
In the planning process: Before a site is developed, failing to adequately assess soil or topography or making incorrect assumptions about frost depths or snow accumulation can create vulnerabilities.
During installation: Refusals occur if a pile takes longer than five minutes to install or doesn’t advance 12 inches within 30 seconds. During installation, conditions that lead to frost heave can also lead to refusal risk, which in turn leads to project delays and budget escalation.
Over time: Even if a site operates successfully and without incident for a year, it can still be vulnerable to frost heave over time. Ice lens growth can cause soil to freeze to foundations, a process known as adfreezing, and create an upward heaving force. If a site wasn’t developed with the variety of unique site conditions in mind, frost jacking may happen years after installation.
How to frost heave-proof a site
Giving frost heave the heave-ho entails a one-of-a-kind approach to every site, following these steps:
Assess the site: Even though silt is a common soil across Illinois, there may be other soils mixed in adding complexity to a site. Developers and EPCs can protect a site by reviewing a report of soil properties and refusal risk estimates. In other words, decision makers need to accurately perform pull tests at multiple spots on the site and extensive geotechnical assessments before moving on to installation.
Build a strong foundation: If a developer and/or EPC selects a foundation before choosing a racking partner, they may miss critical assessments, such as subterranean issues that inform the right foundation options. One of the biggest foundation questions in any site planning process is piles vs. ground screws. Ground screws are uniquely suited to deep freezing regions, are usually more cost effective and faster to install, and ensure the best long-term reliability. Still, there may be unusual conditions that make piles a better choice. A racking partner can offer a break-even analysis to help determine the best site approach.
Prepare for snow load: The majority of frost heave considerations are focused on what’s happening in the ground, but preparing for snow load is also important. Racking that’s not designed to withstand snow load can weaken foundations, increase costs, reduce productivity, and decrease longevity. Snow load preparation may include module tilt, higher module front edge height, reinforced material, and an individual table-based design approach. |
