From pv magazine USA
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.
Most vulnerable sites
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.
What to ask
With all this in mind, developers can protect their sites by asking a partner these questions about their expertise and experience.
Frost Heave
- Where do you get your adfreeze and frost depth values?
- How do you calculate frost heave loads and forces?
- What’s your process of performing soil analysis?
- Have you worked on sites that have experienced frost heave, and if so, how have they performed in the long term?
Snow loads
- How do you calculate snow loads for a project?
- What’s the heaviest snow load you have installed?
- How do you track the weather? Do you have trackers with snow sensors and snow shed features?
Topography
- What slope tolerance can you accommodate?
- What slope tolerance have you installed in?
- Is your racking adjustable during installation?
Installation
- How many foundations do you plan to install per day, and what is the basis for that assumption?
- Have you accounted for refusal risks and costs during the planning phase? How did you arrive at these results?
- Where have you installed projects? What’s your familiarity with the regional and local specifics?
- What are the steps you take to make installations more predictable so costs and timelines don’t escalate?
While solar in Illinois has its share of challenges, these conditions are solvable with careful assessments and customized developments and installations. Since frost heave can occur even years after a commercial operation date, it’s wise to consider the long-term ROI and reliability of racking systems and not just the upfront costs. With weather challenges and frost heave at bay, Illinois is poised to continue its national leadership in utility-scale solar and the clean energy transition.
Ameer Abuhamdeh, currently serving as Innovation Engineer with Terrasmart, brings product development and mechanical engineering experience to his 5-year tenure at Terrasmart. His wind tunnel and product testing have produced innovational solutions for Terrasmart’s ground mount systems team. Abuhamdeh holds a degree from the University of Cincinnati with a Bachelor of Science in Mechanical Engineering.
The views and opinions expressed in this article are the author’s own, and do not necessarily reflect those held by pv magazine.
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