Frost heave can drastically affect a solar project’s life span. When water freezes in subsurface soils, it forms ice lenses that shift and move upward. The movement of this ice causes racking systems and piles to deflect, with costly consequences such as cracked modules and damaged string and ground wiring. Extensive replacement and repairs will be required to bring the system back into operation.
Deploying solar as a viable energy source in frigid climates presents challenges that are not present when developing solar installations in more temperate climates. The tough environmental conditions impose a steep learning curve.
Cold Weather Risks
Frost heave causes the soil volume to expand as water converts to ice. Ice lenses formed by free water drawn to the underside of the frozen soil layer expand and move the frozen surface soils upward. If the bond between the frozen soil and the pile surface is strong enough, it can break the remaining pile uplift resistance of the unfrozen soil layer, lifting the pile.
Contractors working in cold climates are all too familiar with frost heave. A number of projects in Canada’s Ontario solar market are experiencing economic and schedule impacts from this cold weather phenomenon because project teams have neglected to use cold weather mitigation strategies. Contractors with extensive renewable energy experience have found successful solutions. By using differing types of foundation methods, such as helical piles, for example, contractors can prevent the adverse effects of frost heave and avoid costly repairs and lost revenue. Experience with ground mount photovoltaic (PV) foundations in cold climates has shown that bearing-type footings such as helical piles and concrete spread footings perform well in frost-susceptible soils when local recommendations are followed. Owners concerned about frost heave at their project should consider commissioning a geotechnical study of the area to optimize parameters for foundations and help ensure the project achieves useful operation throughout the project life cycle.
Foundation Protection and Testing Strategies
Projects with a life span of 20 to 25 years require additional measures to prevent the effects of frost heave, which can be exacerbated by corroded steel piles. Galvanizing the steel’s surface is a common method of protecting against corrosion and frost heave. The National Weather Service can provide soil temperature depth maps to determine anticipated frost depth.
A well-thought-out pile verification program is essential in areas where frost heave has an impact on the final design. Contractors should have a design verification testing program in place to test and confirm the piles’ capacity, optimize the pile size and embedment depth and delineate areas of potential construction issues, such as soft or obstructed areas. Foundations must be installed under a quality control program that includes visual inspection, torque measuring and production proof testing that tests the anticipated frost load and deflections. Testing foundations before installation allows teams to correct issues early in the construction process. If excessive deflections are encountered, pile remediation can be performed to include deeper embedment with longer piles or spliced piles, larger or multiple helix piles or insulation. Examining each case individually will determine the best remediation type.
Contractors must consider the stress of the movement caused by frost heave on the equipment concrete pad and PV equipment, including on the transformer medium-voltage terminations. Excessive ground movement can be accounted for by providing additional loops in the cable and installing horizontal runs in the PVC conduits. Expansion couplings in horizontal and vertical runs will protect combiner box, re-combiner box and transformer cable terminations.
The moment a pile moves, it creates stress on all aspects of the solar PV system, from electrical infrastructure to foundations. If owners want their projects to meet their target energy outputs for years to come, they must choose a team that understands frost heave and its potentially serious ramifications. Contractors deploying solar energy solutions in ice-prone areas must develop a solution to prevent frost heave, and ensure the processes are in place to guarantee success.