Case Study: PRI Engineering Takes on Arctic Permafrost During Solar Project

The Inuvik Solar Project to reduce dependency on diesel fuel for Inuvik communities

Early last year, the Canadian federal government pledged $8 million to develop eight clean energy projects in the Northwest territories. $800,000 was awarded to Nihtat Energy Ltd., a development company to implement a community clean energy project in Inuvik.

Inuvik is a small town with a population just above 3,000, that borders the east coast of Yukon and is one of the largest consumers of diesel in the Arctic. According to the Canadian federal government, Inuvik consumes the most diesel for heat and power when compared to the rest of the Northwest Territory communities. Construction of the Inuvik Solar Project is expected to reduce the dependency on diesel fuel and redirect the communities to clean sources of energy.

The Inuvik Solar Project will be a ground-mount solar facility with a capacity of 1.62 MWDC. Construction for the proposed grid-connected solar facility commenced in 2021 and is expected to reach completion by 2022.

Permafrost Poses Solar Foundation Design Challenge with Unstable Ground Conditions

PRI Engineering has completed a few other projects in the Canadian arctic, however, this is the first project where permafrost was encountered and needed to be considered for design. Permafrost will form when the ground remains at or below freezing temperatures (0⁰C / 32⁰F) over the course of the entire calendar year. Because permafrost can accumulate anywhere from several inches deep to thousands of feet deep, it is expected that installers will hit a few challenges when these conditions are encountered and not adequately investigated. In permafrost environments, the changing seasons can trigger certain reactions, such as frost uplift in the winter months and downdrag, typically caused by the thawing of the active overburden layer, during the summer months.

Given any situation where permafrost is present, it’s important to consider its active layer. The active layer thaws in the summers and freezes again in the winter and sits atop the permafrost layer.

For example, permafrost that is initially observed at a 100-meter thickness can decrease to 90 meters when the active layer beings to thaw in warmer weather.

Let’s quickly talk about what thawing and frost heave are and how they can disrupt ground conditions. Thawing reduces the effective stress of the soil which can reduce foundation capacity, possibly leading to failure. Thawing also creates an uneven subsurface, which can lead to significant damage to the structure. Frost heave is developed when the active layer freezes in the winter months and puts an “uplift” force on the foundation. This is caused by the expansion of water freezing in the pores of the soil (water expands 9% in volume when turning from liquid to solid).

Constructing on permafrost can get tricky as changes in the active layer will occur and introduce varying load cases, which must be considered for design. Challenges related to access may increase earthwork requirements if construction methodology is not considered when scheduling. Permafrost will impede permeability; therefore, drainage of the overburden layer is somewhat problematic during thawing. As the active layer thaws, if adequate drainage is not provided the layer will become unstable when saturated, meaning winter construction is often preferred, but you then need to be ready to deal with -50oC. That’s why completing a proper geotechnical assessment to identify the risks prior to building is critical. In Inuvik, initial assessments revealed permafrost at depths of approximately 1.5 m. It was clear that we would not be able to go with a traditional solar racking foundation design and the design would need to be modified due to the presence of permafrost.

PRI takes on preliminary foundation design

Typically, pile designs consider frost penetration depths of around 1.8 m to 2.5 m. This is a rough average that we observe for regions in Alberta where the climate tends to run a bit cooler than some of their neighboring provinces. Frost depths are usually deeper in colder climates, especially in regions where temperatures reach below freezing point. The greater the frost depth, the greater the frost load you must contend with. Because this project is located in the Northwest Territories where it is normal to see temperatures taking a nosedive to around -50⁰C, we were prepared to run into anywhere from 10 to 15 meters of permafrost.

A variety of load tests mechanically mimicked snow loads, dead loads, frost uplift, and eccentric wind loads to evaluate the pile’s maximum capacity and optimize our pile designs. To give us an idea of the effect of thawing of the active layer, we calculated downdrag forces and creep settlement of the piles. Tensile pre-production load tests were carried out on installed test piles.

The Solution: to Secure Pile in Place

PRI deviated from the traditional foundation design because of the permafrost layer that was observed at a depth of approximately 1.5 m. Usually, traditional foundation solutions include driven piles, helical piles, and micropiles. For the Inuvik Solar project, we opted to use an adfreeze pile.

With an adfreeze pile, a hole is drilled into the underlying frozen material. We then filled the holes with arctic slurry, which is a mixture of sand and water. The mixture then freezes around the pile, which creates resistance from the design loads.

”We drilled a hole to test the target embedment depth and then backfilled it with the arctic slurry mix. The pile held concentric to the hole as the mixture froze around it.

Jelica Garcia, Geotechnical Project Manager at PRI Engineering

The results from our field investigation tests came out as expected and we were confident to move forward with our completed foundation design.

Working with permafrost can be exceptionally challenging. The biggest threat is when the active layer begins to thaw. As climate change has driven up temperatures, the rate at which the ice in the permafrost begins to melt starts to increase. It takes the right team to form a creative solution that ensures the success of the foundation on the ground. This was a notable first for PRI Engineering, taking on a solar project in permafrost conditions. The team successfully assessed the site and ground conditions and derived a workable solution for a foundation design that would guarantee a strong pile. The remaining work on the Inuvik Solar Project is expected to be completed by 2022.