Evaluating Thermal Regime of Cold Region Roads for Climate Change Adaptation

Nearly a quarter of the Northern Hemisphere is underlain with permafrost, of which discontinuous permafrost is most sensitive to the temperature of its surrounding. Climate change and the implementation of infrastructure have combined effects on the thermal regime of permafrost, leading to thaw settlement that can be detrimental to structures. In Northern Canada and Alaska, permafrost degradation is leading to unprecedented spending on maintenance and rehabilitation of linear infrastructure. Although a number of permafrost protection techniques for road structures have emerged in recent decades, their costs remain high and rate of implementation remains low. This predicament can be attributed to the lack of studies that compare the feasibility and performance of these techniques. There is also only a low number of field studies that examine the in situ thermal regime of permafrost influenced by both climate change and infrastructure, which further contributes to the uncertainty surrounding many of the permafrost protection techniques. In light of these gaps, this study conducted a literature scan for qualitative feasibility comparison of existing permafrost protection techniques, simulated the performance of select techniques in TEMPS for comparison, and examined the in situ thermal regime through a field study along the Ingraham Trail (NWT Highway 4) in Yellowknife, Northwest Territories.
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\nBased on existing literature, reflective surfaces, shading, ventilation pipes, thermosyphons, air convection, geosynthetics, pre-thawing, and heat-dissipating structures and materials are deemed suitable for high-temperature (discontinuous) permafrost, while embankment insulation was deemed suitable for low-temperature (continuous) permafrost. Most heat-dissipating pavement structures, like unilateral heat transfer pavement, may be suitable for both high- and low-temperature permafrost, though further studies are required. It was concluded from this review that most permafrost protection techniques available at the moment are designed for high-temperature, discontinuous permafrost. This is as expected since discontinuous permafrost has the highest risk of degradation in light of climate change and anthropogenic effects.
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\nIn order to simulate the performance of alternative thermosyphon fluids (lithium, potassium, and sodium) and insulative materials (XPS, FGA, and LWCA), a baseline pavement design was adapted from previous literature and verified using PAVEXpress against traffic loading. The structure was then transferred to TEMPS with each thermosyphon fluid of interest inserted into the pavement 80 mm below the driving surface and each insulative material just above the subgrade. Temperatures at specified depths with permafrost protection techniques were calculated by TEMPS and compared to control. A chi-squared test was performed to verify that the difference in results is statistically significant. It was found that the three alkali metals did not make a statistically significant difference to the subgrade temperature compared to control. However, in terms of embankment insulation material, extruded polystyrene (XPS) was found to have the best quantitative performance throughout the year except for about a 1.5-month period during spring thaw when foam-glass aggregates (FGA) have slightly better performance. This indicates that XPS, of the three materials, should be selected for general application, while FGA can be used in areas especially sensitive to spring thaw.
\nThe installation of 10 thermistor strings along the Ingraham Trail allowed for 8.5 months of road temperature data in asphalt, chipseal, and gravel surfaces to be collected down to a depth of 0.3 m. Each thermistor string is made up of one GeoPrecision data logger and two GeoPrecision temperature sensors. This model of thermistor strings was selected due to its popularity, reliability, and relatively reasonable cost. Prior to being installed, each string was tethered to a hardwood dowel using 4 to 5 nylon cable ties to provide rigidity to the assembly. Installation took place in January 2019 over 5 days. Data collected from January 21st, 2019 to October 3rd, 2019 were used in this study.
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\nIn examining average road and air temperatures, asphalt was found to be warmest in temperature, as expected, from March to October (with no data available for November and December). It was however surpassed by gravel between January and February, likely due to the combined effects of climate warming and thermal regime disturbance by the road itself. Chipseal has the highest positive heat balance by surface material type along the Ingraham Trail, meaning it absorbed the most amount of heat, which is different from the expectation that asphalt would be the most heat-absorbent. This indicates the importance of field investigations and is a sign that combined effects of climate change and manmade structures are at play. Chipseal displayed the most positive heat index by month in April and May. It is however slightly surpassed by control in June, July and August. Again, this unusual finding indicates the importance of field investigations and is a sign that combined effects of climate change and manmade structures on permafrost cannot easily be studied otherwise.
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\nWhen examined by time of day, there is nearly no change in heat balance when heat is being extracted (i.e. during winter months) compared to when heat is being induced (i.e. during summer months). When heat is being induced, the greatest fluctuation in heat balance is observed in asphalt. This indicates an increased thermal disturbance to the pavement and subgrade when exterior temperature is high, which will become increasingly frequent with climate change. Examining heat balance against thermal gradient allowed regression equations to be developed for each of the three surface material types, each with a coefficient of determination greater than 0.999. This can be used to predicting extent of thermal disturbance to permafrost subgrade beneath the road embankment when temperatures at two depths are known. Overall, these data demonstrate the importance of field investigations and are a sign that the combined effects of climate change and manmade structures on permafrost cannot easily be studied otherwise.
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\nRecommendations for future work include a systemic literature review to qualitatively identify, compare, and contrast the functions, efficiency, constructability, design requirements, and precedents (if any) of existing permafrost protection techniques. With this, a more accurate quantitative performance evaluation of existing permafrost protection techniques can be conducted so that costs can be more easily justified by jurisdictions wishing to implement a technique. Data along the Ingraham Trail should continue to be collected and downloaded for longitudinal analysis of road and air temperatures as well as heat balance; and the same analyses conducted in this study should be repeated when one full year of data becomes available to verify trends discussed.

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