Understanding Permafrost Processes under the Iqaluit Airport

In 2017, researchers from Laval University conducted a quantitative risk analysis of the sensitivity of Iqaluit International Airport (YFB) to thaw settlement and permafrost degradation. Located in the city of Iqaluit in the territory of Nunavut, Iqaluit Airport is a hub for air transportation in the eastern Canadian Arctic. YFB has a 2,750-m runway and associated aprons and taxiways servicing the community of Iqaluit, Nunavut and outer lying Nunavut communities for which Iqaluit is the transit hub. It was built during the 1940’s, expanded with additional apron area and taxiways in the late 1950’s, 1970’s and 2010’s, and resurfaced and repaired repeatedly throughout the life of the infrastructure. As with most infrastructure in the Canadian Arctic, permafrost conditions were not investigated before construction. The uppermost layer of permafrost, known as the active layer, melts in summer with the depth of thaw dependent on the warmth of the summer. When the depth of thaw extends into ice-rich permafrost, the runway’s surface settles or cracks, requiring expensive repairs. Increasingly warm seasonal temperatures are causing the active layer to deepen thawing more subsurface ice, which brings more headaches for airport managers. To address knowledge gaps at the Iqaluit International Airport, scientists conducted geoscientific research of the airport’s permafrost conditions, its sensitivity to thaw settlement and the physical processes involved in the permafrost’s degradation. The research produced a quantitative risk analysis methodology utilizing geostatistics and reliability analysis methods and cost information to analyze the thaw settlement and soil bridge formation dangers within the embankment.

Understanding and Assessing Impacts

Permafrost conditions were characterized in detail between 2010-18 in preparation for major airport renovations and upgrading in 2018-19. Temperature profiles under asphalt pavement show warmer ground and faster, deeper, and longer thaw penetration than the shoulders and natural terrain, causing increased settlement underlying and adjacent to the asphalt surfaces. The newly acquired geoscientific data on the airport’s permafrost has oriented risk analyses and engineering design implemented during the recent improvements in order to develop a modern infrastructure that is better adapted to the impacts of climate warming. This information was integrated with a cost-benefit analysis and presented as part of the larger quantitative risk assessment that was performed. Risk evaluation is the combination, through multiplication, of, generally, three factors for a single danger: its hazard, consequence, and vulnerability. In the context of a quantitative risk assessment, the description is a limit state equation, which provides a definition of failure, either for ultimate or serviceability states. The probability of exceeding this failure limit is the hazard. The consequence is the cost of repairing the infrastructure damage and, possibly, the damage’s indirect effects on communities. Finally, the vulnerability is the degree to which the infrastructure is affected. Direct cost information came from earned value report data and the construction area calculations on the plans. Human and societal impact factors were determined from rubrics and conversations with stakeholders.

Identifying Actions

To address knowledge gaps at the Iqaluit International Airport, scientists conducted geoscientific research of the airport’s permafrost conditions, its sensitivity to thaw settlement and the physical processes involved in the permafrost’s degradation. Scientists used a range of geoscience data to assess the conditions. This included ecological and surficial sediment mapping. Remote sensing of ground settlement was also used to identify potential hazards. These initiatives were supported by field validations, geophysical surveys and permafrost measurements. The hazard, direct costs, and indirect cost factors were determined from available site data, project documents (plans, earned value reports, project agreement), and stakeholder interviews. The hazard analysis used Monte Carlo simulation and standard permafrost engineering calculations to determine thaw settlements using a stochastic analysis. Human and societal impact factors were determined from rubrics and conversations with stakeholders. The results of the research translated into engineering decisions made by the Government of Nunavut and its partners, that will help prolong the life of a critical piece of northern infrastructure.

Implementation

The actions undertaken as part of the risk assessment of Iqaluit airport include:
  1. A quantitative risk analysis methodology utilizing geostatistics and reliability analysis methods and cost information to analyze the thaw settlement and soil bridge formation dangers within the embankment;
  2. A fragility assessment to determine changes in these risks due to changing mean annual air temperature (MAAT); and,
  3. A cost/benefit analysis for an insulated embankment section based on the fragility analysis results, failure limits and previous repair cycles.
In addition, data from aerial photo interpretation, archival research, geophysics, drilling and coring, ground temperature monitoring, and numerical modelling were integrated in a GIS application and used to study the area. Ground Penetrating Radar was used to delimit cryostratigraphic units and locate features under the embankments, particularly cracks and ice wedges. Bridging hazard was determined through laboratory testing of model ice wedges which was used to correlate the hazard with the probability of bridging occurrence. The risks were then calculated resulting in the highest risks for thaw settlement in the glaciomarine ice wedge (IW) and lacustrine geological settings, and soil bridging. The process of risk assessment consists of five steps: 1) identification and 2) description of dangers (processes resulting in damage); 3) calculation of hazards and consequences; 4) evaluation of risk; and, 5) treatment of risks. Risk evaluation is the combination, through multiplication, of, generally, three factors for a single danger: its hazard, consequence, and vulnerability. In the context of a quantitative risk assessment, the description is a limit state equation, which provides a definition of failure, either for ultimate or serviceability states. The probability of exceeding this failure limit is the hazard. The consequence is the cost of repairing the infrastructure damage and, possibly, the damage’s indirect effects on communities. Finally, the vulnerability is the degree to which the infrastructure is affected.

Outcomes and Monitoring Progress

The resulting risks aimed to illuminate regions of YFB that warrant attention by Arctic Limited Partnership, Transport Canada and YFB management. Research activities identified areas of local and regional ground settlement, and related these observations to permafrost and hydrological processes leading to abandonment of an existing problematic taxiway. The risk and climate fragility were assessed for five thaw settlement cases and the danger of soil bridging over voids in the embankment. The hazard and risk results correlated well with the reported historical observations of permafrost degradation when a 3.5cm settlement serviceability criteria was used. The cases of highest hazards and risks for thaw settlement were at ice wedge locations within the glaciomarine sediments and lacustrine sediments where surficial fissures, settlement and groundwater seepage have been noted. The addition of insulation to the infrastructure’s fill section significantly reduced the hazard and risk of thaw settlement, when compared to the other analysis cases. The climate fragility analysis, using a 2.5°C temperature increase in the period between 2010 and 2050, resulted in steadily increasing or constant hazard for the five thaw settlement analysis cases. The cumulative thaw settlement from this analysis was used to determine the repair cycles necessary after the deflection criteria (3.5cm) was exceeded. The present cost for the five thaw settlement cases mimicked the risk order from highest to lowest with uninsulated ice wedges in glaciomarine sediments and lacustrine sediments having the highest hazard throughout the analysis period. The present cost analysis determined that the costs for an insulated ice wedge in glaciomarine sediments was 19-36% of the present cost for the other four analysis cases making the addition of insulation to the infrastructure’s fill section is a cost- effective measure. But, the mitigation method of increased thermal resistance (thicker fill section or insulation) can only delay, not prevent, permafrost degradation.

Resources

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