Engineering geophysical investigation of permafrost conditions underneath airfield embankments in Northern Quebec (Canada) Richard Fortier & Catherine Savard Centre d’études Nordiques (CEN), Université Laval, Québec, QC, Canada Département de géologie et de génie géologique, Université Laval, Québec, QC, Canada ABSTRACT An increase in air temperature in excess of 3 °C over a 10-year period from 1992 to 2002 was observed in Northern Quebec (Canada). This increase induced widespread permafrost degradation in the discontinuous permafrost zone mainly under the form of localized zones of thaw subsidence affecting the performance of roads and airfields built on thaw-instable frozen ground. In a context of fast population growth and increasing resource development in Northern Quebec, the vulnerability of transportation infrastructure to permafrost degradation raises concern for the safety and economic sustainability of the Inuit communities. An engineering geophysical investigation, including capacitively coupled resistivity (CCR) and ground penetrating radar (GPR) surveys, was carried out in summer 2004 at the airports of Tasiujak, Aupaluk, Kangirsuk, and Quaqtaq in Ungava Bay to characterize the frozen ground conditions underneath the airfield embankments and assess their vulnerability to permafrost degradation. CCR and GPR are complementary non-invasive subsurface investigation tools. CCR is effective for characterizing the permafrost conditions while GPR defines accurately the cryostratigraphic contacts. The combined interpretation of CCR and GPR surveys leads to a high resolution cross–section of permafrost conditions. An example of the subsurface characterization capabilities of combined CCR and GPR surveys is given for Tasiujaq Airport which is considered vulnerable to permafrost degradation. RÉSUMÉ Une augmentation de la température de l’air supérieure à 3 °C a été observée au Québec nordique (Canada) sur une période de 10 ans de 1992 à 2002. Cette augmentation a provoqué une dégradation généralisée du pergélisol dans la zone de pergélisol discontinu sous la forme de secteurs bien définis d’affaissement au dégel qui affecte la performance des routes et pistes d’atterrissage construites sur un sol gelé instable au dégel. Dans un contexte de forte croissance de la population et de développement de l’exploitation des ressources naturelles au Québec nordique, la vulnérabilité des infrastructures de transport à la dégradation du pergélisol soulève des inquiétudes sur la sécurité et le développement durable des communautés Inuites. Un programme d’investigation géophysique qui incluait des profilages de résistivité électrique avec un système couplé capacitivement et des levés de géoradar a été réalisé durant l’été 2004 aux aéroports de Tasiujaq, Aupaluk, Kangirsuk et Quaqtaq sur la côte ouest de la Baie d’Ungava pour caractériser les conditions de la sous-fondation pergélisolée des remblais aéroportuaires et évaluer leur vulnérabilité à la dégradation du pergélisol. Le système couplé capacitivement et le géoradar sont des outils géophysiques complémentaires d’investigation non-invasive de la sous-surface. Le premier outil fournit des informations sur les conditions du pergélisol alors que le second définit précisément les contacts cryostratigraphiques. L’interprétation combinée des levés de résistivité électrique et de géoradar génère des vues en coupe à haute résolution des conditions du pergélisol. Un exemple des possibilités de caractérisation de la sous-surface avec des levés combinés de résistivité électrique et de géoradar est donné pour l’aéroport de Tasiujaq jugé vulnérable à la dégradation du pergélisol. 1 INTRODUCTION Due to the isostatic rebound, once the sediments were exposed to the cold atmosphere, permafrost aggraded. There are fourteen Inuit communities located along the Most infrastructures such as the airports critical to coasts of Hudson Bay and Ungava Bay in Northern maintain year-round access to these communities are Quebec (Canada) for an easy access to the sea since the then potentially built on ice-rich frozen ground and Inuit are sustained in part by sea food. They have vulnerable to permafrost degradation. In a context of fast modern facilities such as roads, airfields, seaports, population growth, increasing resource development and schools, dispensaries, and houses to accommodate the climate warming anticipated over the next decades, the population. These communities are also situated in the vulnerability of transportation infrastructure to permafrost marine transgression of Tyrrell and D’Iberville seas degradation raises concern for the safety and economic following the retreat of the Wisconsin Ice Sheet where sustainability of the communities in Northern Quebec. An frost-susceptible glaciomarine sediments were deposited. engineering geophysical investigation was carried out in 1307 summer 2004 at the airports of Tasiujak, Aupaluk, July 2004 (Figures 3 and 4). The aircraft pilots complain Kangirsuk, and Quaqtaq in Ungava Bay (Figure 1) to about this depression in the airfield making hazardous characterize the frozen ground conditions underneath the the conditions for take-off and landing. Cracks due to airfield embankments and assess their vulnerability to shoulder rotation also affect the embankment sides permafrost degradation. The results of this investigation (Figure 3). The thaw settlement underneath the for the Tasiujaq Airport are presented herein. embankment is taking place in the thaw-instable intertidal diamicton. An increase in air temperature in excess of 3 °C over a 10-year period from 1992 to 2002 was Figure 1. Permafrost distribution and location of Kuujjuaq, Tasiujaq, Aupaluk, Kangirsuk and Quaqtaq in Northern Quebec, Canada. 2 STUDY SITE The Tasiujaq Airport is located on the West coast of Figure 2. Map of Tasiujaq Airport, Northern Ungava Bay in the discontinuous but widespread Quebec, Canada. Location of geophysical surveys. permafrost zone (Figure 1). The airfield is built on a vast marine terrace at an elevation between 29 and 35 m Table 1. Description of the stratigraphic cut SC-1 along above sea level. According to a stratigraphic cut SC-1 the Bérard River. along the Bérard River (Figure 2 and Table 1), the airfield embankment lies on a 2.3 m thick intertidal diamicton Unit Characteristics Depth interval near the ground surface made of ice-poor and saline silts (m) and fine sands overlying 7.9 m thick coarse sand and Organic layer 0 - 0.05 to 0.3 gravel layers over a deep marine clay layer. As observed Fluviatile layer coarse sands 0.05 to 0.3 - 0.7 in the stratigraphic cut SC-1, the intertidal diamicton Intertidal frost-susceptible and thaw 0.7 - 3 contains millimetric thick ice lenses. A permafrost mound diamicton layer instable silts and fine to coarse with a mineral core was levelled at the Northeast end of sands the airfield from 5+050 and 5+105 m in distance before water content: 2 - 20% the embankment construction. The airfield also crosses a pore water salinity: 5 - 18 g/l small lake which is an abandoned Holocene meander of Coarse sand ice-poor coarse sands, 3 - 4 m the Bérard River (Figure 2). layer gravels, pebbles and blocks The surface of the airfield embankment is affected by Gravel layer ice-poor gravels 4 - 10.9 two localized zones of thaw subsidence progressing from water content: 10 - 15% the Southeast shoulder to the airfield centre. The Deep clay layer marine clay > 10.9 subsidence volume of one of these zones was 44 m 3 on 1308 observed at Kuujjuaq, about 110 km Southeast of Tasiujaq (Figures 1 and 5). Following this increase in air temperature, permafrost warming was also monitored at Tasiujaq Airport (Figure 6): 1) 3.1 °C at a depth of 7 m in the field from 1994 to 2004 and 2) 1.8 and 0.8 °C at a depth of 5.4 m in the subgrade underneath the airfield embankment and distances of 5+090 m from 1994 to 2004 and 5+300 m from 1992 to 2004 respectively. The depth of the thawing front on July increased by less than 20 cm over the ten-year monitoring period (Figure 6). Moreover, due to Southwest prevailing winds in winter, there is important snow accumulation on the Southeast shoulder and toe of the airfield embankment (Figures 7 and 8) while the tundra away of the airfield is nearly free of snow following the wind ablation. The thick embankment disrupts the topography and acts as a snow fence. The permafrost degradation is therefore due to not only the increase in air temperature but also the thermal insulation of snow cover. This is not unique to the Tasiujaq Airport (Fortier and Bolduc 2008). Drainage problem observed along the embankment toes (Figure 7) Figure 4. Spatial distribution of thaw subsidence affecting can also cause the permafrost degradation due to the the airfield embankment near the distance 5+230 m, latent heat of water delaying the freeze up and cooling of Southeast side, on July 2004 at Tasiujaq Airport, the active layer in winter. Northern Quebec, Canada. The + symbols identify the location of elevation measurements. Figure 3. Thaw subsidence and cracks affecting the airfield embankment near the distance 5+230 m, Southeast side, on summer 2001 at Tasiujaq Airport, Figure 5. Mean annual air temperature at Kuujjuaq Northern Quebec, Canada. The black camera case gives (meteorological record from Environment Canada, the scale. (photograph by Denis Sarrazin, CEN) http://climate.weatheroffice.gc.ca/index.html). The full curve is a 5 year running average while the full and dashed horizontal lines are respectively the average and ±standard deviation (values in parentheses) over the reference period 1961–1990. 1309 Figure 6. Temperature profiles in the field (HT-304) and in the airfield embankment and subgrade (HT-180 and HT-181) at Tasiujaq Airport, Northern Quebec, Canada. See Figure 3 for the location of thermistor cables HT- Figure 7. Snow accumulation on the Southeast (A) and 304, Northwest (B) shoulders and toes of the airfield HT-180 and HT-181. embankment on April 2005. C) Drainage problem at the Northwest toe of the airfield embankment on July 2004 at Tasiujaq Airport, Northern Quebec, Canada.