Collapse of permafrost and failure of bridges in the community of Pangnirtung, Nunavut

Entzu Hsieh, Alexandre Tchekhovski, Ron Mongeau

Dans les comptes rendus d’articles de la conférence: GEO2011: 64th Canadian Geotechnical Conference, 14th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, 5th Pan-American Conference on Teaching and Learning of Geotechnical Engineering

Session: Climate Change & GeoHazards

ABSTRACT: The community of Pangnirtung is situated on the west side of the Cumberland Peninsula, Baffin Island, and located within the continuous permafrost zone. During the week ending June 16, 2008, heavy rains along with flooding resulted in significant thermo-erosion of the river banks, numerous landslides and cracks in the ground surface down to a depth of 5 m to 7 m. These processes also damaged two bridges across the Duval River. Site reconnaissance and assessments were conducted to determine the causes of intensive development for thermo-erosion and landslides. A drill program was performed across the river valley to assist in determination the subsoil and permafrost conditions for abutments of a new bridge. Based on the results of field investigations and assessments, it was considered that soil erosion due to heavy rains, anomalous climate conditions in 2007/2008 and presence of ice rich materials and frost cracking were the key factors for development of thermo-erosion and landslides. Short term and long term remediation were recommended. A cost effective foundation approach for the new bridge was also provided.

RÉSUMÉ: La communauté de Pangnirtung est située du côté ouest de la péninsule Cumberland, le de Baffin, dans une zone de pergélisol continu. Pendant la semaine se terminant le 16 juin 2008, de fortes pluies avec inondations ont causé une thermo-érosion importante des berges de la rivière, de nombreux glissements de terrain et des fissures depuis la surface du sol jusqu'à quelque 5 m à 7 m de profondeur. Ces processus ont également endommagé deux ponts sur la rivière Duval. Une reconnaissance du site et des évaluations ont été réalisées pour déterminer les causes de cette thermo-érosion et des glissements de terrain. Un programme de forage a été effectué dans la vallée de la rivière pour déterminer les conditions du sous-sol et du pergélisol en vue de la construction des culées d'un nouveau pont. Sur la base des résultats des études de site et des évaluations associées, il est considéré que l'érosion des sols due à de fortes pluies, les conditions climatiques anormales en 2007/2008, de même que la présence de matériaux riches en glace et de fissures de gel, constituent les facteurs clés qui ont menés au développement de thermo-érosion et de glissements de terrain. Des mesures correctives à court terme et long terme ont été recommandées. Une approche économique pour les fondations du nouveau pont a également été fournie. 1. INTRODUCTION The community of Pangnirtung is situated on the west side of the Cumberland Peninsula, Baffin Island, and located within the continuous permafrost zone. The community is divided into the north and south portions by the Duval River. Surficial soil encountered on the south and north banks of the river consisted of coarse to medium grained sand with various amounts of gravel, cobbles and boulders. Fine to coarse grained sand or silty sand with trace gravel was found below the coarse to medium sand. The permafrost temperature was measured in a range between -3°C and -5°C at depth from 10 m to 16 m. During the week ending June 16, 2008, heavy rains along with flooding resulted in significant thermo-erosion of the river banks, numerous landslides and cracks in the ground surface down to a depth of 5 m to 7 m. These processes also damaged two bridges across the Duval River. With both bridges being closed, residents had no access to municipal services such as water reservoir, sewage treatment plant and landfill. High water also knocked down a power pole in the community causing power failures in some areas. These caused a considerable impact to the social life of the community and the Hamlet of Pangnirtung declared a state of emergency after flooding. Site reconnaissance and assessments were conducted to determine the causes of intensive development for thermo-erosion and landslides. A drill program was performed across the river valley to assist in determining the subsoil and permafrost conditions for abutments of a new bridge. 2. SITE CONDITION Pangnirtung is located in about 350 km northeast from the town of Iqaluit and just several kilometres south of the Arctic Circle (Figure 1). The population of the community

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