Ninth International Conference on Permafrost ... - IARC Research

Preservation of the Alaska HighwayEva StephaniDepartment of Geology and Geological Engineering, Université Laval, Québec, Québec, CanadaDaniel FortierInstitute of Northern Engineering, University of Alaska Fairbanks, Alaska, USAYuri ShurDepartment of Civil and Environmental Engineering, University of Alaska Fairbanks, Alaska, USAGuy DoréDepartment of Civil Engineering, Université Laval, Québec, CanadaBill StanleyYukon Highway and Public Works, Yukon Government, Whitehorse, Yukon, CanadaIntroductionRoad construction in permafrost areas affects the thermalregime of frozen soils via removal of the vegetation,compaction of the soil, road cut, and use of black asphaltpavement, for instance. The thermal degradation of thepermafrost causes the ground ice to melt and results inpermafrost thaw settlement, as well as subsidence andcracking of pavement. In many northern areas, roads arenow showing signs of instability as a result of permafrostdegradation, which could be partly due to recent climatewarming. According to the IPCC projection of climatewarming, this situation will undoubtedly be exacerbated inthe future (IPCC 2007).The Alaska Highway is an essential and widely usedcommunication link between Alaska, Canada, and thesouthern United States. The highway has a poor drivingsurface, and some sections of the embankments haveexperienced substantial settlement. Severe pavementsubsidence, longitudinal cracking, and potholes couldeventually threaten the structural integrity of theinfrastructure. Sections of the Alaska Highway built onice-rich permafrost might eventually require relocation orreplacement with a different design, and sections built onpermafrost with a lower volume of ice will require at leastrehabilitation. Alternative designs and mitigation measuresshould be adopted in order to reduce maintenance costs.The Yukon Government has decided to implement a testsection at Beaver Creek, Yukon Territory, Canada (62°20′N,140°50′W). Engineering mitigation measures will be testedto control the degradation of the permafrost. Six techniqueswill be implemented at the Beaver Creek experimental roadsite: (1) Air convection embankment (ACE); (2) heat drain;(3) air duct cooling system; (4) thermo-reflective snowshed; (5) grass-covered embankment; and (6) light-coloredaggregate bituminous surface treatments (BST).The Alaska University Transportation Center is responsiblefor characterizing the stratigraphy at the test site and fordetermining the geotechnical properties of the permafrostprior to the construction of the road test section. Theobjectives of this paper are to outline the thaw-susceptiblenature of the permafrost at the test site and to illustrate thechallenges related to rehabilitation of degraded permafrostunder the road embankment.MethodologyField methodsThermal conditions at the Beaver Creek experimental sitehave been monitored since 1998 by means of thermistorcables. One cable is installed under the center line of the road,one cable is installed in the side slope of the embankment,and one cable is in the natural ground adjacent to the road.Air temperatures also have been monitored.Drilling and coring operations at the test site were realizedduring summer 2007 in the natural ground adjacent to theroad and in the berms. Permafrost cores from 17 boreholeswere collected, sampled, and brought back in a freezer to theUniversity of Alaska Fairbanks for laboratory analyses.Laboratory methodsThe cryostructure and sediment types of soil were identified.Each stratigraphic unit was identified by its pH, electricalconductivity, gravimetric and volumetric ice content, grainsizedistribution, and thaw-settlement potential.Preliminary Results and DiscussionThe mean annual air temperature (1971–2000) at BeaverCreek is -5.5°C (Environment Canada 2008). The localextreme maximum was 32.8°C in 1982, while the localextreme minimum was -55°C in 1971 (Environment Canada2008). The mean annual precipitation is 416.3 mm, of which123.1 mm water equivalent falls as snow (EnvironmentCanada 2008). Monthly mean air temperatures are 11.9°C,14°C, and 11.2°C for June, July, and August, respectively.Air temperatures above and below freezing represent anaverage (1971–2000) of 1532.6 thawing degree-days and3534.2 freezing degree-days, respectively (EnvironmentCanada 2008).The coring operations in the natural ground revealed thepresence of very ice-rich syngenetic permafrost with buriedinactive ice wedges. Three main stratigraphic units wereidentified: Unit 1 (0–0.5 m) is ice-rich peat. Unit 2 (0.5–6.5m) is silt, which is ice-rich at the top (>2.0 m), ice-poorbelow (2.0–3.5 m), and ice-rich in the lower portion (3.5–6.5m). The top of the ice wedge is located in the ice-poor layeraround 2.5 m depth and extends down to at least 6.5 m. Unit3 (6.5–10 m) is an ice-rich diamicton.299