Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tTable 1. Thaw strain values of the stratigraphic units.Unit Ice content Thaw strain1 Ice-rich 0.352 (0.5–2 m) Ice-rich 0.41–0.612 (2–3.5 m) Ice-poor 0.052 (3.5–6.5 m) Ice-rich 0.43–0.54Figure 1. Longitudinal cracking along the shoulder of the AlaskaHighway due to degradation of the underlying permafrost. The 1-mshovel (arrow) gives the scale.AWaterSaturated soilFigure 2. (A) Very ice-rich microlenticular cryostructure typical ofsyngenetic permafrost. (B) Thaw-settlement-potential test showingexcess water upon thawing in the thaw-settlement cell.In 2004, the maximum thaw depth in the natural ground was70 cm. The maximum thaw depth under the center line of theroad was located in the embankment material. However themean annual ground temperature just below the embankmentwas close to the melting point (-0.3°C), and numerousobservations (in 2007) of cracks and depressions affectingthe central part of the road suggest that the thaw depth nowreaches the natural ground where the embankment is thinner.In the side slope of the embankment, the thaw depth waslocated in the natural ground. Deepening of the active layerin the natural ground under the road creates thaw settlementand subsidence of the embankment (Fig. 1) Drilling realizedin 2007 from the berm adjacent to the side slope of theembankment revealed that, locally, the maximum thaw depthcan be located as deep as 2.5 m in the natural ground belowBthe base of the embankment. If submitted to warming, theselarge unfrozen zones will progressively extend towards thecenter line of the road. The water content measured in thethawed layer under the embankment varied between 221%and 357%. This indicates that these zones are supersaturateddue to the melting of ground ice and restricted drainage.The high water content can be explained by the prevalenceof permafrost with an extremely ice-rich micro-lenticularcryostructure. This type of cryostructure is typical ofsyngenetic permafrost and usually has very high settlementpotential (Fig. 2). Thaw-settlement tests indicate thatthe upper part of the permafrost (Units 1 & 2) is highlysusceptible to thermal degradation (Table 1). A thawsettlementtest realized on the ice-poor layer of Unit 2revealed that it is thaw-stable (Table 1). However this layercomprises a widespread network of ice wedges extending inthe underlying ice-rich units. Ice wedges are highly thawsusceptibleand their geometric patterns make them prone tothermo-erosion and deep linear subsidence.ConclusionsDrilling operations revealed the presence of large,unfrozen supersaturated zones under the road embankment.Rehabilitation of thawed permafrost under the roadembankment will necessitate refreezing of the sediments.This could be long, due to the large amount of latent heatto extract from the water trapped in these unfrozen zones.Our study indicates that syngenetic ice-rich permafrost ishighly susceptible to thermal degradation, especially whereice wedges are present.Our preliminary results indicate that a detailed spatialgeotechnical characterization of the permafrost (e.g.,cryostructure, volumetric ice content, grain-size distribution,thaw-settlement potential, thaw-consolidation potential,ground thermal regime) is needed to determine the potentialthaw susceptibility of the permafrost to climate warming andto find the proper engineering solutions to control permafrostdegradation.ReferencesIPCC. 2007. Climate Change 2007: The Physical ScienceBasis. Contribution of Working Group 1 to the FourthAssessment Report of the Intergovernmental Panel onClimate Change, S. Solomon, D. Qin, M. Manning,Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, & H.L.Miller (eds.). Cambridge, UK & New York, USA:Cambridge University Press,300