Structure and properties of ice-rich permafrost near Anchorage, Alaska

• Section of the ice-rich permafrost was studied near Anchorage, Alaska.
• Average volume of visible ground ice in glacio-lacustrine sediments was 42.5%.
• Permafrost temperature is at the thawing point and the permafrost body is degrading.
• Permafrost degradation will lead to significant thaw settlement.
• The thick organic layer and high ice content protect permafrost from rapid thawing.

Geotechnical investigations in the vicinity of Anchorage, Alaska revealed a body of ice-rich permafrost up to 12 m thick and approximately 150 m long. Mean annual air temperature in Anchorage is + 2.2 °C which is unfavorable to the existence of permafrost. However, isolated masses of relic permafrost exist at temperatures close to 0 °C protected from rapid thawing by a thick layer of peat and soils with high ice content. Frozen soils found at the study site include silty clay of glacio-lacustrine origin with numerous layers of segregated ice up to 70 cm in thickness. The average volume of visible ice is 42.5%; and total gravimetric water content is 68%. With an average permafrost thickness of 9.5 m and an average thaw strain of 40%, the thaw settlement of the surface is expected to be at least 3.8 m after degradation of permafrost, an unacceptable deformation for any engineered structure on these soils.