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.