Photo of the nearly complete excavation is attached. Photo courtesy of BGC Engineering/FHWAWhile completing the excavation west of the slide, the contractor unearthed an approximately 90-foot-long (measured roughly NW/SE) by 50-foot-wide, 25-foot-deep mass of ice. It extends completely across the roadway at a point roughly 125 feet west of the west abutment. Going forward, the plan is to over-excavate the ice within the limits of the roadway, ditch line, and backslope insofar as possible. In sections where ice will remain, they will backfill with specially-sized rock to create an Air Convection Embankment (ACE).
An ACE is designed to maintain the ice in a frozen state by capitalizing on the relative buoyancy of warm air. In the winter, cold (and therefore denser) air will sink to the bottom of the ACE, cooling the ice. During the summer, the air at the bottom of the ACE will be cooled by the ice and will be denser than the warmer ambient air. The cool air will remain at the bottom of the ACE, effectively insulating the ice from warmer conditions above. The ACE approach has been successfully used by the Alaska State DOT to preserve permafrost under roadways at locations in Fairbanks and on the Alcan and Dalton highways. Geosynthetic reinforcement will also be used within the roadway to minimize maintenance requirements.
Ground ice removal from the west cut. Photo courtesy of BGC Engineering/FHWA