|Engineering: Excavation Methods|
See also Keeping It Frozen
The Permafrost Tunnel was excavated during the winter months of 196365 by CRREL engineers and scientists. The tunnel initially served as a research study to test new mining technologies and construction methods in frozen soils.
Diagram from U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (1981) CRREL permafrost tunnel.
CRREL Benchnotes 17: 1-7. Hanover, NH: Cold Regions Research and Engineering Laboratory.
CRREL engineers chose the specific tunnel site because of the existing escarpment created by gold mining activities in the 1920s. This site selection enabled the engineers to encounter permafrost in the most efficient manner possible (cross section diagram shown to the right).
During mining, an electric shuttle car was located behind the Alkirk and material was loaded into the shuttle car from the conveyor of the Alkirk. When the shuttle car was fully loaded, the excavation would stop and the shuttle car would pull out of the tunnel where it would unload itself into a spoil pile. The shuttle car would return to the excavation, and the spoil pile was relocated with a wheeled loader.
CRREL engineers ran two ten-hour shifts, finishing with a short maintenance shift. The first 2 meters of the excavation passed through what is known as the active layer, or seasonal thaw zone. In the thawed state this soil would not withstand its own weight without support. Because of this a semicircle of corrugated steel culvert was installed through the active layer and mechanical refrigeration coils placed in the soil to prevent the escarpment from sloughing over the entrance. The corrugated section was buried with soil and a layer of peat was added to help insulate the active layer and tunnel.
As the excavation progressed into solid permafrost, structural support was not required any longer as the frozen strength of the soil is sufficient to support the 15 meters of soil above. However, the active layer and portal insulation alone were insufficient to prevent the temperature from rising above freezing in the summer, so a refrigeration unit was added inside the tunnel to maintain below freezing temperatures. Construction was limited to winter months and in the summer the tunnel was sealed to avoid permafrost thaw and tunnel collapse.
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