Evaluation of the CRREL Permafrost Tunnel; Technical rept

摘要

The Permafrost Tunnel was excavated in frozen silt and consists of a 110-m-long horizontal adit and a 45-m-long winze that extends down to the underlying gravel. Some change has occurred since the excavation was conducted in the mid-1960s, so a team was assembled in the spring of 2006 to assess these changes. Frozen silt deformation was noted in the rear of the adit, and a roof fall of the gravel layer was noted in the room at the bottom of the winze. Both of these were found to be attributable to thermal forcing events and the raising of the overall facility temperature to near-freezing temperatures. Sublimation was also noted throughout the tunnel, but this does not pose a problem for the structural integrity of the facility. The team recommends that the facility temperature be lowered to approximately -5 C, which will decrease creep rates and the weakening of lithologic bonds between soil units. Overall, the facility is safe for continued use by researchers and others.