Drilling boreholes is an important part of any permafrost site investigation and should be carried out with ample lead time to any construction. For low sensitivity, low consequence projects, uncored boreholes that are later used for ground temperature monitoring may be sufficient. Drilling chips already provide some information about the structure of the ground, i.e. ice contents, bedrock, etc. In addition, in-hole geophysics may be performed revealing data about variabilities in water content or density with depth.
Visual inspections of the borehole using downhole cameras can provide valuable information about possible existence of ice lenses or large voids (e.g. Arenson, 2002). If borehole instability prevents keeping the borehole uncased for longer periods, periodical inspections of the borehole bottom are recommended, even though it slows the drilling process.
If either the failure consequences or foundation sensitivity is high, coring of undisturbed permafrost samples is strongly recommended. Coring in mountainous permafrost is very challenging due to the heterogeneity of the soil where large boulders can be locked into an ice matrix. Arenson (2002) or Vonder Muhll (1996) provide some information on drilling and sampling in rock glaciers. The soils at the permafrost base can be coarse and large voids are present where liquid drilling mud would get lost, which would cause further environmental problems. Therefore cold air flushing is to be used. It is crucial that the air is cold enough to extract any heat that is generated during the drilling process. A triple tube system for coring, which results in the least sample disturbance, is the best choice in permafrost.
Long-term monitoring instruments should be installed in all boreholes drilled on site. The actual instrument depends on the project, and some can be combined, such as those to measure temperature and deformations. However, no borehole should be left unused, particularly as boreholes offer an opportunity to make line measurements with depth for a specific point on the surface and several therefore give a three-dimensional overview of ground conditions, which can vary strongly at a given site in mountain permafrost.
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