Geothermal Considerations in Alpine Permafrost

By understanding the geothermal processes involved during the freezing and thawing of permafrost soils, one can better judge the thermo-mechanical response of the ground to changes in temperature and stress.

Mountainous soil and rock properties and conditions are very diverse. Ground ice with a range of origins can be present within a mountain permafrost environment, including glacier ice, compacted snow, segregated ice, ice derived from adfreezing from rain or melt water during thaw, snow avalanches, etc. Consequently, alpine permafrost can be solid rock with ice-filled joints, fine grained soils with low ice contents, ice supersaturated gravels (excess ice), where not all particles are in contact, or dirty ice with some dispersed solid particles distributed within the ice (Fig. 5). Sampling from triple cored, air cooled drilling has shown that air contents of over 20% by volume can exist (Arenson and Springman, 2005b). In contrast to most glacier ice, the ice in permafrost, rock glaciers included, is much older because there is no distinct accumulation and ablation zone as would be found in mountain glaciers.

The time- and temperature dependent mechanical properties of the ice in the soil are the factors that commonly govern the geotechnical properties of frozen permafrost foundations. Knowledge about the ice (e.g. content, structure, distribution) in the ground is a major component in the design. Solid, dry, unfractured bedrock has similar geotechnical properties in a frozen state (e.g. at -10°C) as in an unfrozen condition (e.g. at +10°C). Saturated sand at +10°C and at -10°C on the other hand, with all the pore water frozen, behaves in a different manner.

Figure 5. Different types of frozen ground (a) 1. Ice filled joints, 2. Compact frozen gravel, 3. Ice rich gravel, 4. Dirty ice, 5. Ice lenses in fine grained soils. (b) frozen gravel, CRREL permafrost tunnel, Anchorage Alaska (L. Arenson). (c) ice lenses in frozen silt, CRREL permafrost tunnel, Anchorage Alaska (L. Arenson).

Figure 5. Different types of frozen ground (a) 1. Ice filled joints, 2. Compact frozen gravel, 3. Ice rich gravel, 4. Dirty ice, 5. Ice lenses in fine grained soils. (b) frozen gravel, CRREL permafrost tunnel, Anchorage Alaska (L. Arenson). (c) ice lenses in frozen silt, CRREL permafrost tunnel, Anchorage Alaska (L. Arenson).

The actual ice content is only one factor that influences the mechanical behaviour of a frozen soil. Its structure, i.e. the location, thickness and assembly of the ice lenses or ice-filled pores, is also important. Because permafrost and ice formation may have occurred over hundreds / thousands of years, ancient freeze-thaw cycles at greater depths can result in heterogeneous ice structures with ice rich layers that cannot be identified from the surface. The freezing processes that include the thermal, hydro-geological and geomorphological histories are the driving factors behind this ice lens formation, hence the final structure of a frozen soil.

Three-dimensional effects further influence the geothermal regimes in mountainous environments. In particular, thermal gradients with depth depend upon slope angle, orientation and content as well as on snow, glacier cover or groundwater flow (e.g. Gruber et al., 2004).

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