Mountain Infrastructure and Alpine Permafrost

Typical infrastructure in mountainous environments is listed above. Some structures may have to be tied back with anchors reaching into permafrost. Further hazards to infrastructure, settlements or reservoirs are evoked by slopes located in permafrost that may trigger mass movements during rainfall infiltration or thawing of snow or ice.

Due to the heterogeneity of the mountain environment, with the lower permafrost boundary changing significantly within short distances, structures are often located in zones that would be classified as sporadic permafrost. These are zones where patches of permafrost interchange with zones that are permafrost free. These changes in foundation properties create particularly vulnerable situations for the structures if some part of the foundation is located in permafrost, whereas the other is not.

Shallow or deep foundations are possible in permafrost environments. The design most suitable will depend on the actual situation and the predicted changes over the design life. In addition to geotechnical properties, geothermal properties and any thermal disturbances caused by the structure to be built must be considered during the design. Whenever ice is present in the ground, thermal and/or stress changes affect the ground as a function of time, hence the foundation condition does not remain the same over the lifetime of the structure.

Definition of Permafrost and Other Relevant Terms

According to the Glossary of Permafrost and Related Ground Ice Terms (van Everdingen, 1998 revised May 2005), permafrost is defined as:

"Ground (soil or rock and included ice and organic material) that remains at or below 0°C for at least two consecutive years."

Figure 1 shows additional definitions used in permafrost science and engineering. All these definitions are purely thermal and do not indicate whether ice is present or not. Changes in the geothermal gradient with depth further provide information on past temperatures and possible irregularities with depth, such as presence of taliks.

glossary definition of the active layer (van Everdingen, 1998 revised 2005)

Active layer

Zero annual amplitude (ZAA)

Active layer

Zero annual amplitude (ZAA)

Freezing point depression

Figure 1. Definition of permafrost after van Everdingen (1998 revised 2005) and Muller (1947).

Freezing point depression traditional definition of the active layer (Muller, 1947)

Active layer

Permafrost table

Permafrost thickness

Permafrost base

Figure 1. Definition of permafrost after van Everdingen (1998 revised 2005) and Muller (1947).

Frozen ground on the other hand is defined as:

"Soil or rock in which part or all of the pore water has turned into ice."

The second definition contains more information about the condition of a particular soil, but in contrast to the definition of permafrost, frozen ground is only a snapshot in time. Frozen ground can be found in temperate zones during cold months or induced artificially for ground freezing applications (e.g. Harris, 1995; Pimentel et al., 2007).

When designing and assessing permafrost conditions, it is therefore fundamental to assess the spatial ground conditions and relevant characteristics, keeping in mind that the ground is to be used as foundation material and its properties need to be known for geotechnical and geothermal analysis.

Active Layer

The active layer needs special attention because mechanical and thermal processes within this thin surface layer will often control the design of infrastructure. Ground movements or seasonal changes in strength require a good understanding of the thermo-hydro-mechanical response of this layer. According to the permafrost glossary (van Everdingen, 1998 revised 2005), the active layer is defined as (Fig. 1):

"The layer of ground that is subject to annual thawing and freezing in areas underlain by permafrost."

Arnold et al. (2005), for example, studied the stability of the active layer of a rock glacier in Switzerland. The interlocking effect in the coarse, elongated and angular particles was found to cause significant dilatancy at the low stresses pertaining near the ground surface and hence extremely high strength parameters were obtained. However, sliding over a possible massive ice layer at the permafrost table might be of concern because interface friction is reduced by a factor of two. Analysis of field data in respect of the hydrothermal processes developing within the active layer confirmed that this scenario might well occur (Rist and Phillips, 2005). During snow melt, the ground temperatures increase rapidly (0.6°C/day), which is partly caused by convective heat fluxes as melt water percolates to the permafrost table through the voids in the soil matrix. Seepage may induce down-slope displacements within the active layer during snow-melt.

Harris et al. (2008) and Kern-Luetschg et al. (2008) demonstrate the effect of the freezing of the active layer on solifluction. Progressive soil freezing from the surface down and from the permafrost table upwards forms a closed hydraulic system within the central zone of the active layer. Water migration towards the respective freezing fronts reduces moisture contents in the central zone, and pore water suction increases the effective stress and hence the unfrozen shear strength.

Issues / Hazards in Mountain Permafrost With Regard to Climate Change Uncertainties

Climate change is creating major challenges to the design of new and the assessment of existing infrastructure in mountain permafrost. The challenges are mainly related to the uncertainties that are associated with prediction of long-term climate change scenarios. On the one hand, designs must be economic but on the other hand some assumptions have to be made with respect to changing ground conditions. Climate variability not only involves changes in air temperature, but also changes in precipitation, e.g. rain and snow days, vegetation, e.g. evapotranspiration, wind (speed and directions) and solar radiation, i.e. radiant energy. These changes, in turn, affect the surface energy balance, hence the heat transfer into and out of the ground and therefore permafrost conditions. When incorporating long-term climate change effects, it is crucial that not only air temperature changes are considered, but also other climatic parameters that will influence the surface energy balance.

There are several hazards to mountain infrastructure that are related to permafrost and in particular to changes in climate conditions (Fig. 2). A range of typical hazards is listed below as an aide memoire. However, it is not exhaustive and additions may be required for individual projects.

Long-term changes in ice temperature and thawing o Major changes in soil properties o Thaw consolidation / settlements o Reduction in strength o Changes in soil deformation characteristics, creep rates o Possible changes in failure mechanisms, e.g. blocks becoming unstable from thawed joints

Change in precipitation o Changes in snow loads o Changes in snow drift o Changes in run-off, water pressures o Influence on ground temperatures o Changes in duration and timing of the winter snow cover General ground warming o Change in active layer thickness and composition o Changes in the duration and timing of the active layer o Changes in intensity and duration of frost heave o Changes in soil strength properties o Effect on frost weathering

Slope stability related issues o Influence on unstable slopes, reduced slope stability o Effect of slope orientation o Effect on erosion processes o Possibly affected area, change in soil volumes for mass movements o Rate of movement o Effect on run-out zones

It is important to note that none of the above stated effects will occur alone, but the various combinations that influence each other may form the most critical condition for a specific structure.

Figure 2. Interactions between climate change factors, geohazards and mountain infrastructure.

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