Réf. Harris & al 2001 - A

Référence bibliographique complète
HARRIS, C., DAVIES, M.C. R., ETZELMÜLLER, B. The Assessment of Potential Geotechnical Hazards Associated with Mountain Permafrost in a Warming Global Climate. Permafrost and Periglacial Processes, 2001, 12: 145–156.

Abstract: European mountain permafrost is generally only a few degrees below zero Celsius, and may therefore be highly sensitive to climate change. Permafrost degradation may lead to thaw settlement and reduction in the stability of mountain slopes. Engineering projects within the high mountain zone require careful investigations of potential permafrost-related hazards. This paper summarizes a staged approach to such investigations. Phase 1 involves walkover site survey supported by a desk study to define potential permafrost hazard zones. Data from permafrost distribution maps, topographic and geological maps are integrated, preferably using GIS methodology. If permafrost is possible and is judged to pose a significant threat to the development, phase 2 investigations are recommended whereby field thermal measurements, drilling of exploratory boreholes, and geophysical surveys are undertaken to clarify permafrost characteristics. The resulting data set should form an important component in subsequent engineering design. On a larger scale, a similar approach should be adopted as part of land-use planning within the mountain permafrost zone.

Résumé : Généralement, le pergélisol européen de montagne est seulement quelques degrés en dessous du zéro Celsius et est, de ce fait, très sensible aux changements climatiques. La dégradation du pergélisol peut entraîner des affaissements du sol au dégel et une réduction de la stabilité des pentes de montagne. Des projets d’ingénierie dans la haute montagne exigent donc des recherches soigneuses concernant les risques potentiels liés à l’existence du pergélisol. Le présent article résume une approche en différentes étapes d’une telle recherche. La phase 1 consiste en un examen attentif du terrain, travail suivi par une étude en laboratoire afin de définir les zones potentielles où existerait un pergélisol dangereux. Des données provenant de cartes de la distribution du pergélisol ainsi que de cartes topographiques et géologiques doivent être intégrées en utilisant si possible un système d’information géographique. Si un pergélisol est possible et paraït constituer une menace, une deuxième phase de recherches est recommandée. Elle comprendra des mesures de température sur le terrain, la réalisation de forages d’exploration et des levés géophysiques pour définir les caractéristiques du pergélisol. Les données recueillies constitueront une partie importante du projet d’ingénierie. A une échelle plus grande, une échelle approche semblable devrait être adoptée dans toute étude visant à modifier l’affectation de terrains dans la zone du pergélisol de montagne.

Mots-clés
Hazard assessment, mapping, mountain permafrost, permafrost engineering.

Organismes / Contacts
Department of Earth Sciences, Cardiff University, PO Box 914, Cardiff CF10 3YE, UK 2
Department of Civil Engineering, University of Dundee, Dundee DD1 4HN, UK
Department of Physical Geography, University of Oslo, P.O. Box 1042, Blindern, N-0316 Oslo

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Temperature Permafrost degradation, creep-related processes Mass movements Deep-seated bedrock failures, rock slides, rock fall, mudslide, debris flow, gelifluction

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Mountain chains of Western Europe          

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses

As a consequence of global warming, mean annual air temperatures are predicted to rise (e.g. Deque et al., 1998; Hulme et al., 1999) and snow distribution and duration are likely to change. [...] However, even in a warming world, regional cooling trends may occur, and reduction in snow cover and glacial ice cover may also lead to greater winter cooling in certain areas (Wegmann et al., 1998).


Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations

Long-term monitoring of ground temperatures at Murtèl–Corvatsch in Switzerland has clearly demonstrated the sensitivity of mountain permafrost to changes in both air temperature and the amount and timing of winter snow cover (Vonder Mühll et al., 1998).

Modélisations
 
Hypothèses

European mountain permafrost temperatures are generally only a few degrees below zero, so that a slight shift in energy flux at the ground surface could lead to widespread permafrost degradation. Likely impacts of warming surface temperatures within the permafrost zone include active-layer thickening, basal melting causing permafrost thinning, and hydrogeological changes.


Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
   

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
The physical stability of permafrost terrain, especially on steep mountain sides, is highly sensitive to thermal changes, since thawing reduces the strength of both ice-rich sediments and frozen jointed bedrock. Ice-rich soils undergo thaw consolidation during melting, with resulting elevated pore water pressures (Morgenstern and Nixon, 1971; Harris and Davies, 1998), so that formerly frozen sediment-mantled slopes may become unstable.
Modélisations
 
Hypothèses

Bedrock slopes may also suffer destabilization if warming reduces strength of ice-bonded open joints (Davies et al., 2001) or leads to ground­water movements that cause pore pressures to rise. Warming-induced permafrost degradation is, therefore, likely to lead to increasing scale and frequency of slope failures and may cause thaw settlement damage to foundations (Haeberli, 1992; Haeberli et al., 1997; King and Kalisch, 1998; Arenson and Springman, 2000).

Potential future permafrost changes associated with a warming climate may affect both mountain slopes in their natural state, leading to hazardous changes in geomorphic activity, and structures such as roads, houses, ski facilities etc. which may in consequence suffer damage. In addition, man-made structures may themselves cause changes in permafrost thermal regime, leading to degradation of foundations or local instability.

Likely impacts of warming surface temperatures within the permafrost zone include active-layer thickening, basal melting causing permafrost thinning, and hydrogeological changes. Negative impacts on foundation systems may be due to thaw consolidation, lateral loading or hydrogeological changes. Retaining systems may suffer increase in active earth pressure and increased lateral loading due to sediment accumulation or earth movements.

On slopes, permafrost degradation may lead to deep-seated bedrock failures, creep-related processes, rock slides, rock fall, mudslide or active-layer detachment failures, increased debris flow activity and accelerated gelifluction. Other potential engineering effects include water ingress to subsurface structures, and impacts on services.


Paramètres de l'aléa
Sensibilité du paramètre de l'aléa à des paramètres climatiques et du milieu / Facteurs de contrôle
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 

(4) - Remarques générales

Permafrost is present at higher elevations through the major mountain chains of Western Europe (Scandinavian Mountains, the Alps, Pyrenees and Sierra Nevada) (Haeberli et al., 1993, Harris and Vonder Mühll, 2001).

Mountain permafrost has a complex spatial distribution that depends largely on altitude, radiation and snow distribution in space and time (Haeberli et al., 1983; Hoelzle, 1994; Hoelzle et al., 2001, this issue). Accurate spatial modelling and careful field surveys are therefore necessary to identify areas susceptible to permafrost degradation and ensure that engineering structures and settlements are not adversely affected (Keusen and Haeberli, 1983; Keusen and Amiguet, 1987; Vonder Mühll and Keusen, 1995).

The EU PACE project has, in consultation with engineering practitioners, developed a practical site investigation methodology aimed at determining the geotechnical hazard associated with mountain permafrost in general, and degradation in particular. The proposed methodology differs from traditional hazard assessment approaches, which are generally predicated on the premise that past and present-day distribution, intensity and frequency of hazardous phenomena may be used to define future hazard levels and distributions. The PACE permafrost assessment protocols are not based on assessment of past hazards, since the concern is with future changes rather than forward projection of historical conditions.

Likely impacts of warming surface temperatures within the permafrost zone include active-layer thickening, basal melting causing permafrost thinning, and hydrogeological changes. Negative impacts on foundation systems may be due to thaw consolidation, lateral loading or hydrogeological changes. Retaining systems may suffer increase in active earth pressure and increased lateral loading due to sediment accumulation or earth movements.


(5) - Syntèses et préconisations

An initial approach to the determination of potential hazard zones considered during this research project was to develop a synthetic index of potential permafrost hazard. This could be based on weighted factors applied to a standard grid across the area of assessment. However, it was concluded that such an index depends on subjective factor weightings that may vary from site to site, and generates a potentially misleading ‘hazard score’ that could be interpreted in terms of level of hazard potential. Thus it was decided to develop a ‘decision tree’ approach to guide practitioners through the phase 1 site assessment. This provides criteria against which decisions may be made concerning the need to progress from desk study to more detailed ground investigation. The aim of phase 1 is to determine whether the presence of permafrost is possible within the area of investigation, and if so, the nature of geotechnical hazards that might arise from a perturbation in near-surface thermal conditions.

Références citées :

Arenson L, Springman S. 2000. Slope stability and related problems of Alpine permafrost. In Proceedings of the International Workshop on Permafrost Engineering, Longyearbyen, Svalbard, Norway, 18–21 June 2000. 183–196.

Deque M, Marquet P, Jones RG. 1998. Simulation of climate change over Europe using a global variable resolution general circulation model. Climate Dynamics 14: 173–189.

Davies MCRD, Hamza O, Harris C. 2001. The effect of rise in mean annual temperature on the stability of rock slopes containing ice filled discontinuities. Permafrost and Periglacial Processes, 12: 137–144.

Haeberli W. 1992. Construction, environmental problems and natural hazards in periglacial mountain belts. Permafrost and Periglacial Processes 3: 111–124.

Haeberli W, Guodong C, Gorbunov AP, Harris SA. 1993. Mountain permafrost and climatic change. Permafrost and Periglacial Processes 4: 165–174.

Haeberli W, Wegmann M, Vonder Mühll D. 1997. Slope stability problems related to glacier shrinkage and permafrost degradation in the Alps. Eclogae geologicae Helvetiae 90: 407–414. [Fiche Biblio]

Harris C, Davies MCR. 1998. Pressures recorded during laboratory freezing and thawing of natural silt-rich soil. In Proceedings of the 7th International Conference on Permafrost, Lewkowicz AG, Allard M. (eds). Collection Nordicana, 57: 433–439.

Harris C, Rea B, Davies M. 2001. Scaled physical modelling of mass movement processes on thawing slopes. Permafrost and Periglacial Processes, 12: 125–135.

Hulme M, Mitchell JFB, Ingram WJ, Lowe J, Johns TC, New M, Viner D. 1999. Climate change scenarios for global impacts studies. Global Environmental Change 9: S3–S19.

Keusen HR, Amiguet JL. 1987 Die Neubauten auf dem Jungfraujoch. Geologie, Felseigenschafte Permafrost. Schweizer Ingenieur und Architeckt 30–31: 17–18.

Keusen HR, Haeberli W. 1983. Site investigation and foundation design aspects of cable car construction in Alpine permafrost at the ‘Chli Matterhorn’, Wallis, Swiss Alps. In Proceedings of the Fourth Interna­tional Conference on Permafrost, Fairbanks Alaska, USA. National Academy Press: Washington, D.C; Vol 1, 601–605.

King L, Kalisch A. 1998. Permafrost distribution and implications for construction in the Zermatt area, Swiss Alps. In Proceedings of the 7th International Conference on Permafrost, Lewkowicz AG, Allard M. (eds). Collection Nordicana, 57: 569–574.

Vonder Mühll D, Stucki Th, Haeberli W. 1998. Borehole temperatures in Alpine permafrost: a ten year series. In Proceedings of the 7th International Conference on Permafrost, Lewkowicz AG, Allard M. (eds) Collection Nordicana 57, 1089–1095.

Vonder Mühll, Hauck C, Gruber H, McDonald R, Rus­sile N. 2001. New geophysical method of investigating the nature and distribution of mountain permafrost with special reference to radiometry techniques. Permafrost and Periglacial Processes, 12: 27–38.

Wegmann M, Gudmundsson GH, Haeberli W. 1998. Permafrost changes in rock walls and the retreat of Alpine glaciers: a thermal modelling approach. Permafrost and Periglacial Processes 9: 23–33.