Permafrost-Related Geohazards and Infrastructure Construction in Mountainous Environments
Summary and Keywords
Mountain environments, home to about 12% of the global population and covering nearly a quarter of the global land surface, create hazardous conditions for various infrastructures. The economic and ecologic importance of these environments for tourism, transportation, hydropower generation, or natural resource extraction requires that direct and indirect interactions between infrastructures and geohazards be evaluated. Construction of infrastructure in mountain permafrost environments can change the ground thermal regime, affect gravity-driven processes, impact the strength of ice-rich foundations, or result in permafrost aggradation via natural convection. The severity of impact, and whether permafrost will degrade or aggrade in response to the construction, is a function of numerous parameters including climate change, which needs to be considered when evaluating the changes in existing or formation of new geohazards. The main challenge relates to the uncertainties associated with the projections of medium- (decadal) and long-term (century-scale) climate change. A fundamental understanding of the various processes at play and a good knowledge of the foundation conditions is required to ascertain that infrastructure in permafrost environment functions as intended. Many of the tools required for identifying geohazards in the periglacial and appropriate risk management strategies are already available.
Mountain environments, which constitute assemblages of variable landforms, reach well above the surrounding area and often form of peaks, arêtes, and valleys. They cover approximately 24% of the global land surface (Kapos, Rhind, Edwards, Price, & Ravilious, 2000) and are home to about 12% of the global population (Huddleston et al., 2003). Such an environment is often synonymous with steep slopes, complex geology, variable climatic conditions, and various geohazards, creating challenging conditions for any sort of infrastructure construction. However, as sources of water, energy, natural resource, and agricultural and forest products and as centers of biological and cultural diversity, religion, recreation, and tourism, mountains bear some significance for more than half of the global population (Messerli & Ives, 1997; Price, 2004). In response to the cold average air temperatures in many mountainous areas, permafrost is abundant at low and high latitudes in the northern and southern hemispheres (Brown, Ferrians, Heginbottom, & Melnikov, 1997; Gruber, 2012; Zhang, Barry, Knowles, Heginbottom, & Brown, 1999; Zhang, Barry, Knowles, Ling, & Armstrong, 2003). The long-term mean annual air temperature (MAAT) can be used as an initial proxy to delineate mountain regions where permafrost may occur (Haeberli et al., 2010). It has been shown that in areas with a MAAT below -3°C, the presence of permafrost is likely, whereas at a MAAT of -1°C, the presence of permafrost is limited to specific micro-climatic conditions. The MAAT is only one parameter controlling the potential presence of permafrost in mountainous terrain. The local topography, micro-climatology, vegetation, and surface and subsurface hydrology including snowmelt, sublimation, snow drift, glacial ice extent, and geology are other factors influencing the extent of permafrost and ground ice (Gruber & Haeberli, 2009; Gruber et al., 2015; Haeberli, 2013; Haeberli et al., 2010).
While geohazards are typical for mountainous environments, the increase in MAAT due to global warming is and will change the frequency and, in some cases, the magnitude of slope instabilities in high mountain permafrost terrain (Gruber, Hoelzle, & Haeberli, 2004; Haeberli, 2013; Kern, Lieb, Seier, & Kellerer-Pirklbauer, 2012; Phillips et al., 2009; Springman et al., 2013; Zischg et al., 2012).
Haeberli, Schaub, and Huggel (2016) show that while glacier ice volumes in most cold mountain ranges rapidly decrease due to accelerating global warming, the degradation of permafrost occurs more gradually due to the role of latent heat in the ground. Consequently, many existing glacier landscapes are transforming at a decadal time scale into new landscapes of bare bedrock, loose debris, emerging vegetation, and the formation of new lakes. Landscape changes in permafrost terrain will be slower and more notable at a century time-scale with ice-cemented talus slopes subsiding and active rock glaciers becoming inactive and, over millennial scales, relict. These altered landscapes have a significant potential to generate new geohazards or intensify existing ones. In some cases, such landscapes may indeed result in a decrease in geohazard potential. Risk from ice avalanche or rock slide impact–generated waves and moraine dam breaches may eventually diminish or disappear altogether as the unstable source is depleted or the respective lakes infilled or drained.
Designing, constructing, operating, and maintaining civil infrastructure in mountainous terrain is often an engineering challenge due to topographical constraints, extreme weather conditions, remoteness, geology, hydrology, and various geohazards, including those related to permafrost (Haeberli, 1992; Keusen & Amiguet, 1987; Steiner, Graber, & Keusen, 1996; Tobler, Graf, & Canassy, 2016). The engineering challenges are typically confined to the active layer, which freezes in the winter and thaws in the summer, and the stability of ice-bearing foundations (e.g., Arenson, Phillips, & Springman, 2009; Phillips et al., 2007). Ice viscosity, the tendency of ice-rich frozen ground to creep, the potential for ice segregation in slopes, and the potential for thaw consolidation in response to permafrost degradation introduce substantial uncertainties when designing infrastructure in permafrost environments (Arenson, Colgan, & Marshall, 2014). Permafrost-specific site investigation, construction, maintenance, and monitoring techniques are required to ensure the longevity and the sustainability of a structure.
Climate change is influencing air temperatures, precipitation regimes, and snow cover distributions, all of which are changing active layer thickness and mountain permafrost distribution (Clague, 2008; Haeberli & Beniston, 1998; Harris et al., 2009; Harris, Davies, & Etzelmüller, 2001). The rate and magnitude of these changes, which influence the mechanical properties and the stability and safety of the infrastructure, are difficult to project for the design life of the infrastructure but must be considered during the design phase (BNQ, 2017; Hayley & Horne, 2008; Instanes, 2016; Rykaart, Munoz, & Stevens, 2016). The unprecedented rates in climate change (IPCC, 2013, 2014), together with the formation of new geohazards from rapid changes in the glacial and periglacial environments (Haeberli et al., 2016; Stoffel & Huggel, 2017), exacerbate the design and construction complexity in mountain settings.
Construction activities in mountains where permafrost is of relevance are diverse. They include structures with:
1. Constructed foundations, such as buildings, pylons for power lines or cable cars, communication towers; defense structures, and technical snow-production systems
2. Embankment fills or cuts for roads and railways
3. Dams and dikes, for example, for hydroelectric or mining projects or providing avalanche, debris flow, and rock fall retention basins
4. Pipelines, buried or above ground, for transporting water, sewage, oil, gas, or ore concentrate
5. Underground constructions, including transportation tunnels and shafts
6. Ski runs
Permafrost-associated geohazards that are related to such construction activities are expressed both on-site and off-site. On-site hazards are generally the easiest to identify and manage, as they relate to the underlying permafrost including problems such as subsidence or creep (Bommer, Phillips, & Arenson, 2010; Phillips et al., 2007). Hazards induced by off-site permafrost, in response to degradation of distant permafrost, are more challenging to characterize, project, and design for (Huggel et al., 2012; Stoffel et al., 2014). These could include direct consequences of ice loss such as rock toppling (Gruber & Haeberli, 2007) or form complex process chains, such as a rock slide entering a lake and causing a displacement wave leading to downstream debris flows and/or flooding.
The challenges related to construction activities in mountain permafrost environments are reasonably understood. However, there are very limited guidelines available to help engineers and owners in designing and planning infrastructure in mountain permafrost, (e.g., Bommer, Phillips, Keusen, & Teysseire, 2009) or assessing how geohazards are affecting those. Based on a series of case studies, this article highlights challenges related to human-induced geohazards in mountain permafrost environments. The case studies are followed by the introduction of a framework intended to guide construction-related geohazard risk management.
The impact of infrastructure on permafrost and vice versa has primarily been studied and documented for polar regions, mainly in the light of climate change (ACIA, 2005; Auld et al., 2009; Instanes, 2003; Shiklomanov, Streletskiy, Swales, & Kokorev, 2016). For mountainous regions, only a limited number of studies have been presented (Arenson, Johnston, Quinn, & Wainstein, 2015a; Dall’Amico et al., 2011; Gruber et al., 2015; Haeberli, 1992). Dall’Amico et al. (2011) present case studies related to local ground movements in response to alterations of the thermal regime, and summarize the key processes that result in hazardous effects on infrastructure as follows:
- New active layer formation in response to removal of the original active layer. If the permafrost foundation contains excess ice, large thaw consolidation settlements occur at the surface.
- Earthworks generally compact natural material. This can result in a change of the thermal conductivity due to pore space loss and affect active layer thickness or induce permafrost degradation.
- If level ground is excavated for construction or drilling, the drainage of meltwater is altered, which, in absence of careful water management, can lead to ponding. The water, forming a heat sink, will then cause enhanced local permafrost degradation.
- The effect of embankments on the permafrost foundation. This depends on the material: a porous material can lead to permafrost conservation, whereas compacted soils will increase the thermal conductivity.
These aspects are discussed and further refined using cases studies that are unique to mountainous terrain where gravity-controlled processes are often dominating. The examples and geohazards are categorized related to:
- Changes in the ground thermal regime
- Gravity processes
- Load impact on ice-rich permafrost (including tertiary creep)
- Anthropogenic rock glacier formation
- Heap leach operation
- Changes in bedrock
- Glacier lake outburst floods and permafrost degradation
Changes in Ground Thermal Regime
Any construction activity affects the natural ground surface conditions and the ground thermal regime. Depending on the disturbance and the original conditions, such impact can be negligible or substantial and must be assessed for every situation. The changes in the ground thermal regime originate from changes in the controls of energy flows between the ground and the atmosphere. This includes changes in material properties, surface albedo, snow cover, surface hydrology, and vegetation.
Figure 1, for example, shows how the construction and operation of an access road in mountainous terrain can affect the local permafrost conditions. In response to the road cut, a new active layer is forming under the road. Since the cut is horizontal, the active layer is often completely removed and the excavation may even extent into the permafrost on the upslope side. On the downslope side, the fill material accumulates, allowing the active layer to raise above its original depth and into the placed fill material. This new slope geometry may itself affect the permafrost and slope stability. Permafrost degradation on the upslope side results in road settlements. Because of these uneven conditions, surface water tends to run along the upslope side where it is typically channelized, further enhancing upslope permafrost degradation and potentially undercutting the cut slope by lose particle entrainment and through thermal erosion. Depending on water flow depth and flow frequency, this may lead to additional permafrost degradation below the channel. In unison with changes in the active layer on the cut slope, local instabilities can occur that affect operations and traffic safety and result in increased maintenance.
On the downslope side, the permafrost table can rise due to additional insulation granted by the placed fill. This may increase local slope stability, but it may also affect runoff through the active layer. Natural runoff on top of the permafrost table will be hindered and water diverted to follow the new road cut with the issues outlined above.
Another phenomenon affecting the ground thermal regime is the change in snow accumulation along linear infrastructures, such as roads. Typically, cuts result in an increased snow accumulation due to leeward turbulence and thus decreases in wind velocities. In response to the insulating effect of a thicker snow cover on the permafrost (Luetschg & Haeberli, 2007; Hrbáček, Láska, & Engel, 2016), its degradation can be accelerated. On the other hand, an increased accumulation of snow along an access road requires snow management to keep the road open if the site in question is designed for year-round use or early summer access. Often, the snow is pushed to the side, as illustrated in Figure 1, as hauling and dumping snow is not economical. This can result in additional insulation and increased degradation as often observed along road embankments in the Arctic (TAC, 2010). However, in combination with regular summer road maintenance, the snow may be covered with debris that protects the snow from melting and with time ground ice formation (Figure 1). This artificial permafrost may locally stabilize the berm, but it may also form a new hazard in the future when air and ground temperatures increase and ground ice melts.
Figure 2 shows a platform that was built for exploration drilling at high elevation. The processes described above for road cuts also apply for this platform. Snow accumulated on the platform, and it melts later than at the natural slopes for similar elevations. The changes in the runoff regime caused by the construction of the platform can be noted in the gully erosion that occurred at the edge of the platform. Such incisions penetrate the active layer and can generate new instabilities depending on the local permafrost and hydrological conditions.
Avalanche-protection structures may have similar effects on the ground thermal regime as road cuts that accumulate snow. The accumulation of snow in a snownet (Figure 3; Phillips, 2006; Phillips & Margreth, 2008) or behind a snow fence (Hinkel & Hurd, 2006) alters the surface energy balance and may result in permafrost degradation and changes in local geohazards. In addition to the direct thermal effects from thermal insulation, changes in permafrost response to altered runoff (timing and volume) may be even more critical to the slope.
Gravity-driven deformation of slopes or specific cryoforms are common in mountainous periglacial environments (Arenson, Kääb, & O’Sullivan, 2016). Those can be superficial within the active layer (solifluction/gelifluction) or deeper in the form of creep of ice-rich material. Such deformation can result in hazardous conditions if infrastructure is located on or near a deforming slope. The frozen debris lobes located in the southern Brooks Range of Alaska and described by Daanen, Grosse, Darrow, Hamilton, and Jones (2012), Darrow, Daanen, and Simpson (2013), and Darrow, Gyswyt, Simpson, Daanen, and Hubbard (2016) are examples of how creeping permafrost can be hazardous to linear infrastructure, such as the Dalton Highway and the Trans-Alaska Pipeline (Figure 4). The studies have shown that the creep deformation rates have increased with time, although it is unclear how these forms will behave in the future. The deformation may continue to increase as a combination of ground ice warming and pore pressure increase at the base of the permafrost. The frozen debris lobe may also suddenly fail in the form of a catastrophic landslide if a strength threshold is reached (e.g., tertiary creep strain), or the deformation decreases until the lobes stops from advancing if the ground ice melts and thereby no longer allowing viscous deformation to occur and the inter-particle strength increases.
Creeping rock glaciers can also impact infrastructure that is built across them. Two examples are shown in Figures 5 and 6 where roads and ski runs are impacted and higher-than-normal maintenance is required. For point load foundations, such as a chairlift pylon, flexible systems may be applied that can be adjusted with time (Arenson et al., 2009; Dall’Amico et al., 2011; Philips et al., 2007).
The Ritigraben rock glacier (Figure 6) offers a unique case study on the effect of changes in the surface conditions, in this case from a ski run, on the rock glacier dynamic. Lugon and Stoffel (2010) present 50 years of permafrost creep data and magnitude–frequency (M–F) relationships of debris flows recorded in the Ritigraben torrent originating at the rock glacier front. The authors report no direct coupling between displacement rates of and sediment delivery by the rock glacier and the frequency of small- and medium-magnitude debris flows. In contrast, a direct link between source and sink processes exists in the case of active layer failures originating at the rock glacier snout, triggering debris flow events in the channel. The ski run itself, constructed in 1984, does not seem to have altered the overall debris flow behavior at that site. However, retrogressive erosion in the debris flow source zone in the rock glacier front due to active layer thickening may increasingly affect the ski run on top of the rock glacier.
Surficial deformations from solifluction/gelifluction may also impact infrastructure. They can directly impact ridged foundations (Figure 7) or roads (Figure 8). Over time, the surficial movements have resulted in the poles being inclined to a level that required the building to be abandoned or the road being inaccessible. However, the deformations are typically slow and do not result in catastrophic geo hazards.
Impact of Loads on Ice-Rich Permafrost
Load increases the creep rates of ice-rich material (Arenson et al., 2014). Therefore, new hazards can form if a load is placed on an ice-rich slope. A unique example of such a case is the rapid advancement of a waste rock pile at the Kumtor Gold Mine in Kyrgyzstan (Jamieson, Ewertowski, & Evans, 2015; Torgoev & Omorov, 2014). The placement of waste rock on a slope that likely contains ice-rich permafrost, together with the presence of remnants of a glacier in the valley that was partially buried, resulted in unexpected high creep deformation and requiring the dislocation of parts of mining infrastructure. Torgoev and Omorov (2014) do not provide details of the processes, but it is likely that the acceleration is a combination of an increase in ground temperature, an increase in stresses, local permafrost degradation, and changes in the hydrology. The complex interaction between these processes is expected to be responsible for generating these hazardous conditions.
The placement of high loads on permafrost, and in particular rock glaciers, is mainly known from the mining industry. A prominent case is the placement of waste rock on rock glaciers for the development of an open pit copper mine at Los Pelambres (Azócar & Brenning, 2008). The internal structure of a rock glacier in the area has been studied (Monnier & Kinnard, 2013). While the rock glaciers have been covered for more than 15 years, there are no reports on an acceleration in the deformation of these rock glaciers or the formation of new hazards due to the additional load.
O’Sullivan, Arenson, and Murton (2016) present a theoretical study on changes in creep velocity in response to loading of a protalus rampart with waste rock material. The study illustrates that placing the waste rock from the top via end dumping likely results in an increase in creep velocity and a decrease in the stability of the ice-rich cryoform. On the other hand, if the material is placed from the bottom up, a stable berm is created that would increase the stability of the protalus rampart as it is covered with additional material with time. While the study only considered changes in the stress conditions due to a new load, and hereby ignoring changes in the ground thermal regimes and hydrology, it provides some guidance in how to reduce potential unsafe conditions.
Anthropogenic Rock Glacier Formation
Another unique case was presented by Grebenets, Kerimov, and Titkov (1998) for a waste rock facility on the Rudnaya Mountain in Norilsk, Russia. A waste rock pile transformed into a creeping rock glacier as water infiltrated the course waste rock material and ice started to segregate within the pile. The coarse material likely allowed for natural convection to occur (Pham, 2013; Pham et al., 2013), further cooling the interior of the pile and promoting the aggradation of ice-rich permafrost. Ultimately, average deformations of 40 mm/day were recorded by Grebenets, Kerimov, and Titkov (1998) due to the excess ice that had formed. The authors further concluded that in the event of a failure, the material could travel a distance of more than 1 km and destroy dozens of industrial buildings and structures. As a mitigative measure, several areas were evacuated. The authors are not aware of a recent report on the waste rock behavior, and it is unclear if a change in the dynamic or even a failure had occurred in recent years.
While reports on the formation of a rock glacier due to construction are extremely rare, the example illustrates that depending on the properties of the material placed and the (micro)climatic conditions, new periglacial forms can be generated artificially that could generate new hazards in the future.
Heap Leach Operation
As mining projects reach new heights and scales for operations in mountainous environments, the required infrastructure will also be exposed to new, cold temperature–related phenomena. One such element that may be impacted is a heap leach that is used for chemically extracting precious metal from the ore. Cold-climate heap leaching is not new (see, e.g., Smith, 1997; EBA, 2011); the challenges related to the presence of permafrost are, however, still not commonly considered during the design and operations. The coarse nature of a heap leach can favor natural convection and cooling of the pile. As leachate infiltrates, it may freeze at depth and clog the system because the temperatures in the pile are colder assumed for design. This has significant impacts on the operation and the water balance, potentially resulting in hazardous conditions.
Changes in Bedrock
If possible, foundations for structures should be built in bedrock because it is typically more stable. However, the bedrock may be jointed and fractured, with the discontinuities being cemented by ice or changes in the ground thermal regimes resulting in frost action in areas that had been historically stable. Active layer thickening and subsidence occurred, for example, beneath the northern tower of the Kulmhotel Gornergrat in Zermatt, Switzerland. The settlements occurred as the active layer thickened in response to the heating of the basement for over 20 years (Hof, King, & Gruber, 2003). Heat- and stress-induced differential settlements were also why the new mountain restaurant Pardorama in Ischgl (Austrian Alps) located at an elevation of 2,600 m was built on a three-point foundation, which can be raised using hydraulic pumps and steel plates, thereby compensating for settlement of the underlying bedrock (Haeberli et al., 2010). Dätwyler (2004) report on several cases of bedrock instability due to the loss of ground ice (possibly also induced by climate change) at several cable car stations and mountain huts, causing damage to the buildings in recent years. Not only does the heat from the building alter the ground thermal regime, but also unheated structures, such as pylons or garages, change the permafrost in response to changes in the surface heat fluxes and induce subsidence (e.g., the Schilthorn and Gemsstock summits in Switzerland; Arenson et al., 2009). Tobler et al. (2016) reported on the construction of the new summit station that will form Europe’s highest cable car station at the Klein Matterhorn in Switzerland. The cut into permafrost bedrock (Figure 9) will result in new hazards, such as rock fall, that must be managed during construction and operation, as a new ground thermal regime slowly develops in adjustment to the changed bedrock topography and changes to the heat transfer between bedrock and the atmosphere.
Tunnels in permafrost may also affect the ground thermal regime because air or water can warm frozen rock zones, as was observed during the drilling of access tunnels in the Chli Titlis (Haeberli, Iken, & Siegenthaler, 1979), Klein Matterhorn (Rieder, Keusen, & Amiguet, 1980), or the Jungfrau east ridge (Wegmann, 1998). The heat transferred into the tunnel of the Klein Matterhorn tunnel by 490,000 visitors every year and generated by about 70,000 elevator movements in transporting people to the mountaintop caused the bedrock temperatures in the tunnel to rise from about −12°C in the early 1980s to −8°C in 1999 and further to −3°C by 2005. In addition, the refreezing of melt water in the lift shaft has required remedial ventilation measures (Keusen & Haeberli, 1983; King & Kalisch, 1998).
Glacier Lake Outburst Flood and Permafrost Degradation
Haeberli et al. (2016) have illustrated that glacier lake outburst floods can result from newly forming lakes due to impact/flood waves caused by ice/rock avalanches from nearby steep mountain slopes with degrading permafrost. These hazards, which originate in the glacial and periglacial environments, are often referred to as glacier lake outburst floods (GLOF). An example of infrastructure being potentially affected by rapid glacier shrinkage and a GLOF, but also of potentially affecting the GLOF hazard, is the Kumtor mine site in Kyrgyzstan (Jamieson et al., 2015; Janský et al., 2009; Petrakov et al., 2016). Figure 10 shows a thermokarst lake that formed within the ice-cored end moraine of Petrov Lake, which is situated upstream of various mine infrastructure. The degradation of permafrost in the moraine dam can increases the hazard for a dam failure via different scenarios: (1) active layer increase may allow water flow through the dam at lower elevation; (2) warmer ground conditions may allow the formation of a talik, which could eventually lead to localized seepage in the talik and through the moraine dam as the talik thickens and widens; and (3) an increase in temperature reduces the strength of the frozen dam. Lake level management is a risk-mitigating strategy as it reduces the potential for the naturally occurring permafrost degradation to generate a potentially catastrophic flood event.
An ice avalanche that occurred on the northwest flanks of Mount Edith Cavell, located in Jasper National Park, Alberta, Canada, which triggered a small GLOF affecting an access road and parking lot typically frequently visited by tourists, is another example demonstrating hazards to infrastructure from changes in the glacial and periglacial environment (Quinn et al., 2014; Wedgwood, 2014). About 125,000 m3 of ice fell close to a vertical kilometer onto a lower glacier and then into a full tarn triggering the damaging flood (Figure 11). Multiple factors were considered relevant in triggering the ice avalanche, including rock fall on the glacier, likely caused by warm air temperatures prior to the event. The ice avalanche originated from a steep and likely polythermal hanging glacier with its stability governed, in part, by the ground thermal regime in its bed. Similar observations have been made in the Caucasus (Haeberli et al., 2004) and in the Peruvian Cordillera Blanca (Carey, Huggel, Bury, Portocarrero, & Haeberli, 2012).
Infrastructure being affected by a GLOF is not a new phenomenon and has been reported for many sites, including nearby Cathedral Mountain, located in the southern Rocky Mountains of British Columbia, about 7 km east of the community of Field, BC, Canada (Arenson et al., 2015b; Jackson, 1979). Cathedral Glacier is situated at the top of a gully, known as Cathedral Gulch, where debris flows can be triggered by large water volume releases from natural water reservoirs on top of and adjacent to the glacier (Figure 12). However, while Arenson et al. (2015b) indicate that this event is isolated, characterized by the presence of permafrost in the debris flow source zone and the rapid changes occurring in the glacier, the case could be symptomatic for other such events that appear to be occurring more frequently in recent years.
Continued glacial retreat has also exacerbated GLOF potential in many mountain environments. A particularly relevant example is that of Lake Palcacocha near the city of Huaraz Peru. On December 1941, approximately 1,800 people lost their lives in Huaraz due to a GLOF that originated at Lake Palcacocha. Now, glaciers have receded further and the lake has increased in area and volume. Somos-Valenzuela, Chisolm, Rivas, Portocarrero, and McKinney (2016) simulated the complex process chain consisting of avalanches entering the lake, wave generation, propagation and moraine overtopping, flood propagation downstream, and impact on populated areas. However, glaciers continue to shrink in the Cordillera Blanca despite increases in precipitation since the early 1980s (Schauwecker et al., 2014), and continuing retreat may, over time, evacuate certain watershed portions from glacial ice. This may result in a reduction of the risk of ice avalanche–triggered impact waves. At the same time, in some cases the diminishing effect of glacial buttressing may lead to rock slides where unfavorable structure prevails and frost action–related weathering develops. This example illustrates the complexity associated with climate and associated glacial changes in high mountain environments that can threaten tens of thousands of people downstream.
Mountain Permafrost Hydrology
There are few studies available on mountain permafrost hydrology (e.g., Krainer, 2011; Krainer, Chinellato, Tonidandel, & Lang, 2011), and none of the available publications report on hazards related to infrastructure. However, with many geohazards, water often plays a key role in the development or the triggering of a geohazard. Therefore, some major aspects are discussed here.
Mountainous permafrost zones can receive water from several sources, including direct precipitation, runoff from adjacent slopes, glacier- and snowmelt, and groundwater. Water output may occur through surface runoff, subsurface discharge, subsurface seepage, sublimation, vapor flux, evaporation, and ground ice melt. Because of this complexity in surface hydrology and hydrogeology, water discharge from permafrost areas can not simply be attributed to the melt of ice from the frozen ground alone. In fact, ground ice may be hundreds to thousands of years old (Haeberli et al., 1999), having no influence on the yearly water balance or late summer runoff. Nevertheless, the presence of permafrost affects surface hydrology, and its seasonal characteristics may be summarized as follows:
– Late winter: water within the active layer is frozen and any observed discharge originates entirely from groundwater flow systems at the permafrost base or in taliks.
– Late spring/early summer: the thawing front penetrates the ground; melting snow and ice recharges the upper portion of the permafrost, creating a seasonal aquifer perched on top of the frozen core.
– Summer: the majority of the free water within the active layer has discharged and most of the discharge observed, other than runoff from rainfall, originates from the permafrost base or from taliks. Melt water from snow patches in depressions on the surface continues to run off through the summer.
– Late summer/early fall: the active layer has reached its maximum and the freezing front starts to penetrate downward from the ground surface. Under degrading conditions, ground ice in the permafrost may start melting, contributing to the runoff. Any remaining water may accumulate in the central portion of the active layer. Under cold permafrost conditions, two-sided freezing may occur, in which case the active layer also freezes from the permafrost table upwards (Harris et al., 2008).
The case studies presented in the previous section highlight the fact that construction activities invariably impact the permafrost regime. However, the type, frequency, magnitude, and temporal impact of this disturbance varies significantly depending on the construction activity, the foundation conditions, and local climatic conditions. This controls the changes in frequency, magnitude, and possibly the intensity of existing geohazards, the formation of new geohazards, or the disappearance of known geohazards. Figure 13 summarizes the complexity of the impact of construction on geohazards in permafrost environments. This schematic may help in identifying such geohazards, allowing their timely risk management.
First, the above- or below-ground structure, such as a tunnel or a cavern, or a road embankment or building changes the ground thermal regime. For example, above-ground structures change snow accumulation pattern and runoff, affecting the surface energy balance and ground temperatures. With time, this may result in permafrost degradation, including ground warming and thaw, or permafrost aggradation (including ground cooling and freezing). With respect to geohazards, the former situation is often more critical than the latter.
Permafrost degradation is first reflected in increase in active layer depth, which, for ice-rich permafrost, can result in substantial thaw consolidation settlements or mass movements where ice cohesion is lost. The changes in geohazards are often triggered by a change in the surface or subsurface hydrology. Permafrost degradation can create new flow paths, or construction-induced changes in the snow accumulation can result in changes in snow melt runoff volumes and timing.
If the construction changes permafrost aggradation, for example, in response to convective cooling or removal of an insulating snow cover or repeat compaction by snow cats of ski runs, the active layer decreases, ice can segregate, and new geomorphic processes (e.g., solifluction) or landforms (e.g., rock glaciers) may form. In most cases, permafrost aggradation also changes the runoff regime in response to the change in the active layer. For example, a thinner active layer can saturate more quickly than a thicker one, leading to more shallow landslides for the same hydroclimatic events than previously. At the same time, the magnitude (controlled by thickness) may decrease in response to active layer thinning.
Active Layer Detachments
As the active layer increases, potentially ice-rich layers at the permafrost table start to melt. Depending on the site-specific drainage conditions, pore pressures may increase, thereby reducing the strength of this layer. This, often in combination with rain water, snow melt, or enhanced glacier runoff, can trigger an instability in the active layer.
Active layer increase and changes in runoff can impact the frequency and/or the magnitude of debris flows. With an increase in the active layer, hitherto unknown debris volumes may become available for debris flow entrainment and transport. Historic frequency–magnitude relationships based on quasi-stationary conditions may no longer be applicable. This tendency may occur in parallel with more frequent and higher-magnitude extreme precipitation events (e.g., Prein, Rasmussen, Chaghai, Clear, & Holland, 2017). In June 2013, for example, a high-intensity/duration rainstorm resulted in a series of damaging debris floods in the Bow River Valley, Alberta, Canada (Jakob et al., 2014). One debris flow was initiated at the front of a rock glacier, exposing frozen debris, which will continue to slough as a new active layer forms (Figure 14).
Figure 15 shows the extreme complexity to which debris flows are subject in a changing climate. While changes in precipitation extremes are important because they are the dominating triggering mechanism, other watershed-scale changes can also drastically change either the frequency or the magnitude of debris flows. This includes widespread beetle infestation that leads to tree mortality, wildfires changing the runoff regime, and permafrost and glacial changes that may affect the availability of readily erodible sediment. While the individual links are understood in theory, the quantification of periglacial geohazards in a changing climate has thus far received comparatively little attention.
An increase in bedrock temperature can thaw ice in frozen joints or decrease their strength as the ice warms (Draebing, Krautblatter, & Dikau, 2014; Günzel & Davies, 2006; Krautblatter, Funk, & Günzel, 2013). This can result in increased rock fall activity or major rock instabilities (rock slides and rock avalanches). The former can be mitigated using rock anchors or other techniques, such as thermosyphons, to promote heat extraction. In the case of large (>10,000 m3) rock masses, stabilization is typically not feasible. Heavy rock anchoring has been demonstrated to be a successful mitigation measure at the Klein Matterhorn (Schoeneich, Dall’Amico, Deline, & Zischg, 2011). Infrastructures at the Jungfraujoch (Swiss Alps), including an underground train station, restaurant, and research station, have been successfully retained due to ongoing maintenance efforts (Keusen & Amiguet, 1987).
Should a rock fall frequency–magnitude relationship exist for the site, it may no longer be valid in light of the thermal changes in the contributing rock face. This may affect the maintenance frequency of existing rock fall mitigation structures or the construction of new ones.
Differential Settlements and Thermokarst
The heterogeneity in the ground ice conditions often found in mountainous terrain, together with the irregular change in the surface conditions (i.e., snow drifts), result in equally heterogeneous ground ice melt and thaw settlements. Such settlements can initiate stresses in rigid foundations that were not included in the design or require higher than anticipated maintenance. Differential settlements may further accelerate local degradation as water can pond or find new pathways forming a heat sink and associated taliks. With time, the construction can therefore form larger thermokarst that may result in new hazards.
Mitigative measures may include the removal of ice-rich soils. However, the excavation of ice-rich frozen soil is challenging, often with very limited success (e.g., construction of the Grande Dixence dam in the ice-rich permafrost of a rock glacier at Prafleuri, Valais Alps, Switzerland; Haeberli, 1992; Haeberli et al., 2010).
Depending on the changes in the surface conditions or the physical properties of material being placed, permafrost may aggrade. Ground ice may accumulate via segregation, and formerly stable slopes may start to creep under gravity. Snow management can also result in the formation of ice-rich permafrost if accumulated snow is covered by debris, thereby insulating it during the summer.
In addition to the potential impacts from a construction, climate change must be considered when evaluating the changes in existing or the formation of new hazards. The main challenge is related to the uncertainties associated with the projections of medium- (decadal) and long-term (century-scale) climate change. Most engineering designs are cost-sensitive, and assumptions must be made as to potential changes in ground conditions. Changes are not limited to those in air temperature, but also include 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, like the actual construction, affect the heat transfer into and out of the permafrost. Therefore, it is important that not only projected changes in air temperature be considered but also other climatic and edaphic parameters that will influence the surface energy balance when evaluating how much an infrastructure changes hazards in the long term.
Climate extremes, including projected changes, as well as differences in seasonal trends should be evaluated when assessing the effects of climate change. For example, an increase in the frequency and magnitude of extreme precipitation during summer storms in combination with increased warming of summer temperatures can result in substantial changes to debris flow hazards and thus risks compared with only average changes. The uncertainties associated with these projections are, however, much greater and much more complex. Such an effort is not warranted for all infrastructures. A standard, recently developed for site investigations in permafrost environments in Canada (BNQ, 2017), provides guidance on how to include climate change in the design of a foundation. Some practical advice on constructions in mountain permafrost or a schematic approach in evaluating direct and indirect geohazards in a changing climate can be found in Bommer et al. (2009) or Arenson et al. (2015b).
Designing, constructing, operating, and maintaining infrastructure in mountain permafrost terrain is an engineering challenge. Any construction activity has the potential to alter the ground thermal regime, requiring that the potential consequences to foundations and possible changes to geohazard frequency, magnitude, and intensity be evaluated and treated appropriately. To date, no detailed guidelines exist that help practitioners, owners, and regulators to carry out such assessments systematically. The economic and ecologic importance of mountainous environments for tourism, transportation, hydropower generation, as well as natural resource extraction mandates that all direct and indirect interactions between infrastructure and geohazards be evaluated. This is particularly pertinent in the light of ongoing and unprecedented rates of climate change in which past conditions may no longer be a rational basis for future decisions. The case studies presented in this article illustrate the potential complexity of such infrastructure/permafrost interactions. A profound understanding of the various processes and a thorough knowledge of the foundation conditions are of paramount importance to predict changes in the ground thermal regime, hydrology, and associated geohazards and risks. Guidelines are starting to be developed to address this complexity. Tools for the identification and quantification of permafrost-specific geohazards are available. These, in combination with thorough site investigations and modeling, allow implementation of appropriate risk management at the design stage with the goal of reducing future losses and ascertaining that infrastructure functions as designed.
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