Permafrost can be found on approximately 20% of the earth’s surface and can be defined as “perennially frozen ground, a naturally occurring material with a temperature colder than 0° C (32° F) continuously for two or more years” (Encyclopaedia Britannica, 2009). It can be located in regions further than approx 60o north or in alpine regions. It can reach a thickness of up to 1000 metres, as it does on the North Slope of Alaska. It extends through as much as 50% of Canada and 80% of Alaska (Clark, 1988 p32). Figure 1.1 below shows the distribution of permafrost in the Northern hemisphere. There are four main types of permafrost: isolated, sporadic, discontinuous and continuous.
This is specifically important in the construction and engineering considerations when designing and building lifts in resort areas. The popular mountain sport industry has witnessed an explosion of growth in the last 60 years. This demand for snow has resulted in the construction of purpose built ski resorts, such as Avoriaz and Flaine in the French Alps, and the expansion of lift infrastructure to support this growth. This article focuses on the challenging conditions that permafrost can have on construction, in the artic. Future articles will focus more specifically on the engineering of lift systems.
Permafrost creates challenging conditions for construction and, as Figure 1.1 demonstrates, there are a lot of regions inhabited by humans which are affected by permafrost. Permafrost engineering has to deal with specially frozen ground phenomena related to the frost heave and thaw, for example: frost mound, icing, thaw settlement, gelifluction and subsidence. When those geological phenomena threaten the safety of construction and the stability of the engineering, it becomes an engineering hazard.
Figure 1.1: Map showing Artic Permafrost Boundaries
Thompson (1977, p.39) argues that there are several man-induced permafrost changes. Exploitation of the tundra for economic and social gain alters both the energy exchange and terrain characteristics. The clearing of vegetation changes the albedo of the tundra and therefore alters the net radiation. Where roads and airstrips are built, the absorption of solar radiation is amplified due to the dark absorbent materials used which have a very low albedo. This leads to a thawing of the permafrost in summer time. However in winter, roads and airstrips actually reduce the heat in the ground as there is no insulating snow pack on top of the tundra, this results in rapid long-wave heat loss and permafrost thickness increases. Crawford and Johnston (1971 p248) conducted research into this and found that “At Inuvik in the Mackenzie Delta an airstrip was constructed on a fill 2.4 – 3.7 m thick. Temperature measurements by the Division of Building Research have shown that the permafrost has actually risen into the embankment at least 0.6m”.
When constructing a new settlement in a permafrost region, it is important that the natural equilibrium between the soil and permafrost is maintained. If permafrost begins to melt then the building may subside, collapse or deform. Subsidence can have potentially lethal affects: if a gas pipe subsides, fire can be caused and if an oil storage depot cracks, the surrounding environment will be severely damaged. There are two forms of construction: passive, where every attempt is made not to disturb the permafrost or active, where the permafrost is destroyed. Active construction techniques are only found in areas where the active layer of the permafrost is very thin. In order to construct a settlement in the high Arctic, passive construction is required.
Before construction work begins it is very important to take a thorough survey of the area and research the following;
• Sites of proposed buildings and structures
• Utility lines
• Type of soils and vegetation
• Topography
• Energy source
Figure 2.1 shows the range of qualified professionals required to carry out a successful survey prior to construction. There is a wide range of professions required in order to thoroughly establish a location to construct a new settlement and reduce the risk from permafrost hazards.
Figure 2.1: The range of qualified professionals required to carry out a successful survey
Once a suitable location has been established there are different techniques that can be implemented to reduce the potential hazard caused by permafrost.
When building sites are underlain by permafrost, the foundation system must ensure that any warmth emanating from the structure does not induce thawing of the permafrost layer. The simplest and cheapest way to do this is accomplished by elevating the building above the ground surface on a pile or adjustable foundation system. The resulting air space ensures that heat from the structure will not penetrate the active layer and cause permafrost warming. Thousands of structures ranging from single-family residences to large living quarters and apartment blocks are currently supported on pile foundations, including many residential structures in the permafrost zones of Alaska, Canada and Scandinavia, and many apartment buildings in Siberia. Figure 3.1 shows an apartment block in Russia supported on piles to stop heat radiation to the permafrost.
Larger buildings carrying heavier loads such as factories, water plants and fuel storage facilities cannot be supported off the ground using piles due to the massive cost and instability. Therefore a different method for preventing heat radiating from the buildings is required. Methods used to control this are thermosyphon or air-duct cooling systems. Several large sections of tubing are inserted beneath a building and covered in a layer of loose material. The tubing allows air flow to ventilate through the base of the building, cooling the surrounding loose material (Figure 4.1). The loose material is used to keep the structure further away from regions of permafrost on the ground. “To enhance the cooling effect of air ducts, an experiment was conducted to investigate the impact of a temperature-controlled ventilation system where one or both ends of the air ducts are installed with shutters that open and close automatically with changes in air temperature (Figure 4.2). The shutter has a temperature sensor and a control unit. It closes when the ambient air temperature is higher than a pre-set value. The temperature data measured from the inner walls of the ventilation ducts indicates that ducts with shutters are 1 °C colder than those without” (Wei et al, 2009).
Figure 4.1 and 4.2
A thermosyphon is a method of passive heat exchange based on natural convection. Cool liquids are circulated around the underside of the structure in a closed loop circuit, without requiring an external pump, cooling the foundations and reducing heat radiation into the permafrost. The thermosyphon cooling system aims to simplify the liquid pumping and heat transfer by avoiding the cost, operation reliability and complexity problems linked to conventional pumping systems.
Utilities such as gas, electricity and sewage are normally installed into the property underground in separate pipes. This is not possible when permafrost is found in the earth below a dwelling because utilities would freeze and laying the pipes would destroy the permafrost and cause a heat rise in the surrounding ground. One of the solutions to this problem is the utilidor; insulated tubes carrying water and electricity into a house and removing sewage housed in corrugated steel and placed on stilts above ground level to reduce heat radiation. One of the first widespread uses of utilidors was in Inuvik (1959), although this caused tension between the “natives” and new residents, as Farish and Lackenbauer (2009) found. “Social partitioning was also spatial, enforced by the geographical groupings of native residents in the north part of the town, far from government housing built for civil servants [Figure 5.1]. The ‘serviced’ end of town contained ‘modern furnished apartments and house units’ attached to Inuvik’s famous utilidors”. Figure 5.2 shows how the utilidor system links all buildings to one central utilidor providing them with sewage removal and water. When constructing a new settlement one of the first things I would construct is an utilidor system that links all of the buildings in the settlement. Utilidors are expensive and account for a large proportion of construction costs but are essential to a modern town built in a permafrost region.
Figure 5.1
Figure 5.2
With the current global awareness and fears about climate change, renewable energies are being considered for new constructions to decrease their dependability on fossil fuels. Lowland Arctic regions can be exposed and receive sufficiently high average wind speeds to make wind-diesel power plants economically viable. In Weis and IIinca’s report (2008) into wind-diesel power plants in the Arctic, they found that “wind generators are suitable in remote Canadian communities only when an average annual wind speed of at least 6.0 m/s is present”. The average wind speed of some remote Canadian communities are between 7 and 13m/s and linking these wind speeds with Weis and IIinca’s report, It would therefore be viable to use wind-diesel power plants to provide energy for the new settlement. This would provide the settlement with an environmentally-friendly energy source and make the settlement more self-dependent, as it won’t have to rely on fuel being transported from elsewhere, which can be problematic during the harsh winter conditions. By reducing the output of CO2, the effects of global warming will begin to slow down and temperatures in the Arctic can begin to stabilise, thus reducing the permafrost thawing rate and preventing the hazards associated with changing the heat balance in permafrost.
Transport links have to be created to a new settlement. Roads, railways and airstrips all cause permafrost thawing unless correctly constructed. A buckled rail, cracked road and an uneven airstrip are all potential fatal hazards that can be caused by permafrost thawing. The simplest and most cost-effective method is to build an embankment underneath the road, railway or airstrip. This lifts the transport link away from the permafrost and the embankment thickness can be adjusted to ensure that seasonal thawing is contained within the embankment itself, thus avoiding thawing of the underlying permafrost. Ventilation pipes [Figure 4.2] can be installed when the embankment is being constructed to remove some of the heat and keep the permafrost frozen. Ventilation pipes are used to cool the Qinghai–Tibet Railway.
I conclude that after the survey has been completed and an appropriate location has been established for the construction of a new settlement, then there are specific engineering techniques to avoid permafrost engineering hazards. Buildings in the settlement will have to be constructed using either piles, an embankment or some form of cooling system such as ventilation tubes or a thermosyphon. An utilidor system will be installed and connected to every building in the settlement to extract waste and provide utilities. To power the settlement, a wind-diesel power plant will be used to ensure sustainability. Finally transport links will be built on embankments pre-fitted with ventilation tubes in order to reduce thermal radiation melting the permafrost.
References
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Qihao, Y., July 2008. Investigation of embankment with temperature-controlled ventilation along the Qinghai–Tibet Railway. Cold Regions Science and Technology, Volume 53, Issue 2. pp193-199
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Weis, T.M, & Iiinca, A., July 2008. The utility of energy storage to improve the economics of wind–diesel power plants in Canada. Renewable Energy, Volume 33, Issue 7. pp1544-1557
