Changes in the ground thermal regime are the major cause of problems with permafrost infrastructure. Efforts should therefore be made to limit the changes in the ground temperatures and in particular to prevent phase changes in the ground. By moving the active layer into soils that are non-frost susceptible, frost heave and thaw settlements can also be avoided. Figure 18 shows various sketches of suitable foundations in permafrost environments. The various methods are described below.
Special thermal insulation (e.g. Ulitsky et al., 2003) can be used in the design to protect the permafrost ground under a structure, e.g. roads or buildings. Cheng et al. (2004) recently presented a discussion on the existence of maximum and minimum embankment heights and the applicability of thermal insulation. Generally, insulating materials reduce the thermal flux due to their low thermal conductivity and thickness. The effect can either be obtained by using special materials, such as polystyrene (~0.03 W/m°C, e.g. Andersland and Ladanyi, 2004), foam glass (~0.04 W/m°C) or sufficient air space (air: ~0.02 W/m°C). Examples of insulation placement are given in VTT (1987).
Systems capable of extracting heat from the ground without additional energy can be incorporated into the design. Thermosyphons, thermoprobes or air-duct cooling systems offer such a possibility (McKenna and Biggar, 1998; Smith et al., 1991; Wen et al., 2005), however, they have mainly been installed in Northern regions and their efficiency in alpine environments is still to be tested. Passive cooling using gravity-driven air convection is an additional possibility (Fig. 19). Due to the difference in air density between warm and cold air, cold air sinks to the bottom of the coarse-grained layer, resulting in a cooling effect (Arenson et al., 2006; Goering, 1998; Goering et al., 2000; Ma et al., 2006). The main
Designs that involve active cooling should be considered as the last option. Similar to artificial ground freezing (Harris, 1995), heat is actively extracted from the ground in order to cool it and keep the temperatures below freezing point.
Significant amounts of cooling energy may be required, depending on the project. Convective heat fluxes from water and air must be considered and the design has to include redundancies as well as emergency scenarios and plans in the event of failure of the system. Active cooling may be a temporary option during construction or to accelerate cooling after thermal disturbances caused by construction activities.
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