The Influence of Water Flow on Ground Freezing

#1400, 633 - 6th Avenue SW Calgary, Alber ta, Canada T2P 2Y5 Tel: +1 403.269.2002 Fax: +1 403.266.4851 Email: [email protected] Website: www.geo-slope.com Cold air Radiator surface area removes heat Heat flow Vapour Liquid The Influence of Water Flow on Ground Freezing Considering heat transfer processes in soils can be important to many engineering problems. In northern climates where there is permanently or seasonally frozen ground, the presence of engineering projects can have both positive and negative impacts. Potential damage to permafrost can be a serious problem, but it can sometimes be reversed or prevented using heat pipe (thermosyphon) technology. A schematic representation of a thermosyphon is illustrated on the right. In other situations, ground may need to be artificially frozen in order to ensure the success of the engineering project. Artificial ground freezing can be induced by using refrigeration equipment to circulate cold brine through pipes to freeze barrier walls around proposed mine shafts, subway tunnels or building foundation excavations. The photograph shown highlights a series of ground freezing pipes filled with brine that were installed around the perimeter of an underground ore body. Freezing the perimeter was necessary in order to control high water pressures that would have hampered conventional mining techniques. In any ground freezing scenario, the presence of flowing water can significantly delay or even prevent the development of ice due to heat addition by the moving water. While the TEMP/W finite element model has been an industry standard for ground thermal analysis for many years, it has only recently been coupled with the SEEP/W model so that the heat flow that occurs with moving water can be appropriately considered. To illustrate the importance that moving water has on heat flow, consider the following hypothetical example which is shown in plan view. The figure on the left shows the temperature contours and heat flow vectors between two adjacent freeze pipes. The location of the phase change temperature isotherm (i.e., freeze-thaw line) is indicated by the blue contour line. The figure on the right shows the corresponding water flow vectors as the water passes through the increasingly narrow unfrozen gap between the two pipes. Note that as the water moves through the smaller cross-sectional area it has to increase in velocity. If the velocity increases too much,

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The Influence of Water Flow on Ground Freezing