The Arrangement of Grains

When soils are frozen, water migrates toward the freezing front, and in doing so creates concentrations, or "segregations" of water ice and the entrained soluble components (Ostroumov et al. 2001). In doing so, ice lenses can form, depending on the grain size of the material, moisture content and rate and direction of freezing. The 9% expansion of water on freezing can displace soil particles, creating cryo-turbation, frost heave, and the development of fissures and joints, particularly in silty clay soils (Eigenbrod 1996). On melting, ice lenses can transform into cavities, reducing the bulk density of the media and possibly creating macropores. The porosity of silty soil subjected to freeze-thaw cycling may increase with hydrocarbon content (White and Coutard 1999), but the void ratio of fine grained soils may also reduce under freeze-thaw (Chamberlain and Gow 1979). Small vertical cracks link together to form vertical polygons in clays (Chamberlain and Gow 1979), and it may be that these cracks increase permeability in the vertical direction. However, an additional mechanism has been proposed to account for permeability increases in soil materials which do not exhibit cracking, and that is rearrangement of the clay particles within the voids defined by the sand and silt grain boundaries. Prior to freezing, loose clays lie in the voids, but as a consequence of the effective stress imposed by freezing, the clay particles rearrange into a denser packing and possibly also align themselves to create a greater permeability (Chamberlain and Gow 1979). Fine-grained materials can exhibit an increase in permeability of several orders of magnitude following freeze-thaw cycling (Eigenbrod 1996).

It remains unclear whether or not these effects also occur in coarser PRB media. Initial tests in the laboratory (Gore, unpublished data) show that the water-saturated bulk density of two types of clinoptilolite zeolite does not change with up to 50 freeze-thaw cycles, but the bulk density of a granular activated carbon (PicaCarbâ„¢, Pica Inc.) tends to reduce with repeated reorganisation of the grains by flotation. No evidence of long-lived cracking or macropore development was observed in the sand-sized PRB media. However, it is possible that rearrangement of finer particles in the voids may occur following freezing, increasing permeability in some places but decreasing permeability in other places where finer particles enhance the formation of pore ice (Fourie et al. 2007). Alternatively, freeze-thaw shattering may increase the total amount of fine particles, leading to a reduced permeability. X-ray computed tomography, whereby hundreds of X-ray images are used to construct a three-dimensional rendering of the grains and pore spaces with micron-scale resolution, holds great promise for the understanding of the interaction of contaminants and fluid flow, grain behaviour and the development of cracking and segregation ice in permafrost areas (de Argandona et al. 1999; Torrance et al. 2008). Further studies of the hydraulics of contaminated soils and PRB materials are crucial to understanding the behaviour of PRB in areas of freezing ground.

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