Researchers interested in frost action in soils agree on the description illustrated in Fig. 4.3 on how ice lenses grow and cause frost heave. Nevertheless, when it
comes to the degree of water saturation of the unfrozen soil below the freezing front there are two different conceptions. Some researchers believe that the unfrozen soil is fully saturated while others believe it is unsaturated. Of course, these discrepancies in understanding lead to different explanations of the driving force of the capillary rise of water as well as different opinions on how to run laboratory experiments. Andersland and Ladanyi (2004), for example, Konrad and Morgenstern (1980) and Nixon (1991) give equations where it is obvious that full saturation is assumed for the unfrozen soil. Accordingly, Konrad (1990) refers to experiments where the specimen freezes from below and free access to water is permitted at the top. This, of course, gives full saturation of the unfrozen soil. In contrast to this view, Miller (1980) discusses frost heave as freezing of unsaturated soil and references experiments where water is fed in at the bottom of the specimen and ice lenses are fed through capillary rise in unsaturated soil. Accordingly, Penner (1957) freezes unsaturated soil and Hermansson and Guthrie (2005) present laboratory experiments where freezing and frost heave takes place at a height more than 0.5 m above the level of the water supply. It should also be noted that Hermansson and Guthrie (2005) describe testing where the specimen heaves significantly without addition of any external water at all. This, of course, contradicts the assumption that the soil below the freezing front is fully saturated. The expansion without addition of water is suggested to be an effect of air entering the soil.
In agreement with the laboratory experiments Hermansson (2004) described a field study where the depth to the groundwater table is 6 m. Under such a thoroughly drained condition, it is reasonable to assume that the soil is far from saturated. Despite this Hermansson reported 80 mm of frost heaving over a period of 2 months.
The conclusions from these studies are twofold,
• Firstly, frost heave does not require full saturation; and
• Secondly, even a well drained soil might experience a significant frost heave.
In addition to the different understandings about the importance of saturation, there are also two different schools when proposing equations to describe frost heave (Hansson, 2005). One school neglects the liquid water pressure and the other one neglects ice pressure. The first school, characterised by “Miller-type” models, develops models describing the frost heave on a microscopic scale while the second school, characterised by “hydrodynamic” models, handles equations for the redistribution of water up to the freezing front, supplying the frost heave. No computer code is known that handles both processes realistically.
Heat transfer in soils involves convection, radiation, vapour diffusion and conduction. For pavements, conduction is the most important factor. During warm and sunny summer days though, natural convection should not be neglected. The heat transfer is closely associated with water movements — evaporation pulls water through the soil to the evaporation surface. Freezing also drives water movement as water is drawn to the freezing front in soils which have moderate pore sizes and moderate permeability.
Frost susceptible soils always experience frost heave at freezing even if there is no saturation. Drains will lower the heave by reducing the water content but a frost susceptible soil will always hold enough water for a significant heave. Chapter 13 describes some drainage techniques that can help to address these problems. Interested readers are also directed to the reports on frost and drainage, mostly in the context of seasonally frost affected roads, available from the ROADEX project (Berntsen & Saarenketo, 2005; Saarenketo & Aho, 2005).
