Farhad Salour Doctoral Thesis

2.3.
 
Pavement behaviour in cold regions 
In cold regions, frost action is one of the main sources of excess moisture presence in 
pavement structures. Frost penetration in unbound materials and subgrade soils can 
result in frost heave in the pavement structure and accumulation of water in the 
pavement structure in the form of ice lenses which in turn cause weakening during the 
spring-thaw (Hermansson and Guthrie, 2005). There are two conditions required for 
frost action to occur in a pavement structure: a sufficiently low temperature for 
sufficient length of time that can result in a phase change of pore water and the 
presence of moisture and free transfer of the moisture to the freezing front (Coussy, 
2005; Doré and Zubeck, 2008; Hermansson et al., 2009). 
All the general sources of moisture presence in pavement structures, mentioned earlier, 
can contribute to supplying frost action in pavements. These two conditions of 
temperature and moisture presence are generally fulfilled in pavements in cold regions. 
However, the severity and extent of the frost action can vary depending on the 
geographical position and the climate of the region. 

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Frost action mechanism 
Frost progression in the pavement structure disrupts the thermodynamic equilibrium of 
the pavement system, thus introducing a thermal gradient. This leads to a series of 
complex processes that includes the coupled effects of thermal and chemical potential 
at soil particle-water-ice interfaces that act alongside the soil particle mechanical contact 
forces (Henry, 2000; Doré and Zubeck, 2008). When frost penetrates the pavement 
structure, the interstitial moisture in the soil mass begins to convert into ice crystals, 
thus reducing the unfrozen moisture content in the pores. The latent heat of the pore 
water crystallization causes the temperature to slightly increase near the freezing point 
before it begins to decrease again, which results in the freezing of most of the remaining 
free interstitial moisture. At this point, only the hygroscopic moisture (the water held 
tightly on the surface of soil colloidal particle) remains unfrozen. This complex 
procedure in the pavement system creates three distinct zones along the pavement 
profile: the bottom part of the pavement profile remains at temperatures above the 
freezing point and in a completely unfrozen moisture phase; the transitional zone (with 
partly frozen interstitial water) in which the free moisture and the ice crystals coexist 
with a proportion profile that is governed by the temperature profile; and finally, the 
upper part of the pavement profile, where all the water except for the hygroscopic 
moisture is in a completely frozen state (Doré and Zubeck, 2008; Hermansson et al., 
2009). The intermediate zone, which can be up to several decimetres thick, plays a 
significant role in attracting excess moisture into the pavement structure. The moisture 
attraction mechanism is described in the following paragraphs. 
Formation of ice crystals in soil mass pores exerts pressure on the remaining unfrozen 
moisture. Since ice crystals occupy more space than the liquid moisture, they create 
smaller radii soil particle-water-ice interfaces. The contractile skin which is the water 
film that is in contact with the ice crystals works as a membrane in tension that resists 
the ice crystal pressures. This generates a negative pressure in the interstitial moisture 
which results in higher suction in the soil mass. The suction gradient between the top 
and the bottom of the intermediate zone creates a condition in which moisture flows 
upward towards eventual ice lenses along this zone. The upward flow of the moisture in 
the intermediate zone occurs as long as the partly frozen zone remains permeable. The 
permeability of the intermediate zone usually varies from unfrozen soil permeability at 
the bottom of the pavement profile to the eventually impermeable zone at the top of 
the pavement where pores are filled with water crystals with no free moisture paths. 
Since the free moisture passage is not fulfilled in the system due to the impermeability 
of the soil at the top of the pavement, and eventually the negative pressure gradient 
exceeds the overburden pressure, soil particles separate and ice lenses grow so that the 
moisture from the underlying intermediate zone can be extracted. The moisture 
mitigation to the growing ice lenses continues at the frozen fringe, resulting in 
expansion of the ice lenses. 

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At the beginning of the process, the ice lenses form at shallow depths in a frozen fringe 
with a sharp temperature gradient. Therefore, heat is extracted rapidly, the system cools 
and the hydraulic conductivity at the frozen fringe is rapidly reduced. The freezing front 
develops downward and another ice lens forms at a location where the negative 
pressure and the overburden pressure become equal. Both the thermal gradient and the 
cooling rate of the frozen fringe are reduced with depth, which results in slower and 
longer ice lens growth and therefore thicker and more widely-spaced ice lenses. This 
procedure continues as far as the heat is effectively extracted from the frozen fringe 
(Doré and Zubeck, 2008). The mechanism of formation of ice lenses in freezing 
pavement structures is illustrated in Figure 8.