Farhad Salour Doctoral Thesis

4.2.
 
Seasonal variation in thin flexible pavements structural stiffness 
Most of the thin flexible pavement structures in cold regions experience considerable 
seasonal variations in stiffness (Simonsen and Isacsson, 1999). The conceptual overall 
seasonal variation in bearing capacity of the pavements in cold regions includes a 
significant increase in stiffness during the winter period. Besides the stiffness variation 
of the asphalt concrete layer due to seasonal temperature changes, pavement unbound 
materials also exhibit considerable seasonal variations due to the freezing and thawing 
effects (Saarenketo and Aho, 2005). 
Frost penetration in the pavement structures creates strong bonds between the 
unbound particles as the moisture surrounding the particles freezes. This results in a 
very stiff matrix of granular material and subgrade soil particles (Simonsen et al., 2002). 
Therefore, during the frost period, pavements exhibit high overall stiffness and can 
usually support heavy traffic axle loads without any problem. This increase in the overall 
stiffness of the pavement system due to frost penetration depends on many factors such 
as the depth of the frost zone, material properties and the pre-freezing moisture 
content. 
The frost depth and length of the frost period depends on the geographical position 
and the climate of the road section. This condition usually lasts throughout the winter 
period. However, short thawing periods can occur if the air temperature rises above 
zero degrees Celsius (0°C). The surface intermittent thawing can penetrate up to a few 
decimetres in depth and cause a temporary decrease in the unbound granular layer 
stiffness. The length of the surface thawing can vary from a few hours up to a few days, 
depending on the weather condition. As the air temperature drops to below freezing 
temperature, the surface thaws refreeze and the layers regain their high stiffness. 

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The major thaw weakening phase, often referred as structural thaw weakening, occurs 
when thawing penetrates deeper into the pavement structure or in the subgrade soil. 
During this period all the segregation ice lenses that were generated during the freezing 
period convert back into the liquid form as the temperature deeper in the pavement 
structure rises to above freezing values. A considerable amount of excess moisture is 
usually released in the pavement structure. Since thawing is a top-bottom process, the 
moisture content of the thawed sections above the thawing ice lenses can be 
significantly high since draining paths are trapped (Simonsen and Isacsson, 1999; 
Saarenketo and Aho, 2005). As this section is surrounded by the asphalt concrete layer 
at the top and the thawing ice lenses at the bottom, hydrostatic pore pressures can be 
built up due to heavy traffic load passages. This can cause pumping of the fines of the 
material as moisture flows to the sides and therefore loss of support. 
The spring-thaw period is usually followed by a “recovery” period during which the 
bearing capacity of the pavement system is recovered. In granular base and subbase 
layers, the stiffness of the material is usually regained shortly after the spring-thaw 
period is over as the excess moisture drains and the layer recovers. This is due to the 
relatively high permeability in the granular materials. However, in subgrade soils, the 
recovery period usually takes place over a much longer time, especially if the fines 
content of the material is high (e.g. moraines, silty and clayey materials). Full recovery of 
subgrade soils can take from a few weeks up to a few months, depending on the 
material permeability and the drainage condition of the pavement structure. Figure 10 
illustrates the principal changes in overall stiffness of pavements in cold regions.