Farhad Salour Doctoral Thesis

Figure 19.
 
Illustration of frost depth and thaw penetration in the pavement structure in 
2010. 
Figure 20. 
FWD deflection basin during thawing (Left) and recovery periods (Right) 
corresponding to 50 kN impact load. 
The surface deflection data were later used to backcalculate the stiffness of the 
unbound layers with respect to thaw penetration in the pavement structure. Figure 21 
represents the backcalculated layer stiffness together with the volumetric moisture 
measurements for the subbase granular layer and the subgrade. This figure shows the 
considerable effect of the moisture content on the resilient stiffness of the unbound 
pavement materials. Using the unbound materials in situ moduli and their respective 
measured moisture content, a 
M
R
-Moisture model was presented (Figure 22). The 
model data fell on a unique curve and exhibited promising agreement with the 
predictive
M
R
-moisture model (given in Equation 8) that is used in MEPDG (ARA, 
2004) for considering the moisture effects in unbound pavement materials. 

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38 
Figure 21.
 
Backcalculated layer stiffness versus volumetric moisture content data for 
granular layer (left) and subgrade material (right). 
Figure 22. 
Backcalculated stiffness and field moisture data for granular layer (left) and 
subgrade material (right) versus the MEPDG predictive model. 
Main findings in Paper I 

Even though the test site was equipped with a deep drainage system, 
considerable moisture content variation was observed in both in the granular 
layer and the subgrade. 

A clear correlation was observed between the thaw penetration and the 
measured volumetric moisture content in the unbound layers during the spring-
thaw period. The highest annual moisture content in the subgrade was also 
measured during the spring-thaw period. 

The overall stiffness of the pavement structure was considerably affected by the 
spring-thaw effect. The subgrade exhibited greater sensitivity to moisture 
content variation during the spring-thaw compared to the unbound granular 
layer. 

Backcalculation of the FWD data showed evident correlation between the 
measured moisture content and the unbound layer and subgrade stiffness. 

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39 

During the spring-thaw, the stiffness of the subgrade was reduced by 63% while 
this was 48% for the granular layer. 

The backcalculated unbound layer and subgrade stiffness and their measured 
corresponding degree of saturation fell on a unique curve that was in agreement 
with the model proposed in MEPDG. 
In