Farhad Salour Doctoral Thesis

Figure 26.
Layer moduli backcalculation from multilevel FWD loads for granular 
stiffness and their respective moisture content measurement. 
Figure 27.
Layer moduli backcalculation from multilevel FWD loads for subgrade 
stiffness and their respective moisture content measurement.
 

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42 
In all the FWD performed and regardless of the moisture content, higher stiffness for 
the granular material was measured for increased impact load levels, demonstrating the 
stress hardening behaviour of the material. However, for the subgrade material the 
response to the multilevel loads depended on the moisture state of the material. For all 
the FWD measurements conducted during the draining condition in which the material 
was in an unsaturated state, the subgrade exhibited stress softening behaviour. Lower 
stiffness values were measured with an increase in the FWD load level. Similar softening 
behaviour is usually observed in triaxial tests in fine-grained material. 
During the period in which the drainage was clogged, the subgrade was in a fully 
saturated condition and the backcalculated stiffness was constant regardless of the 
FWD load level. This can be clarified by the known behaviour of fine-grained soils in 
an undrained condition. In saturated undrained states, the effective confinement 
pressure remains constant since an increase in the confinement stress is counteracted by 
the pore-water pressure increase. 
Main findings in Paper II 

From the summer measurements on the spring-thaw-recovered structure, it was 
observed that the moisture condition of the unbound layers was to a great 
extent dependent on the groundwater level at the pavement section. The role of 
the drainage system in reducing the moisture content of the unbound layers was 
significant. 

Similar moisture-stiffness behaviour was also observed along with manipulation 
of the drainage system and groundwater level. 

From backcalculation of the FWD measurements with multilevel loads, the 
granular layer exhibited a stress-hardening response and higher impact loads 
resulted in higher material stiffness. 

The silty-sand subgrade also exhibited in situ stress dependent behaviour. A 
stress-softening response was observed when the subgrade was in an 
unsaturated state. However, in saturated conditions, the subgrade showed no or 
negligible stress dependency and identical backcalculated stiffness was obtained 
regardless of the impact load level.