Farhad Salour Doctoral Thesis

7.3.
 
Paper V 
This paper spans a bridge between the field-based moisture measurements and FWD 
test data (Paper II) and the laboratory-based suction-controlled resilient modulus 
predictive model (Paper III). 
The multi-level loads FWD tests that were conducted on the instrumented test section 
in Torpsbruk were backcalculated using the ERAPave nonlinear multilayer-elastic 
computer program. Depending on the subgrade moisture content measurements at 
different depths at the FWD test dates, the subgrade layer was divided into different 
sublayers in the backcalculation model and the stiffness model parameters for each 
sublayer were backcalculated. 

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Furthermore, the resilient moduli of the Torpsbruk subgrade sublayers at their 
corresponding measured moisture contents (suctions) on the FWD dates were 
calculated using the laboratory-based suction-resilient model developed in Paper III. 
The backcalculated subgrade stiffness obtained for all of the FWD test dates and load 
levels were compared with the resilient modulus results obtained from the 
laboratory-based suction-resilient modulus model (Figure 30). 
Figure 30. 
Laboratory-based resilient modulus versus backcalculated moduli using 
nonlinear model in ERAPave. 
Both the field measurements and the laboratory tests confirmed the stress and moisture 
dependence of the subgrade material modulus. In general, good agreement was 
observed between the in situ backcalculated stiffness and the suction-model based 
moduli. 
Main findings in Paper V 

The subgrade sublayers degree of saturation varied between ~ 45 to ~100% 
during the field FWD tests. For this range of variation, the matric suction 
accounted for about 30% of the subgrade resilient modulus variation. 

The enhanced predictive suction-resilient modulus model in Paper III seemed 
to satisfactorily capture the moisture content variation effects on the resilient 
modulus. 

In general, the subgrade moduli calculated by the enhanced predictive 
suction-resilient modulus model and the backcalculated stiffness obtained from 
the FWD field data showed good agreement, concluding that the 
modulus-suction model could efficiently capture the seasonal variation effects.