Farhad Salour Doctoral Thesis

7.2.
 
Paper III and IV 
The effects of moisture content and suction on the resilient modulus of two silty sand 
subgrade soils (Luleå and Torpsbruk subgrades) were investigated using a custom-built 
triaxial cell that was adopted for RLT testing for unsaturated soils. The system was 
capable of individually controlling/measuring the pore-air and pore-pressure (and 
therefore matric suction,
m

) of the specimen during the RLT tests using high precision 

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pressure/volume controllers (PVC) and the soil suction measuring device. Schematic 
diagram of the RLT test apparatus is shown in Figure 28. 
Figure 28. 
Schematic diagram of the suction-controlled RLT test apparatus. 
For both of the subgrade soils, the resilient modulus tests were conducted at a wide 
range of suction (moisture content). From the RLT tests data, it was observed that the 
resilient modulus of subgrades were highly dependent on the net bulk stress, deviator 
stress and the matric suction (Figure 29). 
Figure 29. 
Resilient modulus of the Luleå subgrade soil at different suction states 
(degrees of saturation). 

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A literature survey was conducted and several recent suction-incorporated resilient 
modulus models were summarized (Parreira and Goncalves, 2000; Khoury and Zaman, 
2004; Yang et al., 2005; Liang et al., 2008; Cary and Zapata, 2011; Nowamooz et al., 
2011; Ng et al., 2013). 
The dataset from the suction-controlled resilient tests were then used to calibrate 
several recent predictive models that account for matric suction of the soil and evaluate 
the capability of them in capturing the resilient response of the material under different 
load and moisture conditions. 
Main findings in Paper III and IV 

The resilient modulus of the subgrades increased with increasing net bulk stress 
and decreased with increasing deviator stress (octahedral shear stress). 

Resilient modulus increased significantly with increasing matric suction. 

Change in the resilient modulus with varying net bulk and deviator stress was 
generally more significant at higher suction values (the degree of nonlinearity of 
the material stress-strain relationship was more significant at higher suction 
levels). 

The resilient modulus predictive models that combined the three fundamental 
stress state variables (net confining pressure, deviator stress and matric suction) 
performed more satisfactorily in capturing the resilient response of silty sand 
subgrade soils and their variations due to seasonal changes in the moisture 
content. 

Given the goodness of fit of the suction-resilient predictive models, it can be 
concluded that considering matric suction as an independent stress state variable 
in the models might be a rational approach for incorporating moisture effects in 
the predictive models.