Farhad Salour Doctoral Thesis

8.
 
Summary and Conclusions
In cold regions, climatic factors can significantly affect the mechanical behaviour of 
unbound materials which can influence the overall performance of a pavement. 
Climate-related pavement deterioration imposes considerable maintenance and 
rehabilitation efforts on road authorities as well as on public expenditure. In pavement 
unbound materials, moisture content and temperature (sub-zero temperature) are the 
two main climatic parameters influencing the mechanical behaviour of unbound layers. 
Environmental factors, their interaction with pavements and their influence on material 
behaviour are highly complex in nature. Despite the considerable number of studies 
that have been conducted during past years, fundamental understating of material 
behaviour and modelling with respect to environmental factor effects is still lacking. 
Many studies on stress and moisture dependency of unbound materials are conducted 
through laboratory experiments which may not fully represent the field conditions. The 
research work presented in the first part of this thesis intended to provide more insight 
into pavement in situ environmental factors, unbound material stiffness behaviour and 
their interaction through field studies. For the field investigation, the Torpsbruk test site 
was instrumented with moisture, temperature and groundwater level probes and the 
mechanical behaviour of the pavement was assessed using an FWD device with 
multilevel impact loads. The pavement environment and structural behaviour were 
assessed during a spring-thaw period as well as a case study in which the groundwater 
level at the test site was changed by manipulating the drainage system. 
The second part of this thesis was mainly based on laboratory investigations through 
RLT tests using a modified triaxial chamber that allowed for pore suctions 
measurements throughout the test. The laboratory investigation of moisture effects on 
pavement unbound materials were based on developing an enhanced approach in 
predicting the stiffness of the material using principles of soil mechanics for partially 
saturated conditions and incorporating soil suction in the resilient modulus constitutive 
models. 
The permanent deformation properties of the subgrade materials and their sensitivity to 
moisture content variation have been less studied despite their importance in formation 
of rut depths in thin flexible pavements. The study conducted here showed the 
feasibility of using the multistage RLT test for evaluating the permanent deformation 
properties of fine-grained subgrades. Furthermore, predictive models that were based 
on multistage RLT tests and the time-hardening approach were developed. This can 
significantly reduce the time and effort required for conducting permanent deformation 
tests. 
The work summarized in this thesis covers different aspects of moisture effects on 
pavement unbound materials, using both field and laboratory studies. The main findings 

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from each individual study are given in the summary of the respective appended papers. 
In brief, some of the major finding of the studies can be summarized as the following. 
From both the field measurements (spring-thaw and groundwater level variations) and 
the laboratory-based studies it was observed that the moisture content has a significant 
effect on the mechanical response of the unbound materials and subgrade. In general, 
as moisture content increased, the stiffness of unbound materials and their resistance to 
withstand accumulation of permanent deformation decreased. The field measurements 
at the instrumented test section in Torpsbruk indicated the feasibility and potential of 
using in situ measurements to better understand the seasonal environmental effects on 
pavement performance. Even though field studies and instrumentation can be quite 
challenging and costly, the outcome of these studies can be very valuable and of great 
interest for developing response models (i.e. moisture-stiffness models). Field 
measurements can provide in-depth information about the stress and moisture 
dependency of the material that is so far mainly studied in the laboratory and may not 
fully represent in situ conditions. 
Concerning seasonal variation in the resilient modulus due to moisture changes, it was 
observed from the laboratory study (using RLT tests on two silty sand subgrades) that 
matric suction could be used and incorporated into the prediction models. Matric 
suction is the state variable with highest relevance to unsaturated soil mechanics and is 
highly dependent on the soil structure moisture content. Since moisture content is the 
main environmentally driven factor affecting the behaviour of unbound layers with a 
high fines content and subgrade soils, matric suction can be incorporated into the 
resilient modulus models. 
Regarding the permanent deformation study, the multistage RLT tests and modelling 
procedure based on the time-hardening approach were selected. This allowed for more 
comprehensive study of the material stress dependency compared to the conventional 
single stage RLT procedures. The multistage RLT test procedure enabled testing the 
material under a wider range of stresses with considerably less laboratory effort and 
time. This study also showed the significance of moisture content on the permanent 
deformation behaviour of the two different silty sand subgrade materials that were 
tested.