Farhad Salour Doctoral Thesis

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Moisture Influence on Structural 
Behaviour of Pavements 
Field and Laboratory Investigations 
 
 
 
Farhad Salour 
Doctoral Thesis 
 
 
KTH, Royal Institute of Technology 
School of Architecture and the Built Environment 
Department of Transport Science 
Division of Highway and Railway Engineering 
Stockholm 2015 

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Doctoral dissertation to be defended in room F3, Lindstedtsvägen 26, Royal Institute of 
Technology (KTH), Stockholm, Sweden, on 10
th
of April 2015, at 10:00. 
Faculty opponent: 
Prof. Guy Doré 
Evaluation committee members: 
Prof. Leena Korkiala-Tanttu 
Prof. Sven Knutsson 
Assoc. Prof. Staffan Hintze 
© Farhad Salour 
Doctoral Thesis (2015) 
Division of Highway and Railway Engineering 
School of Architecture and the Built Environment 
KTH Royal Institute of Technology 
SE-100 44 Stockholm 
SWEDEN 
TRITA-TSC-PHD 15-003 
ISBN 978-91-87353-67-3

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i 
Abstract 
The structural behaviour of pavements in cold regions can considerably be affected by 
seasonal variation in environmental factors such as temperature and moisture content. 
Along with the destructive effect of heavy traffic loads, climatic and environmental factors 
can considerably contribute to pavement deterioration. These factors can influence the 
structural and functional capacity of the pavement structures which, as a result, can trigger 
and accelerate pavement deterioration mechanisms. Studies on the influence of variation 
of the environmental factors on the response and behaviour of pavement materials have 
shown that proper consideration to these factors must be given in realistic pavement 
design and analysis. 
In flexible pavement structures, particularly with a thin hot mix asphalt (HMA) layer, 
unbound materials and subgrade soil largely contribute to the overall structural behaviour 
of the pavement system. In unbound materials, moisture content and its variation can 
significantly affect pavement layer stiffness and permanent deformation characteristics. 
Therefore, the moisture condition of pavements and its influence on the mechanical 
behaviour of pavement materials has been of interest among the pavement research 
community. A proper understanding of moisture transformation in pavement systems and 
its effects on pavement performance are important for mechanistic pavement design. 
The present summary of this doctoral thesis is based on four main parts. The first part of 
the thesis covers field measurements and findings from a test section along county road 
126 in southern Sweden and consists of two journal papers (paper I and II) tackling 
different aspects of the research topic. This test section is located in a relatively wet 
ground condition and consists of a thin flexible pavement structure with a deep drainage 
system. It is instrumented with subsurface temperature, volumetric moisture content and 
groundwater probes. The mechanical response of the pavement structure was investigated 
using Falling Weight Deflectometer (FWD) measurements. The second part of the thesis 
(paper III and IV) are based on laboratory experiments and investigates different recent 
approaches that have been proposed to apply principles of unsaturated soil mechanics for 
incorporating seasonal variation of moisture content into the resilient modulus models 
using matric suction. The third part of the thesis (paper V) builds a bridge that spans 
between the laboratory and field investigations with an attempt to evaluate one of the 
predictive models presented in Paper III. The fourth part of the thesis (paper VI) mainly 
focuses on the laboratory-based investigation of the permanent deformation characteristic 
of subgrade soils. In this part, the permanent deformation characteristics of two different 
silty sand subgrade soils were investigated and modelled using the data obtained from 
repeated load traxial tests. 
Paper I mainly focuses on the spring-thaw weakening of the pavement structure. The 
environmental data collected using different sensors and the FWD tests were used to 

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ii 
investigate variations in moisture content with thaw penetration and its influence on the 
stiffness of unbound layers and the pavement’s overall bearing capacity. Using the 
backcalculated layer stiffness and corresponding in situ moisture measurements in the 
unbound layers, a degree of saturation-based moisture-stiffness model was developed for 
the granular material and the subgrade. 
In Paper II, the drainage system of the structure was manually clogged during a three 
month period in summer to raise the groundwater level and increase the moisture content 
of the layers. Along with the subsurface groundwater level and moisture content 
monitoring, the structural response of the pavement was studied. In this research work, 
the FWD tests were conducted at three different load levels. The stress dependent 
behaviour of the unbound granular layer and the subgrade soil were further studied using 
the multilevel loads FWD test data. Additionally, parameters of a nonlinear 
stress-dependent stiffness model were backcalculated and their sensitivity to in situ 
moisture content was studied. 
In Paper III and IV, series of suction-controlled repeated load triaxial (RLT) tests were 
conducted on two silty sand (SM) subgrade materials. Several resilient modulus prediction 
models that account for seasonal moisture content variation through matric suction were 
summarized and after optimizing the model parameters, the capability of the prediction 
models in capturing the material response were evaluated. 
In Paper V, an attempt was made to evaluate the proficiency of one of the suction-resilient 
modulus models using the field moisture content and FWD measurements from the 
Torpsbruk test site. The backcalculated subgrade stiffness dataset at different moisture 
contents were compared with resilient modulus models obtained from the suction-resilient 
modulus predictive model. 
Paper VI presents an evaluation of several permanent deformation models for unbound 
pavement materials that incorporate the time-hardening concept using a series of 
multistage repeated load triaxial (RLT) tests conducted on silty sand subgrade materials. 
The permanent deformation tests were conducted at four different moisture contents with 
pore suctions measurement throughout the test. The effect of moisture content (matric 
suction) on the permanent deformation characteristics of the materials and the predictive 
model parameters were further investigated.