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Evaluating a Model for Seasonal Variation of Silty Sand Subgrade Resilient Modulus with FWD Tests
KTH, School of Architecture and the Built Environment (ABE), Transport Science, Highway and Railway Engineering. Swedish National Road and Transport Research Institute, VTI, Sweden .
University of Iceland.
Arizona State University.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The stiffness of unbound pavement material is one of the main input parameters in mechanisticdesign and analysis of pavement systems. This material property is usually moisture dependentand therefore, most of the unbound pavement layers exhibit seasonal variation in stiffness as thepavement moisture regime changes throughout the year. Therefore, this variation should be takeninto account in any realistic pavement design. In unbound materials with high fine content,change in moisture content can result in change in the stress state due to suction effects. In thisstudy, an enhanced predictive resilient modulus model that accounts for seasonal variation bymeans of suction measurement is presented. A silty sand subgrade was tested using a modifiedRepeated Load Triaxial (RTL) system under different moisture (suction) conditions and a set ofresilient modulus model regression parameters were determined. The capability of the model tocapture seasonal moisture variation effects was further evaluated using field data. A series ofFalling Weight Deflectometer (FWD) tests with multi-level loads were conducted on aninstrumented pavement structure where the moisture content of the subgrade was changed bymanipulating the pavement drainage condition. The resilient moduli obtained from the modelwere compared to the backcalculated stiffness data obtained from FWD tests conducted atdifferent moisture conditions. Overall, a good agreement was observed between thelaboratory-based resilient modulus and the backcalculated stiffness. The resilientmodulus-suction model could efficiently capture the moisture content effects.

Keyword [en]
Resilient modulus, backcalculation, falling weight deflectometer, seasonal variation, moisture content, silty subgrade
National Category
Infrastructure Engineering
URN: urn:nbn:se:kth:diva-162084OAI: diva2:796852

QS 2015

Available from: 2015-03-20 Created: 2015-03-20 Last updated: 2015-03-24Bibliographically approved
In thesis
1. Moisture Influence on Structural Behaviour of Pavements: Field and Laboratory Investigations
Open this publication in new window or tab >>Moisture Influence on Structural Behaviour of Pavements: Field and Laboratory Investigations
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. xiv, 61 p.
Falling Weight Deflectometer (FWD), backcalculation, unbound material, subgrade, seasonal variation, moisture content, spring-thaw, drainage, pavement stiffness, field test, pavement instrumentation, resilient modulus, permanent deformation, repeated load triaxial test, unsaturated soil, matric suction.
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering; Transport Science
urn:nbn:se:kth:diva-162076 (URN)978-91-87353-67-3 (ISBN)
Public defence
2015-04-10, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)

QC 20150324

Available from: 2015-03-24 Created: 2015-03-20 Last updated: 2015-03-24Bibliographically approved

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