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Resilient modulus modelling of unsaturated subgrade soils: laboratory investigation of silty sand subgrade
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.
2015 (English)In: International Journal on Road Materials and Pavement Design, ISSN 1468-0629, Vol. 16, no 3, 553-568 p.Article in journal (Refereed) Published
Abstract [en]

In flexible pavement structures, stiffness of unbound granular layers and subgrade soil significantly contribute to the overall performance of the pavement system. The stiffness of pavement unbound materials is widely characterised by the resilient modulus, M-r, which is obtained from repeated load triaxial (RLT) tests. Although pavement unbound materials are usually in partially saturated conditions and experience seasonal moisture content and therefore suction variations in the field, their stiffness is conventionally characterised using the total stress approach in which the effect of soil suction is not taken into account. Thus, an enhanced approach in predicting the stiffness of subgrade soils has to account for the partially saturated conditions and incorporate soil suction (i.e. matric suction) in the M-r constitutive models. In this study, several M-r prediction models that take into account the effect of pore suction were investigated. The M-r data set from an experimental investigation of two silty sand subgrade soils that was conducted using a suction-controlled RLT testing apparatus was used to optimise the parameter of these models. The capability of the M-r predictive models in capturing the resilient modulus behaviour of the silty sand subgrade soils and its variation due to seasonal changes in the moisture content (soil suction) were evaluated. It was observed that the M-r models that combine the three fundamental stress state variables (confining stress, deviator stress and matric suction) performed better in capturing the resilient modulus behaviour of the subgrade materials.

Place, publisher, year, edition, pages
2015. Vol. 16, no 3, 553-568 p.
Keyword [en]
moisture content, seasonal variation, unsaturated subgrade, resilient modulus, matric suction, repeated load triaxial test
National Category
Infrastructure Engineering
URN: urn:nbn:se:kth:diva-162082DOI: 10.1080/14680629.2015.1021107ISI: 000355728600004ScopusID: 2-s2.0-84930753258OAI: diva2:796844

QC 20150626

Available from: 2015-03-20 Created: 2015-03-20 Last updated: 2015-06-26Bibliographically 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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