Mechanical Characterisation of Unsaturated Sandy Subgrades Based on Suction-Controlled Repeated Load Triaxial Testing
Time: Fri 2025-11-07 14.00
Location: public video conference, Kollegiesalen, Brinellvägen 8, Campus
Video link: https://kth-se.zoom.us/j/69642823274
Language: English
Subject area: Civil and Architectural Engineering, Building Materials
Doctoral student: José Everton , Byggnadsmaterial
Opponent: Associate Professor Jean-Pascal Bilodeau, Université Laval, Quebec City, Canada
Supervisor: Adjunct Professor Sigurdur Erlingsson, Byggnadsmaterial
QC 20251014
Abstract
Flexible pavements constitute the majority of the Swedish and global road networks. In Sweden, the asphalt bound layers are typically thin, making the pavement structure sensitive to seasonal variations that influence its mechanical response—from the asphalt bound layers to the unbound granular layers and the subgrade. The present research focuses on subgrade soils, which exhibit moisture-sensitive and stress-dependent behaviour, in some sense similar to the behaviour of the unbound granular layers. However, unlike the latter materials, the mechanical response of fine-grained soils is primarily controlled by matric suction, which binds particles together.
The objective of this study was to characterise how variations in moisture/suction and stress state affect the mechanical response of subgrade soils, and to develop predictive models that accurately represent these effects. Three sandy soils from southern Sweden—classified as a poorly graded sand and two silty sands (with fines ranging from 4–39%)—were tested under suction-controlled Repeated Load Triaxial (RLT) tests.
The research integrates two complementary aspects of subgrade performance: resilient behaviour and permanent deformation. The resilient modulus (MR) results demonstrated that increased matric suction (lower moisture) leads to stiffer response, while the influence of deviatoric stress varied with soil type. Suction- and effective-stress-based models provided improved MR predictions, particularly for the finer soils. The permanent deformation (PD) analysis revealed that low matric suction (higher moisture content) significantly increased plastic strains, with silty sands showing greater resistance to deformation than the poorly graded sand across a range of possible moisture conditions.
For modelling the PD behaviour, the study linked the resilient and plastic responses under different stress conditions in a suction-controlled environment, providing a unified understanding of subgrade behaviour across seasonal moisture variations. A model was proposed to predict PD under unsaturated and near-saturation states, complementing the MR-predictive models and enhancing the mechanistic modelling of unsaturated sandy subgrades in pavement design applications.