Resilient modulus of unsaturated subgrade soil: experimental and theoretical investigations

Abstract

The resilient modulus, M R , of subgrade soil is an important stiffness parameter for analysing fatigue cracking in either the asphalt or concrete layer of a pavement. Although subgrade soil is often unsaturated and subject to seasonal variations of moisture content and hence suction in the field, effects of soil suction on the resilient modulus are generally not accounted for in existing testing methods. In this study, M R values of a subgrade soil under various stress and suction conditions were investigated using a suction-controlled cyclic triaxial apparatus. To enhance the accuracy of measurements, Hall-effect transducers were employed to monitor the local axial and radial deformation of each specimen. It was found that M R increases with number of load applications when a soil contracts, but decreases slightly when a soil dilates. When suction increases, the soil response tends to change from contractive to dilative due to suction-induced dilatancy. Moreover, the measured M R is highly dependent on the stress state. It decreases with cyclic stress due to the nonlinearity of the soil stress-strain behaviour, but increases significantly with suction due to the presence of water tension. At the same stress and suction conditions, M R measured along the wetting path is generally larger than that measured along the drying path. A new semi-empirical equation representing the stress-dependency of M R is proposed and was verified using experimental results of four different soils.Résumé : Le module de résilience, M R , du sol de fondation est un paramètre important de la rigidité pour l'analyse des fissures de fatigue dans l'asphalte ou la couche de béton d'une chaussée. Même si le sol de fondation sur le terrain est souvent non saturé et soumis aux variations saisonnières de teneur en eau, et conséquemment de succion, les effets de la succion du sol sur le module de résilience ne sont généralement pas considérés dans les méthodes d'essais existantes. Dans cette étude, les M R d'un sol de fondation soumis à différentes conditions de contraintes et succion ont été étudiés à l'aide d'un appareil triaxial cyclique contrôlé par la succion. Pour améliorer la précision des mesures, des capteurs à effet de Hall ont été utilisés pour suivre les déformations axiales et radiales de chaque échantillon. Il a été observé que M R augmente avec le nombre d'applications de charges lorsque le sol se contracte, mais diminue légèrement lorsque le sol se dilate. Quand la succion augmente, le comportement du sol tend à changer de contractif à dilatant en raison de la dilatation induite par la succion. De plus, le M R mesuré est fortement dépendant de l'état des contraintes. Il diminue avec les contraintes cycliques en raison de la non-linéarité du comportement en contrainte-déformation du sol, mais augmente significativement avec la succion à cause de la présence de la tension de l'eau. Pour les mêmes conditions de contrainte et succion, le M R mesuré en mouillage est généralement plus grand que celui mesuré en séchage. Une nouvelle éq...

Predicting the Variation of Resilient Modulus with Respect to Suction Using the Soil-Water Characteristic Curve as a Tool

“…1 (i.e., MEPDG 2004) and semi-empirical model Eq. 6 (i.e., Ng et al 2013) are chosen to provide comparisons between the predicted and measured M R values of the fine-grained soil (i.e., Oklahoma clayey soil). Eq.…”

“…Eq. 6 was developed using extensive test results on a decomposed tuff from Hong Kong which is A-7-6 soil according to the AASHTO M-145 soil classification system (Ng et al 2013). Khoury and Zaman (2004) and the M R predicted using Eqs.…”

“…Numerous experimental investigations have shown that the seasonal moisture variations and the resultant suction fluctuations have a significant influence on the M R (e.g., Khoury and Zaman 2004, Yang et al 2008, Gupta et al 2007). Many investigators have proposed various approaches for predicting the M R taking account of the influence of suction or moisture content (e.g., Yang et al 2005, Cary and Zapata 2011, Sahin et al 2013, Ng et al 2013.…”

“…The first one is a widely used empirical relationship proposed by Mechanistic-Empirical Pavement Design Guide (2004) for both coarse-and fine-grained soils. The second one is a semi-empirical model proposed by Ng et al (2013) that is based on comprehensive experimental investigations. The strengths and limitations of the two selected models are discussed.…”

“…However, the relationship between the M R and moisture content is highly soil-type dependent and is difficult to quantify (Ng et al 2013). In addition, several factors or parameters such as the soil structure or fabric, compaction effort, climate and groundwater table fluctuation also influence the M R .…”