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ARTICLE Modelling resilient modulus seasonal variation of silty sand subgrade s

ARTICLE Modelling resilient modulus seasonal variation of silty sand subgrade soils with matric suction control1 Farhad Salour, Sigurdur Erlingsson, and Claudia E. Zapata Abstract: The resilient modulus of unbound materials is an important parameter in the mechanistic design of pavements. Although unbound layers are frequently in a partially saturated state, a total stress approach is conventionally used in modeling the material behaviour, and therefore pore pressure effects are not considered. In ﬁne-grained unbound materials, the satura- tion state can affect their mechanical behaviour due to pore pressure effects. In this study a modiﬁed test procedure and a predictive resilient modulus model that takes into account the subgrade soil matric suction as a stress state variable is presented. Two different silty sand subgrade materials were tested in unsaturated conditions using a series of repeated load triaxial tests under controlled pore suction conditions to study its inﬂuence on the resilient modulus. The test data were further used to obtain the resilient modulus model regression parameters that account for moisture content variations through the matric suction parameter. Generally, the prediction model could effectively capture the resilient modulus behaviour of the subgrades with respect to changes in the normal stress state and the matric suction. Given the completeness of this method, this prediction model is recommended as an improved approach in capturing the moisture content effects on the material stiffness properties. Key words: subgrade, resilient modulus, unsaturated soil, matric suction, environmental effects, moisture content. Résumé : Le module de résilience de matériaux non reliés est un paramètre important dans la conception mécanistique des chaussées. Même si les couches non reliées sont fréquemment dans un état partiellement saturé, une approche de contrainte totale est généralement utilisée pour la modélisation du comportement des matériaux, donc les effets des pressions intersti- tielles ne sont pas considérés. Pour des matériaux non reliés a ` granulométrie ﬁne, l’état de saturation peut affecter leur comportement mécanique en raison des effets des pressions interstitielles. Dans cette étude, une procédure d’essai modiﬁée et un modèle prédictif du module de résilience qui considère la succion du sol sous la chaussée comme une variable de l’état de contrainte sont présentés. Deux sables silteux utilisés comme sol sous la chaussée ont été testés dans des conditions non saturées a ` l’aide d’une série d’essais triaxiaux répétés réalisés en conditions de succion dans les pores contrôlée aﬁn d’étudier son inﬂuence sur le module de résilience. Des données d’essais ont été aussi utilisées pour obtenir les paramètres de régression du modèle du module de résilience qui considère les variations de teneur en eau a ` travers le paramètre de succion. De façon générale, le modèle de prédiction pouvait représenter efﬁcacement le comportement du module de résilience des sols sous la chaussée relativement aux changements dans l’état de contrainte normale et de succion. Ce modèle de prédiction est recom- mandé en tant qu’approche améliorée pour représenter les effets de la teneur en eau sur les propriétés de rigidité des matériaux, puisque cette méthode est complete. [Traduit par la Rédaction] Mots-clés : sol sous la chaussée, module de résilience, sol non saturé, succion, effets environnementaux, teneur en eau. Introduction A large part of the operating road network around the world consists of ﬂexible pavement structures with a relatively thin bi- tuminous surface layer (≤100 mm). In thin ﬂexible pavement structures, the function of the supporting unbound layers be- comes very signiﬁcant as they greatly contribute to the overall stiffness of the pavement system. Therefore, both large elastic and plastic deformations in unbound granular layers and subgrade soil contribute to pavement deterioration and surface distresses. It is known that moisture content is an important factor affect- ing the mechanical behaviour of unbound pavement materials (Saevarsdottir and Erlingsson 2013; Erlingsson 2010). Recent stud- ies on the improvement of the design and maintenance methods of low volume roads have shown that, in most cases, the problem is related to the accumulation of excess water content in the unbound layers or the subgrade soil (Saarenketo and Aho 2005; Aho and Saarenketo 2006; Charlier et al. 2008; Laloui et al. 2008; Salour and Erlingsson 2013a). This is a common issue in regions where pavement structures are subjected to signiﬁcant seasonal environmental effects, such as high precipitation, groundwater level variations, and freeze–thaw actions in cold regions. There- fore, proper consideration of the inﬂuence of moisture content on the stress state and hence, on the mechanical behaviour of the unbound materials, should be given in any sustainable pavement design and performance prediction model (Santha 1994; Andrei 2003; ARA 2004; Doucet and Doré 2004; Zapata et al. 2007; Cary and Zapata 2010; Rahman and Erlingsson 2012). One of the important input parameters in ﬂexible pavement design is the stiffness of the unbound layer materials. This material Received 24 December 2013. Accepted 5 May 2014. F. Salour. Pavement Technology, Swedish National Road and Transport Research Institute, VTI, 581 95 Linköping, Sweden; and Division of Highway and Railway Engineering, Royal Institute of Technology, KTH, SE-100 44 Stockholm, Sweden. S. Erlingsson. Pavement Technology, Swedish National Road and Transport Research Institute, VTI, 581 95 Linköping, Sweden; and Faculty of Civil and Environmental Engineering, University of Iceland, IS-107 Reykjavik, Iceland. C.E. Zapata. School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ 85287-5306, USA. Corresponding author: Farhad Salour (e-mail: farhad.salour@vti.se). 1This article is one of a series of papers published in the special issue “Celebrating 50 years — Thematic issue on unsaturated soils”. 1413 Can. Geotech. J. 51: 1413–1422 (2014) dx.doi.org/10.1139/cgj-2013-0484 Published at www.nrcresearchpress.com/cgj on 7 May 2014. property is widely characterized by the resilient modulus (MR) using repeated load triaxial (RLT) testing under different stress state and moisture conditions. (e.g., when studying seasonal ef- fects). This soil property depends on many factors such as grada- tion and texture, stress state, compaction energy, dry density, number of load cycles, and moisture content (Lekarp et al. 2000). Despite the fact that unbound pavement materials are generally in partially saturated states and are tested in unsaturated condi- tions in the laboratory, the effect of pore pressures are usually not taken into account and a total stress approach is used when mod- eling their behaviour. From unsaturated soil mechanics, it is known that variation in the moisture content of ﬁne-grained sub- grades results in changes in the stress state of the soil. This can be explained by changes in the soil pore pressure (i.e., the matric suction, m). Matric suction is deﬁned as the pressure difference between the pore–air and the pore–water phases in the soil ma- trix. Therefore, a comprehensive description of ﬁne-grained un- bound materials stiffness behaviour requires an effective stress approach in which the pore pressures (suction) are measured or controlled during the load application (Yang et al. 2005; Cary and Zapata 2011; Nowamooz et al. 2011). Using this approach, moisture content effects can be incorporated into the stiffness predictive models by integrating matric suction as a stress state variable into the models (Khoury and Zaman 2004). This will further allow accounting for seasonal moisture variations and their effect on the material stiffness. In this study, a series of suction-controlled resilient modulus laboratory tests on compacted subgrades in an unsaturated state were conducted using a modiﬁed testing procedure based on the axis-translation technique (Fredlund et al. 2012). Two different silty sand subgrade materials, frequently encountered in Nordic countries due to the heterogeneous sedimentation process of glacial origin, were tested. The objectives of this study were to implement a modiﬁed suction-controlled resilient modulus test- ing system for unsaturated subgrades, and to investigate the in- ﬂuence of the matric suction (moisture content) on the resilient modulus of silty sand subgrade materials. Furthermore, a resilient modulus model that accounts for seasonal moisture changes by incorporating matric suction as a stress state variable was cali- brated and evaluated. Moisture in pavement unbound materials The moisture balance in the pavement system is continuously evolving over time. During the construction phase, pavement un- bound materials are usually compacted at values close to their optimum moisture content and maximum dry density conditions. However, this moisture content will change to a natural equilib- rium state that is greatly dependent on environmental condi- tions, material properties, and the groundwater table level. In a study that was carried out by Zapata et al. (2009), 44 subgrade soil samples were collected from 30 pavement sections throughout the United States. The collected samples had in situ degrees of saturations (Sr) varying between 38% and 97%, with an average value of 79.1%. The moisture content can still deviate from the equilibrium state due to seasonal climatic factors such as rainfall, groundwa- ter table variations, and freeze–thaw actions. Figure 1 shows the monitored moisture content in the subgrade layer over a 1 year period (July 2010 to June 2011) in a test road section in southern Sweden (Torpsbruk). The test section in Torpsbruk usually expe- riences a total frost depth close to 1.2 m in winter. The measure- ments were done using the time domain reﬂectometry technique. Figure 1 is based on in uploads/Ingenierie_Lourd/ content-server.pdf