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Effect of confining stress on the transient hydration of unsaturated GCLs
Effet de la contrainte de confinement sur l'hydratation transitoire de GCLs insaturés
Siemens G.A., Take W.A., Rowe R.K., Brachman R.
GeoEngineering Centre at Queen’s-RMC, Queen’s University, Kingston, Canada
ABSTRACT: Geosynthetic clay liners (GCLs) are often used within composite landfill liner systems in combination with a
geomembrane to provide an effective barrier. In order for the GCL to function as a barrier, however, it must hydrate from its initially
low moisture content. With the geomembrane above the GCL limiting moisture uptake from the surface, the GCL must hydrate by
taking moisture from the foundation soil below. Numerical simulations of hydration showed that in an isothermal closed system, the
foundation soil sets the suction value towards which the GCL migrates. In this paper the impact of normal stress on the rate of
hydration and equilibrium moisture content is investigated numerically. The laboratory tests were completed with 2 kPa normal
stress, however, in a landfill, GCL hydration could occur at significantly higher normal stresses. Therefore the question arises
whether additional normal stress will aid or hinder GCL hydration. The results showed the impact of normal stress is to constrain
swelling of the GCL which will reduce the equilibrium moisture content as well as reduce the time to achieve equilibrium. This
provides additional motivation to cover the composite barrier system in a timely manner within the construction process.
RÉSUMÉ : Les géosynthétiques bentonitiques (GSB) sont souvent utilisés en combinaison avec une géomembrane dans la conception
des sites d'enfouissement pour fournir une barrière étanche. Pour assurer sa fonction d’étanchéité, un GSB doit s’hydrater suite à
l’installation puisqu’il possède un faible taux d’humidité initial. La géomembrane qui recouvre le GSB limite la reprise d'humidité du
GSB. L'hydratation doit donc se faire en utilisant l'humidité du sol au-dessous de la barrière. Des simulations numériques de
l’hydratation ont démontré que dans un système fermé isotherme, le sol de fondation fixe la valeur de succion vers laquelle l’humidité
migre vers le GSB. Dans cet article, les effets de la contrainte normale sur la vitesse d'hydratation et sur la teneur en humidité à
l'équilibre sont étudiés numériquement. Les essais en laboratoire ont été réalisés avec une contrainte normale de 2 kPa. Cependant,
dans un site d'enfouissement, l'hydratation du GSB pourrait se produire à des contraintes normales beaucoup plus élevées. Les
résultats ont montré que l'impact de la contrainte normale est de contraindre le gonflement du GSB, ce qui réduira la teneur en
humidité à l’état d’équilibre ainsi que le temps nécessaire pour atteindre l’état d’équilibre.
KEYWORDS: Geosynthetic clay liner, hydration, numerical simulations, parametric study, landfill, barrier systems
1 INTRODUCTION
Geosynthetic clay liners (GCLs) are often used in combination
with a geomembrane (GMB) as a composite landfill liner
system. For a GCL to function as a hydraulic barrier, however,
it must hydrate from its initially low moisture content. The
source of this moisture cannot be from the atmosphere as the
GMB barrier is installed on top of the GCL. Instead, a GCL
must hydrate with moisture from the foundation soil below the
GCL. This is not to say that the GMB does not play a
significant role in defining the hydration behaviour of a GCL. If
the surface of the GMB is left exposed to solar radiation during
construction of the landfill liner, peak daily temperatures have
been observed in the range of 60-75 °C for black GMBs (Pelte
et al. 1994; Chappel et al., 2012; Rowe et al., 2012). These
daily thermal cycles have been shown in the laboratory to
suppress the ability of GCLs to significantly hydrate (Rowe et
al. 2011).
If best practice is followed and the GMB is covered in a timely
fashion with a granular protection layer or leachate collection
system, the combination of a lack of direct contact with solar
radiation and the thermal insulation provided by these layers of
granular material will result in a much less severe thermal
regime allowing GCL hydration to occur. Because these layers
are designed to be as thin as possible (i.e. maximising landfill
volume), GCL hydration in this scenario could be viewed as
occurring under low confining stresses and relatively isothermal
conditions. Research quantifying the rate of GCL hydration
under these conditions was performed by Rayhani et al. (2011)
which showed that GCLs readily hydrate from the subsoil under
these conditions with the rate of hydration and final equilibrium
water content of the GCL varying on the initial moisture content
of the foundation soil.
The key to quantifying the moisture uptake and retention
behaviour of GCLs is the material’s unsaturated water retention
curve (WRC). The water retention behaviour of GCLs at low
confining stresses has recently been quantified using
measurements of GCL suction using high capacity tensiometers
and relative humidity sensors (Beddoe et al., 2010) for four
geotextile-encased GCLs containing granular bentonite (Beddoe
et al., 2011). The results of this study have indicated that the
method of GCL manufacture, in particular the presence or
absence of a scrim reinforced nonwoven carrier geotextile and
thermal treatment, can have a significant impact on the water
retention behaviour of GCLs. Using the water retention curves
(WRCs) defined by Beddoe et al. (2011), Siemens et al. (2012)
performed an unsaturated numerical modelling parametric study
of the experimental isothermal hydration experiments of
Rayhani et al. (2011). The results of this analysis are shown
schematically in Figure 1. The underlying foundation soil was
observed to act as a suction boundary condition. As a result, the
soil followed a drying path, whereas the GCL followed its
wetting path until an equilibrium suction value was reached.
Due to the relatively small mass of water required to hydrate the
GCL compared to the mass of moisture held in the soil column,
the equilibrium suction value was observed to be that
represented by the foundation soil’s initial moisture content.
