3427
Understanding the effects of high temperature processes on the engineering
properties of soils
Comprendre les effets des procédés a haute température sur les propriétés des sols
Zihms S.G., Switzer C., Karstunen M., Tarantino A.
University of Strathclyde, Glasgow, United Kingdom
ABSTRACT: High temperature processes such as in situ smouldering and thermal remediation techniques can achieve rapid removal
of organic contaminants from soils in much shorter time periods than traditional remediation technologies. Thermal remediation
processes use heat or heated water to volatilise the contaminant within the soil to enable its extraction. High temperatures affect the
particle size distribution, mass loss, mineralogy and permeability of the soil. In sandy soils, the particle size decreases with increasing
temperature due to a mobilisation of fines, which is likely due to the bond of fines to the sand grains being affected by temperature. In
clayey soils, the overall particle size increases with increasing temperature due to aggregation and cementation of the clay fraction.
Permeability seems to be affected by treatment type rather than temperature alone, comparing heat treated and smouldered samples
showed an increase of sand permeability by approximately two magnitudes. This study illustrates the effects of high temperature and
smouldering processes on soil characteristics and dynamic behaviour. Monitoring during and after aggressive remediation is advisable
so that rehabilitation measures can be implemented before site redevelopment.
RÉSUMÉ : Des procédés a haute température tels que la combustion lente in situ et des techniques de traitement thermique peuvent
achever une élimination rapide des contaminants organiques des sols en beaucoup moins de temps que les technologies de traitement
traditionnelles. Les procédés de traitement thermique utilisent la chaleur ou de l’eau chauffée pour vaporiser les contaminants dans le
sol pour permettre leur extraction. Des températures élevées affectent la distribution granulométrique, la perte de masse, la
minéralogie et la perméabilité du sol. Dans les sols sablonneux, la taille des particules décroît avec l'augmentation de température due
à une mobilisation des particules les plus fines, probablement dû à la liaison de ces particules aux grains de sable, affectée par la
température. Dans les sols argileux, la taille des particules augmente avec l'augmentation de température due à l'agrégation et la
cimentation de la fraction argileuse. La perméabilité semble être affectée par le type de traitement plutôt que par la température
uniquement, des échantillons traités par la chaleur ont montré une augmentation de la perméabilité du sable d’environ deux ordres de
grandeur par rapport à ceux traités par combustion lente.Cette étude montre les effets des températures élevées et des procédés de
combustion lente sur les caractéristiques du sol et sur son comportement dynamique. Il est conseillé d’utiliser un système de
surveillance pendant et après traitement agressif afin que les mesures de réhabilitation puissent être appliquées avant le
réaménagement du site.
KEYWORDS:
Thermal behaviour of soils, smouldering remediation, high temperature
1. INTRODUCTION
Soils can be exposed to elevated temperatures naturally through
wild, forest or peat fires or through thermal remediation
processes designed to mitigate contamination by hazardous
organic chemicals. Most research on soil properties and their
heat dependency is based on forest fires and therefore
concentrates on erosion rates, ground stability and nutrients
affected by fire severity. The effects of exposure to
temperatures up to 500°C have been studied widely (Are et al.,
2009; Certini, 2005; Rein, 2009; Rein et al., 2008). Literature
published on heat treatments of clay evaluates the effects of
temperatures up to 1000°C (Tan et al., 2004). Exposures of 200
– 850°C have been observed in soils during wildfires (Certini,
2005; DeBano, 2000; Mataix-Solera and Doerr, 2004; Rein et
al., 2008). Moderate (300-400°C) and high (>450°C)
temperature processes, such as hot water extraction, thermal
desorption, soil heated vapour extraction, incineration or
smouldering are used widely to treat contaminated soils
(Araruna Jr et al., 2004; Chang and Yen, 2006; Gan et al., 2009;
Kronholm et al., 2002; Lee et al., 2008; McGowan et al., 1996;
Pironi et al., 2011; Pironi et al., 2009; Switzer et al., 2009;
Webb and Phelan, 1997). Most research on soil remediation
techniques focuses on the remediation result and less on the
effects the process has on the soil properties itself. In some
cases, the effects on soil properties may be a criterion for
selection of the remediation technique (Chang and Yen, 2006;
Pironi et al., 2011) or the soil properties may influence the
results (Webb and Phelan, 1997). There is little research on the
effects of thermal remediation processes on soil properties
(Araruna Jr et al., 2004; Pironi et al., 2009). Based on the
observations of soil erosion and subsidence after wildfires,
further understanding of the effects of high temperature
remediation processes must be developed.
The maximum temperatures observed in contaminant
remediation vary by the process that is used. With the exception
of smouldering remediation, all of these remediation techniques
use heat or heated water to volatilise the contaminant within the
soil to enable its extraction. Maximum temperatures for these
technologies are typically adjacent to the heat source with more
moderate target temperatures of 80-100
o
C achieved within the
wider treatment zone. The contaminant must be collected and
treated (Chang and Yen, 2006; Gan et al., 2009; Kronholm et
al., 2002; Lee et al., 2008; McGowan et al., 1996; Webb and
Phelan, 1997). These processes maintain high temperatures in
the soil for weeks to months or longer. In contrast, smouldering
remediation uses the contaminant itself as fuel for the
combustion reaction (Pironi et al., 2011; Pironi et al., 2009;
Switzer et al., 2009). In laboratory studies, the soil particles are
exposed to high temperatures on the order of 1000°C for coal
tars and 600-800°C for oils for up to 60 minutes. Field scale
efforts may result in exposure durations on the order of hours or
longer.
Elevated temperatures have been shown to alter the
mineralogical composition of soil. These effects have been
studied extensively in relation to the effects of wildfires on soil
properties. Colour change in soils has been observed after
wildfire and after smouldering remediation. In most cases it
