Page 254 - Analysis and Design of Energy Geostructures

Thermohydromechanical behaviour of soils and soil structure interfaces 229

volumetric behaviour of coarse-grained soils under nonisothermal conditions. Despite
the inherently different nature of the processes that govern the behaviour of soils
depending on their mineralogy, the following common underlying aspects can be
highlighted for both fine- and coarse-grained soils.
If the soil particles were fixed, the thermal expansion of each soil constituent
would produce a global dilation of the solid skeleton, with an associated increase in
the pore size and the interparticle distance (François, 2008). However, soil particles are
not fixed. Therefore, particle rearrangement can occur with an increase in temperature
and is associated with the phenomenon of thermal collapse. In particular, while the
volumetric behaviour of NC soils appears to be driven by particle rearrangement
(whose effect is predominant with respect to the thermoelastic expansion of the soil
particles), the behaviour of OC soils appears to be dominated by the thermoelastic
expansion of the soil particles (with a minimum influence of particle rearrangement)
(Di Donna and Laloui, 2013). In this context, the thermoplastic deformation of
soils upon heating is associated with an unstable configuration of the solid particles,
while the thermoelastic deformation of soils is associated with a particularly
stable configuration of the particles. The presence of unstable voids is considered to
facilitate the occurrence of thermal collapse in coarse-grained soils (Sitharam, 2003). In
any case, the significance of the thermal collapse phenomenon appears to depend on
the magnitude of the pores size (i.e. void ratio) prior to heating, because greater pores
represent a higher potential for collapse (Di Donna and Laloui, 2015). Further consid-
erations specifically apply to fine- and coarse-grained soils.
In fine-grained soils, the magnitude of the thermal collapse phenomenon appears
to be proportional to the plasticity index, I p , that is an indicator of the significance of
chemical interactions in fine-grained soils (Demars and Charles, 1982; Abuel-Naga
et al., 2006; Di Donna and Laloui, 2015). Data available in the literature on this aspect
are collected in Fig. 5.16 and Table 5.1. Despite the scatter between the data, which
prevents from an unequivocal relation between the plasticity index and the volumetric
strain of the soil per unit temperature change, a more significant volumetric deforma-
tion per unit temperature change characterises soils with a higher plasticity index, I p .
In other words, the intensity of the irreversible part of deformation appears to be inde-
pendent of the stress state in the NC range for fine-grained soils, but dependent on
soil type and plasticity (Plum and Esrig, 1969).
Other phenomena contributing to the thermal collapse phenomenon in fine-
grained soils appear to be (1) the degradation of the adsorbed water layer with an
increase in temperature that tends to form larger voids (Fleureau, 1979; Pusch, 1986),
(2) the modification of the contact forces network due to the differences between the
rigidities and the thermal expansion of the different minerals involved (Kingery et al.,
1976) and (3) and the changes in the equilibrium between the Van der Waals