Page 104 - Analysis and Design of Energy Geostructures

76 Analysis and Design of Energy Geostructures

material density. For soils the thermal conductivity markedly depends on (1) mineral-
ogy, (2) dry density, (3) water content and (4) gradation (see, e.g. Brandon and
Mitchell, 1989; Alrtimi et al., 2016; Vulliet et al., 2016). For concrete the thermal
conductivity markedly depends on (1) aggregate types and sources (and thus mineral-
ogy), (2) dry density, (3) water content and (4) mix proportioning (see, e.g. Morabito,
1989; Lanciani et al., 1989; Neville, 1995; Kim et al., 2003). Mineralogy is representa-
tive of the origin and chemical composition of the material. Dry density is representa-
tive of the compaction state of the material. Water content yields information on the
wetting of the medium. Gradation is a feature typically related to the granulometric
curve of soils representing their particle size distribution (the shape of particles being
complementary information to gradation for characterising the morphology of the
material). Mix proportioning represents the proportions of the ingredients used in the
mix design of concrete.
The significance of the aforementioned factors in the estimation of the thermal
conductivity of geomaterials, such as soil, rock and concrete, can be shown by refer-
ring to Table 3.2, which presents values of thermal conductivity, λ i , summarised by
Loveridge (2012) for common constituents i of geomaterials:
1. Among the considered minerals, Quartz can have a thermal conductivity of up to
approximately five times that of the others. For this reason, soils, rocks and con-
cretes characterised by different mineralogy can have markedly different values of
thermal conductivity.

Table 3.2 Thermal conductivity of geomaterials constituents.
Material Thermal conductivity, λ i [W/(m C)]

Air 0.024
Water 0.6
Feldspar 1.4 2.5
Plagioclase 1.5 2.0
Mica 1.6 3.5
Amphibole 2.8 4.8
Garnet 3.1 5.5
Olivine 3.2 5.0
Pyroxene 3.5 5.7
Calcite 3.6
Chlorite 5.2
Quartz 7.7
Source: Data from Banks, D., 2012. An Introduction to Thermogeology: Ground Source Heating and Cooling. John
Wiley & Sons, Côté, J., Konrad, J.-M., 2005. A generalized thermal conductivity model for soils and construction
materials. Can. Geotech. J. 42 (2), 443 458 and Midttømme, K., Banks, D., Kalskin Ramstad, R., Sæther, O.M.,
Skarphagen, H., 2008. Ground-source heat pumps and underground thermal energy storage: energy for the future.
NGU Spec. Publ. 11, 93 98, after Loveridge, F., 2012. The Thermal Performance of Foundation Piles Used as Heat
Exchangers in Ground Energy Systems (Ph.D. Thesis). University of Southampton.