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228 Analysis and Design of Energy Geostructures

Figure 5.15 Volumetric behaviour of coarse-grained soils subjected to thermal cycles. Redrawn
after Ng, C.W.W., Wang, S.H., Zhou, C., 2016. Volume change behaviour of saturated sand under ther-
mal cycles. Géotech. Lett. 6 (2), 124 131.

Ng et al. (2016). These results show that the application of cyclic thermal loads
under drained conditions to coarse-grained soils involves, after a first heating
cooling cycle (in which a contraction of the soil matrix during heating may be
observed), an expansive volume variation of such materials upon heating and a
contractive volume variation upon cooling. Experimental results that corroborate
the previous considerations are shown in Fig. 5.15 with reference to the data of
Ng et al. (2016). The observed cyclic effects are due to the irreversible contraction
induced by the first cycle of heating and cooling. Soil response becomes stiffer
during the second thermal cycle.

5.4.6 Micromechanics of the volumetric behaviour of soils under
nonisothermal conditions
The effect of temperature on the volumetric behaviour of soils at the macroscale is a
combination of microscale processes that govern the interactions between the soil
phases. Predominantly chemical (besides physical) interactions between soil phases
govern the volumetric behaviour of fine-grained soils under nonisothermal conditions.
In contrast, predominantly physical interactions between soil phases govern the