Page 235 - Analysis and Design of Energy Geostructures

210 Analysis and Design of Energy Geostructures

Then, the strength of soil concrete interfaces under nonisothermal conditions is considered: in
this context, the purpose is to comment on the response of interfaces between fine-
and coarse-grained soils and concrete to thermal and mechanical loads. Finally, ques-
tions and problems are proposed: the purpose of this part is to fix and test the under-
standing of the subjects covered in this chapter by addressing a number of exercises.

5.2 Idealisations and assumptions

A key aspect related to the development of laboratory tests on core samples of soil,
similar to other materials, is the representativeness of the obtained results with respect
to the heterogeneities that characterise the tested material and the problem itself across
scales. Based on the considerations proposed in Part B of this book, the concept of
Representative Elementary Volume (REV) is employed in the following. The REV
concept allows describing the behaviour of heterogeneous materials such as soils, rocks
and concrete as if they were homogeneous, with the possibility to account for hetero-
geneities at greater scales than the scale characterising the REV.
In this context, the addressed materials are assumed multiphase systems charac-
terised by one solid phase and one fluid phase. The previous assumption involves
neglecting materials under partially saturated conditions [the thermohydromechanical
behaviour of soils under partially saturated conditions has been investigated, for
example by Bolzon and Schrefler (2005), François and Laloui (2008) and Gens (2010)]
and accounting for soils that are either dry or fully saturated with water. When soils
fully saturated with water are investigated, drained conditions are assumed upon
thermal loading. These conditions are the result of testing procedures that do not cause
excess pore water pressures upon thermal loading, despite the water present in the
pores of soil matrices having a thermal expansion coefficient that is approximately 10
times greater than that of the solid particles and involving interactions with the solid
phase. The considered conditions are representative of most energy geostructure appli-
cations because of the sufficiently slow rates of thermal loads applied to such structures
that ensure drainage of the water embedded in the soil pores and negligible pore water
pressure build-ups (Rotta Loria and Laloui, 2017).
Yet while the deformation and strength of soils and soil structure interfaces under

nonisothermal conditions is commented hereafter for temperature levels between 2 C

and 90 C, temperature values between 2 C and 45 C should be considered represen-

tative for energy geostructure applications (Rotta Loria, 2019). In other words, results
referring to temperature variations higher than 45 C are not representative of current

energy geostructure applications, but they provide additional information on the sub-
ject matter treated herein.