Page 238 - Analysis and Design of Energy Geostructures

Thermohydromechanical behaviour of soils and soil structure interfaces 213

the surface area of the solid particles, while the magnitude of body forces is propor-
tional to the volume of the solid particles. As particle size decreases, surface forces
diminish with the square of the particle diameter, whereas volume forces diminish
with thecubeof the particlediameter.
Chemical interactions between particles inherently involve a higher sensitivity of
the material to temperature compared to a material whose interactions between
particles are predominantly physical. The previous fact implies a higher sensitivity to
temperature of fine-grained soils compared to coarse-grained soils.

5.3.2 Effective stress
Different descriptions and interpretations of the mechanics of soils can be performed
by means of the relevant stress to which deformations are associated. In principle, it is
very unlikely that a stress expression is applicable to the full range of porous materials
(Skempton, 1960, 1961; De Boer and Ehlers, 1990). The reason for this is that the
adequate combination of total stresses, pore pressures and other interaction forces
depends on the constitutive features and the internal structure of the material, losing
in this way its desired universality (Gens et al., 2004). In practice, the effective stress
expression proposed by Terzaghi (1943) may be considered valid for most energy
geostructure applications. The effective stress represents the relevant stress that governs
the macroscopic behaviour of porous materials, based on the principle that the external
stress applied on a saturated porous medium with a fluid (e.g. water) is supported by
the combination of the pore fluid pressure and the effective stress, that is
0
ð5:1Þ
σ 5 σ ij 2 p w δ ij
ij
where σ ij the total stress tensor, p w is the pore (water) pressure and δ ij the
Kroenecker’s delta (equal to 1 if i 5 j and to 0 otherwise). Provided that the assump-
tions governing the formulation of Eq. (5.1) hold, the deformation of porous materials
is caused by a change in the effective stress.
The effective stress initially defined by Terzaghi (1936) may also be defined as the
vector sum of all the interparticle forces in a given direction divided by the total area
being considered (Mitchell and Soga, 2005). Based on the previous definition, a
dependence of the effective stress on the physicochemical interactions between parti-
cles can be remarked. These interactions vary with the application of mechanical and
thermal loads to soil structures.

5.3.3 Preconsolidation stress
The preconsolidation stress (or pressure) is the maximum vertical stress that a soil has
ever supported and is a key parameter to characterise the mechanical behaviour of
soils. Soils retain a memory of the maximum stress that they have supported