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252 Analysis and Design of Energy Geostructures

In the case of rough interfaces, the sliding component is almost null as shearing occurs
inside the soil (Shakir and Zhu, 2009).
In the development of the shearing and sliding mechanisms of soil structure inter-
faces, a major influence is also played by the nature (e.g. monotonic or cyclic) of the
loading. The magnitude of shearing inside the soil increases with the amplitude of
cyclic loading but decreases with continuing cycles. The percentage of sliding (or slip-
page) between the soil and structural material increases with the number of loading
cycles.
The volumetric behaviour of soil structure interfaces changes at each cycle due
to the continuous rearrangement of grains (Di Donna et al., 2015). A global contrac-
tion cycle after cycle and the reduction in the normal effective stress are typically
observed and are responsible for cyclic degradation. The cyclic degradation phenom-
enon is particularly critical in coarse-grained soils. According to Mortara et al.
(2007), the cyclic degradation is not recovered in the postcyclic phase when dealing
with soilsadjacenttosmoothconcrete, whileitispartially recoveredinthe case of
rough concrete. Yet, cyclic degradation increases with increasing normalised rough-
ness, while postcyclic degradation decreases.
Irrespective of the nature of the loading, the constant volume envelope of soil
structure interfaces is unique. An example of this evidence is proposed in Fig. 5.38
with reference to the data presented by Di Donna et al. (2015).

5.8 Strength of soil structure interfaces under nonisothermal
conditions
5.8.1 Strength of sand concrete interfaces
The strength of structural interfaces with coarse-grained soils under nonisothermal
conditions is characterised by insensitivity to temperature variations. This evidence is
related to the limited sensitivity of coarse-grained soils to temperature variations, both
in terms of volumetric and deviatoric behaviour.
The typical response to monotonic shearing under CNL conditions of a

sand concrete interface at 60 C (first heated at the desired temperature and then
sheared) and at the ambient temperature of 20 C is presented in Fig. 5.39 with refer-

ence to the result of Di Donna et al. (2015). No thermally induced effect characterises
the deviatoric behaviour of the considered interface because the curves at 20 C and

60 C are comparable (cf. Fig. 5.39A). The same conclusion can be drawn for the vol-

umetric behaviour of the interface (only the results under 100 and 150 kPa are avail-
able) (cf. Fig. 5.39B). Similar comments can be established considering the response
of the considered interface subjected to cyclic shearing at different temperatures