Page 283 - Analysis and Design of Energy Geostructures

258 Analysis and Design of Energy Geostructures

with sand while globally appears to be on the safety side for interfaces with clay
because it increases the interface strength (Di Donna et al., 2015).
When calculating the capacity of geostructures in terms of effective stress, the angle
of shear strength of the interface is included while the adhesion is neglected.
Therefore, a decrease of the strength of interfaces with fine-grained soils for an
increase in temperature would arise from capacity formulations due to the decrease in
the angle of shear strength of such interfaces. However, the actual increase in the shear
strength of the considered interfaces with temperature by means of the contribution of
the adhesion supports the use of a constant value of interface angle of shear strength
(e.g. determined at ambient temperature) in capacity formulations.
A cyclic mechanical degradation of the interface shear stress induced by the ther-
moelastic expansion and contraction of the structural material at the interface with the
soil may be observed. This effect is similar to the cyclic degradation phenomenon
caused by cyclic mechanical loads applied to interfaces with conventional structures. It
is typically more pronounced for coarse-grained soils compared to fine-grained soils
because of the more significant volumetric cyclic contraction of the interface in the
former case compared to the latter. In the case of interfaces with initially NC fine-
grained soils, the cyclic volumetric contraction is even reduced by the increase in tem-
perature, as the soil first undergoes a thermal collapse thus reducing the soil potential
of collapse during shearing (Di Donna et al., 2015). Cyclic degradation effects may be
considered in the analysis and design of energy geostructures. These effects are
assumed to play a central role in the characterisation of the deformation of energy
geostructures, especially for situations in which significant mechanical loads are
applied. The reason for this is that, together with subsequently applied cyclic thermal
loads, significant mechanical loads (even if approximately constant over time) may
involve noteworthy degradation effects. When cyclic effects are considered limited in
magnitude or absent, they can be omitted in the analysis and design of energy
geostructures.

References
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Experimental evaluation of engineering behavior of soft Bangkok clay under elevated temperature.
J. Geotech. Geoenviron. Eng. 132 (7), 902 910.
Abuel-Naga, H.M., Bergado, D.T., Lim, B.F., 2007. Effect of temperature on shear strength and yielding
behavior of soft Bangkok clay. Soils Found. 47 (3), 423 436.
Agar, J.G., 1984. Geotechnical Behaviour of Oil Sands at Elevated Temperatures and Pressures. (Ph.D.
thesis), Univerity of Alberta, Edmonton, AB.
Agar, J.G., Morgenstern, N.R., Scott, J.D., 1986. Thermal expansion and pore pressure generation in oil
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Agar, J.G., Morgenstern, N.R., Scott, J.D., 1987. Shear strength and stress strain behaviour of Athabasca
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