Page 270 - Analysis and Design of Energy Geostructures

Thermohydromechanical behaviour of soils and soil structure interfaces 245

zone of soil between structures and surrounding ground masses. The thickness of these
settings depends on soil and structure properties, but it is generally considered to vary
from 5 to 10 times the average particle diameter (Boulon and Foray, 1986; Uesugi
et al., 1988). In the context of energy geostructures, a characterisation of soil struc-
ture interfaces is key for addressing phenomena such as mass transfer, heat transfer and
deformation that may result from the influence of loads arising from the geostructure.
In the following, the considered characterisation is investigated with a focus on the
influence on the deformation of energy geostructures.
The exploitation of geothermal energy through energy geostructures involves the
transfer of seasonally and daily cyclic thermal loads in addition to approximately con-
stant mechanical loads to the ground, which result in a mobilisation of the shear
strength of interfaces. The two following aspects of this problem can be distinguished
(Di Donna et al., 2015):
1. Cyclic expansion and contraction of the geostructure during heating and cooling
and the resulting mechanically cyclic mobilisation of the interface shear strength.
2. The direct effect of temperature on the soil at the interface and the resulting
response of the soil structure interface at different temperatures.
The mechanical mobilisation of the interface shear strength [i.e. aspect (1)] has
been extensively addressed in the framework of offshore foundations with respect to
the influence of the cyclic mechanical loads that are typically applied to such structures
as a result of wind and wave actions (De Jong et al., 2003, 2006; Mortara et al., 2007).
The influence of the mechanical solicitation of interfaces caused by mechanical loads is
considered comparable to the one caused by thermal loads in the framework of energy
geostructures. The direct effect of temperature on the soil at the interface (i.e. aspect
2) has been investigated in the framework of energy geostructures (Xiao et al., 2014;
Di Donna et al., 2015; Yavari et al., 2016; Yazdani et al., 2019).

5.7.2 Structure roughness
The structure roughness is commonly identified through the normalised roughness,
R n , and is defined as follows (Uesugi and Kishida, 1986):

R n 5 R max ð5:6Þ
D 50
where R max is the maximum vertical distance between the highest and lowest peaks of
the structure asperities over a fixed length and D 50 is the soil mean grain size (cf.
Fig. 5.32).
Depending on the interface materials and conditions, it is possible to identify a crit-
ical value of roughness, R cr , such that if R n , R cr the interface is considered smooth,