Page 298 - Analysis and Design of Energy Geostructures

274 Analysis and Design of Energy Geostructures

loading (De Beer, 1988). Various pile installation techniques do exist and increasing
combinations of such techniques are observed in practice. So-called displacement piles
and nondisplacement piles can be broadly distinguished according to a classification crite-
rion based on the pile installation technique. Displacement piles are installed by driv-
ing them into the ground and include, without being limited to, so-called driven
piles. During installation, a marked disturbance of the ground is generated because of
the penetration of the piles and noteworthy ground displacements in the radial direc-
tion are observed (Lancellotta, 1995). To highlight this phenomenon the term ‘dis-
placement’ is employed to characterise such foundations. Nondisplacement piles are
installed by excavating the ground for a successive filling with (reinforced) concrete
and include, without being limited to, so-called bored piles. During installation the
boring process causes lateral stresses in the ground to decrease with only a partial
recovery upon concreting and limited ground displacements are observed (Lancellotta,
1995). To highlight this phenomenon the term ‘nondisplacement’ is employed to
characterise such foundations.
Another classification criterion refers to the pile bearing behaviour. Piles subjected
to loads that primarily influence the longitudinal (e.g. axial) pile response are charac-
terised by the generation of shear stresses along the shaft and normal stresses at the toe
ensuring equilibrium. These stresses contribute with those mobilised by the cap in the
overall equilibrium of the foundation. So-called predominantly end-bearing piles and
friction (or floating) piles can be broadly distinguished according to a classification crite-
rion based on the pile bearing behaviour and associated contributions of load-carrying
capacity provided by the shaft and the toe. Piles that penetrate a relatively soft layer of
soil to found on a firmer stratum are generally referred to as end-bearing piles because
the normal stresses mobilised at the pile toe (i.e. by the end-bearing) represent the
higher contribution to the pile load-carrying capacity. Piles that do not found on a
particularly firm stratum are generally referred to as floating piles because they derive
most of their capacity from the shear stresses mobilised at the shaft. In fine-grained
(e.g. cohesive) soil, the shaft capacity of both displacement and nondisplacement piles
is generally paramount. In coarse-grained (e.g. noncohesive) soil, the overall pile
capacity is more evenly divided between shaft and base.

6.4 Temperature variations

Notable temperature variations can develop in energy piles as a consequence of their
geothermal operation. For given boundary conditions, the rate of these temperature
variations depends on the thermal power applied to the energy piles as well as on the
thermohydraulic properties of both the pile and the surrounding ground. For given
applied thermal power and thermal conductivity of the grouting material characteris-
ing energy piles, lower values of thermal conductivity of the ground and groundwater