geothermal piles are pile foundations equipped with...

Corso di Fondazioni - prof. Gianpiero Russo - Università di Napoli Federico II Pali

Geothermal piles are pile foundations equipped with absorber pipes to allow heat exchange with the surrounding ground.

Thermal expansion or contraction of the concrete induces thermal strains and stresses that bring new challenges for the design of such structures.

To simplify the understanding: here an example ofthe strain (a) and load (b) development along the pileshaft for a pile free to move at the head and atthe pile toe in the case of strong ground resistanceand weak ground resistance.

In the example the null point is located at middepth and correspond to the zone where thethermally induced compressive load, in the caseof heating, (tensile load in the case of cooling) getits maximum value


To simplify the understanding: herean example of the strain (a) and load(b) development along the pile shaftfor a pile not free to move at the head and at the pile toe …….


• A positive base reaction acts upwards;• The mechanical load P is given as positive

but acts downward;• Upward displacements are taken as

positive;• Kh is a positive quantity so that the

positive head reaction acts downward, in order to have the same sign convention as the mechanical load P

The sign convention usually adopted in the analysis is as follows

• Upward shaft friction is taken as positive;


The geothermal pile can be divided into two parts separated by the null point. The part above the null point is the ‘‘upper part of the pile’’, while the ‘‘lower part of the pile’’ is the part below it.


When the pile is heated:

• The mobilised resistance at the head of the pile,Qh, increases because the pile head heaves.

• The mobilised shaft friction along the upper partof the pile, Qs,mob,up, decreases because axialdisplacements occur in the upward direction.Negative friction can develop depending on the magnitude of the

displacements.

• The mobilised base resistance, Qb,mob, increasesbecause thermally induced axial displacements in thelower part of the pile occur in the downwarddirection.The ultimate base reaction may be reached depending on themagnitude of the displacements.

• The mobilised shaft friction along the lower partof the pile, Qs,mob,low, increases because axialdisplacements occur in a downward direction.The ultimate shaft friction may be reached depending onthe magnitude of the temperature increase.


When the pile is cooled:

• The mobilised resistance at the head of the pile,Qh, decreases. The capping reaction of the raftoccurs in the upward direction and pulls on the pilehead as it settles.

• The mobilised shaft friction along the upper partof the pile, Qs,mob,up, increases because the axialdisplacements occur in the downward direction.

• The mobilised base resistance, Qb,mob, decreasesbecause the pile tip heaves. If the pile tip heave islarge enough that the contact between the pile base and thesoil is broken (i.e. higher than the elastic unloadingdisplacement at the pile base), the base reaction reacheszero.

• The mobilised shaft friction along the lower partof the pile, Qs,mob,low, decreases because axialdisplacements occur in the upward direction.


To simplify the understanding: here an example ofthe strain (a) and load (b) development along the pileshaft for a pile free to move at the head and atthe pile toe in the case of strong ground resistanceand weak ground resistance.

In the example the null point is located at middepth and correspond to the zone where thethermally induced compressive load, in the caseof heating, (tensile load in the case of cooling) getits maximum value



Three tests on differentpiles in different soilconditions with differenttemperature changes ….

Ref. Thermo-mechanical behaviour of energy piles. Géotechnique vol.62 n°6-503-519

“When a pile is heated or cooled, the thermally induced axialstress inside the pile was between about 50% and 100% of thetheoretical fully restrained values. The latter provides a safeupper bound for estimates of stress change in design.However, the mechanisms by which this effect is mitigated need to beunderstood in order to develop more refined design guidance and to avoidexcessive conservatism.”

“The mobilised shaft resistance profile for a mechanicallyloaded pile may undergo significant changes during thermalloading “

“By ensuring that design concrete stresses are not exceeded, conventionalfactors of safety for skin friction and end bearing are maintained, andfoundation settlements are limited, the heating and cooling of energypiles are unlikely to have any detrimental effect on buildings.”

Ref. Thermo-mechanical behaviour of energy piles. Géotechnique vol.62 n°6- 503-519



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geothermal piles are pile foundations equipped with...

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