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

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1 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.

2 To simplify the understanding: here an example of the strain (a) and load (b) development along the pile shaft for a pile free to move at the head and at the pile toe in the case of strong ground resistance and weak ground resistance. In the example the null point is located at mid depth and correspond to the zone where the thermally induced compressive load, in the case of heating, (tensile load in the case of cooling) get its maximum value

3 To simplify the understanding: here an example of the strain (a) and load (b) development along the pile shaft for a pile not free to move at the head and at the pile toe.

4 The sign convention usually adopted in the analysis is as follows Upward shaft friction is taken as positive; 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

5 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.

6 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 part of the pile, Qs,mob,up, decreases because axial displacements occur in the upward direction. Negative friction can develop depending on the magnitude of the displacements. The mobilised base resistance, Qb,mob, increases because thermally induced axial displacements in the lower part of the pile occur in the downward direction. The ultimate base reaction may be reached depending on the magnitude of the displacements. The mobilised shaft friction along the lower part of the pile, Qs,mob,low, increases because axial displacements occur in a downward direction. The ultimate shaft friction may be reached depending on the magnitude of the temperature increase.

7 When the pile is cooled: The mobilised resistance at the head of the pile, Qh, decreases. The capping reaction of the raft occurs in the upward direction and pulls on the pile head as it settles. The mobilised shaft friction along the upper part of the pile, Qs,mob,up, increases because the axial displacements occur in the downward direction. The mobilised base resistance, Qb,mob, decreases because the pile tip heaves. If the pile tip heave is large enough that the contact between the pile base and the soil is broken (i.e. higher than the elastic unloading displacement at the pile base), the base reaction reaches zero. The mobilised shaft friction along the lower part of the pile, Qs,mob,low, decreases because axial displacements occur in the upward direction.

8 To simplify the understanding: here an example of the strain (a) and load (b) development along the pile shaft for a pile free to move at the head and at the pile toe in the case of strong ground resistance and weak ground resistance. In the example the null point is located at mid depth and correspond to the zone where the thermally induced compressive load, in the case of heating, (tensile load in the case of cooling) get its maximum value

9 Three tests on different piles in different soil conditions with different temperature changes. Ref. Thermo-mechanical behaviour of energy piles. Géotechnique vol.62 n

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13 When a pile is heated or cooled, the thermally induced axial stress inside the pile was between about 50% and 100% of the theoretical fully restrained values. The latter provides a safe upper bound for estimates of stress change in design. However, the mechanisms by which this effect is mitigated need to be understood in order to develop more refined design guidance and to avoid excessive conservatism. The mobilised shaft resistance profile for a mechanically loaded pile may undergo significant changes during thermal loading By ensuring that design concrete stresses are not exceeded, conventional factors of safety for skin friction and end bearing are maintained, and foundation settlements are limited, the heating and cooling of energy piles are unlikely to have any detrimental effect on buildings. Ref. Thermo-mechanical behaviour of energy piles. Géotechnique vol.62 n

14 When a pile is heated or cooled, the thermally induced axial stress inside the pile was between about 50% and 100% of the theoretical fully restrained values. The latter provides a safe upper bound for estimates of stress change in design. However, the mechanisms by which this effect is mitigated need to be understood in order to develop more refined design guidance and to avoid excessive conservatism. The mobilised shaft resistance profile for a mechanically loaded pile may undergo significant changes during thermal loading By ensuring that design concrete stresses are not exceeded, conventional factors of safety for skin friction and end bearing are maintained, and foundation settlements are limited, the heating and cooling of energy piles are unlikely to have any detrimental effect on buildings. Ref. Thermo-mechanical behaviour of energy piles. Géotechnique vol.62 n

15 It is not the end