Performance-Based Design of Energy Pile Foundations-Article

Article

Pdf Summary

The paper develops a framework for understanding the coupled thermal and mechanical behaviour of energy piles—foundations that also act as geothermal heat exchangers—and proposes a performance-based design approach.<br /><br />Thermal loads produce patterns of stress, shear and displacement that differ from those caused by conventional mechanical loads. Heating and cooling can move different parts of a pile in opposite directions, while the pile head, base, surrounding soil and nearby piles constrain its response. These restraints affect thermally induced stresses and displacements. Soil type and loading history also matter: coarse-grained soils and soil–concrete interfaces are generally only mildly temperature-sensitive, whereas fine-grained soils can undergo temperature-dependent, sometimes irreversible volume changes. Interactions between nearby operating piles can increase deformation, so isolated-pile analyses may not fully represent group behaviour.<br /><br />The central design conclusion is that thermal effects can generally be omitted from ultimate limit state checks, provided appropriate structural provisions are used. Ultimate limit state design can therefore follow conventional pile-design procedures for mechanical loads. Energy piles should have sufficient reinforced-concrete compressive strength and minimum longitudinal reinforcement to promote ductile behaviour.<br /><br />At serviceability limit states, both thermal and mechanical effects must be considered. Checks should address pile and group settlement or heave, differential movement and angular distortion, compressive and tensile stresses, and crack control. Analyses should account for restraint conditions and group effects; stresses should be checked along the pile length, while displacement checks should focus on pile-head movement. The paper also proposes partial factors and load-combination guidance for thermal actions.

Keywords

energy piles

geothermal heat exchangers

coupled thermal-mechanical behaviour

performance-based design

thermal loads

serviceability limit state

ultimate limit state

pile settlement

differential movement

thermally induced stresses