The behaviour of clay surrounding pile foundation heat exchangers

Pile foundation heat exchangers (PFHX) are increasing in popularity as an alterna- tive to conventional air conditioning systems in large, commercial buildings. PFHX provide a sustainable means of building cooling and are compatible with the struc- ture of pile foundations given the constant ground temperatures. However, increased understanding of the behaviour of clay subject to seasonal temperature cycles is re- quired to ensure that PFHX can safely operate for the building’s design life.

This study has developed a conceptual model for the microstructural behaviour of clay subjected to heating and cooling cycles. This conceptual model accounts for the effect of temperature on the clay particles, adsorbed double layer and pore water within the clay; identifying how this microstructural behaviour influences the bulk behaviour of the clay and therefore the observed response in the pile foundation.

The conceptual model has been supported through thermal triaxial experiments on kaolin and Durham clays. Testing considered the change in sample volume, critical state friction angle and stiffness of two varying plasticity clays. The model has been further strengthened in its application to existing pieces of similar literature for both experimental and field study findings.

This study concludes by providing an understanding of how this conceptual model applies to the real world response of PFHX installed in low and high plasticity clays, detailing the impact of the initial heating and cooling cycle on the longer term pile head displacement. These results support the use of PFHXs and recommend extending their temperature ranges to increase their efficiency without detrimental impact on the load bearing performance of the clay.