Experimental Study of the Uplift Resistance of Energy Piles in Soft Clay

Geothermal piles present a sustainable energy conservation option in situations where deep foundations are required to support substantial structural loads in relatively poor ground. The piles then play dual role of load transfer elements to the subsurface strata and heat exchangers delivering and/or extracting heat to/from the surrounding soil. The thermal cycles may induce additional loads which lead to the contraction/expansion of the pile along its length. These loads may also affect the pile-soil interface by modifying the soil mechanical characteristics at that location. At present, this impact is not fully understood, particularly when combined with effects associated with the large-scale loading of the surrounding soils. This paper attempts to explore the implications of thermal cycling on the uplift resistance of piles in slightly overconsolidated clays. Laboratory tests on scaled models are presented in which piles are loaded with a sustained static uplift load that is roughly 30% of the ultimate capacity, subjected to heating/cooling cycles, and then loaded to failure. The testing program consists of one control pile and two thermal piles. Results indicated that the vertical deformation of the geothermal piles due to sustained loading increased due to heating and cooling whereas the ultimate capacity decreased compared to the control pile. These observations point to the importance of catering for thermal effects on the serviceability and ultimate capacity in normally to slightly overconsolidated clay.