Settlement behaviour of geothermal energy pile under cyclic thermo-axial loads

Geothermal energy pile (GEP) foundation is a new type of sustainable geostructure that can be used as an alternative solution to the energy demand for heating and cooling of built structures. However, due to limited information of this system, the installed piles have generally been over-designed to lower the risk of the system failing. This paper presents the findings of the research carried out to evaluate the performance of laboratory scaled GEP model (model pile) under the effects of the cyclic thermal loads on the settlement behaviour of the model pile with and without the application of axial load. A small-scale model pile of 19 mm diameter and 300 mm length (150 mm embedded length) was used in the experimental work while kaolin was chosen as the model soil. The model soil was compacted at 90% maximum dry density (1.4625 Mg/m3) with optimum moisture content (17%) to obtain ‘firm’ consistency, in a container of 450 mm height and 270 mm diameter. Strain gauges were installed along the pile to monitor the temperature. The ultimate load, Qu of model pile was determined as 480 N. It is found that two cycles of thermal load decreased the settlement; the higher the values, the lower the settlement due to pile expansion and soil heaves. For thermo-axially loaded pile with two cycles of thermal load, the reduction was not significant as the effect of settlement due to axial load had caused much more settlement. For the thermo-axial loads of 50°C-100 N, 17% of the settlement at failure, sf occurred after the application of axial load. When two cycles of thermal load were applied from 29°C to 50°C, the settlement occurred reduced to 16%sf. From this study it can be concluded that the effect of two cycles of cyclic thermal loads from 29°C to 50°C on pile subjected to 21% of Qu in firm clay, is negligible. The pile could function satisfactorily as designed. However, the application of higher axial loads and cycles of thermal load may need to be studied as it could potentially cause hazard to the building due to the excessive pile settlement.