Performance of geothermal energy piles under thermo-axial loads

Abstract

The geothermal energy pile system is a sustainable geostructure system designed to meet the cooling demands of a building by storing excess heat from the building into the soil, acting as a heat sink. Thermal loads stored in the soil will cause thermally induced settlement, and this factor must be considered in the geotechnical design process. This study aims to develop a preliminary geothermal energy pile laboratory testing system in single gravity condition, to produce the load-settlement curves generated by thermo-axial load tests. The model energy pile behaviour was studied using a physical model consist of a soil container (450 mm height and 270 mm diameter), model pile (15 mm diameter and 150 mm embedded length), and an axial and thermal load control system provided by a pneumatic cylinder and a temperature bath, respectively. Axial loads (100 N and 200 N), thermal loads (35°C and 40°C) and combinations of both loads (thermo-axial loads) were applied to the model pile. The axial load values were chosen based on the ultimate capacity of the model pile in soft soil (about 0.5 and 0.25qu). The kaolin soil used during model testing was classified as silt of intermediate plasticity (MI). The model soil was compacted at 80% and 90% maximum dry density, which were classified as having soft and firm consistency. The model pile behaviour was monitored with a linear variable displacement transducer, load cell and wire thermocouple connected to a data logger to monitor the pile head settlement, applied axial load and pile temperature. The pile response to thermo-axial loads appears to be thermo-elastic and is attributed to soil consistency and magnitude of thermal load applied to the pile. Thermal loads induced small settlement values, whereby the highest value was about 1% of the pile diameter (0.15 mm). In general, firm soils produce lower thermally induced settlement, due to higher restraint at the pile head compared to soft soils. For 35°C thermal loads, the resulting settlement did not exceed the limiting settlement, which is defined as 10% of the model pile diameter (1.5 mm). The highest thermo-axial settlement obtained was 1.66 mm for thermo-axial load of 40?C and 200 N (global factor of safety (FOS) of 1.9) in soft soil, and the settlement at 100 N axial load (global FOS of 3.8) amounts to 1.59 mm. Consequently, in firm soil and for thermo-axial load of 40°C and 200 N (global FOS of 2.3), the thermo-axial settlement is 1.54 mm. To ensure that the thermo-axial settlement does not exceed the limiting settlement, the recommended global FOS used for soft soil and firm soil should be more than 4.0 and 2.5, respectively. A laboratory scale model of an energy pile system, specifically for cohesive soils in single gravity conditions has been developed. This laboratory model is able to produce reliable thermo-axial load-settlement curves in varying soil consistencies, initial axial loads and also varying thermal loads.