Small-scale physical modelling of vertically loaded, cyclically thermally-activated helical piles

To investigate the use of thermally-activated helical piles in shallow geothermal energy systems, a 1-g modelling study was conducted. Helical piles with either a single- or double- helix were installed in a medium dense, dry sand, and subjected to mechanical, thermal only and thermo-mechanical loading. The results indicate that during the thermal tests (1 – 3 cycles), a small upwards residual displacement was observed and pile head movements ranged between about 90% and 100% of the free expansion of the pile shaft above the shallowest helix, suggesting that the helices fixed the shaft and little restraint was offered by the surrounding soil. In the thermo-mechanical tests (30 thermal cycles), the pile head developed irrecoverable settlement as a function of the number of helices (more helices, less settlement) and initial load (higher load, greater settlement). No significant alteration in pile axial stiffness or resistance was found for piles with zero mechanical load that underwent only a few thermal cycles; however, an increase in stiffness and resistance, beyond that due to inherent variability in the test setup, was observed for piles with an initial load and following a large number of thermal cycles. The testing of thermally-activated helical piles in sand has confirmed that the response is similar to conventional piles and that thermal ratcheting effects can be managed by the application of suitable margins of safety in design and/or the use of multi-helix piles.