Energy pile provides a mean to reduce energy consumption for space heating and cooling, while functioning as a support for superstructure. Despite of the environmental benefits of energy pile, some countries are still reluctant in implementing energy pile. This is because of knowledge gap on the influence of temperature cycles on energy pile ultimate and serviceability limit states. This research aims to fill the knowledge gap by conducting centrifuge and numerical modelling of floating energy piles embedded in saturated dense sand. Factors such as construction effects, pile length and radius, magnitude of working load, and thermal cycle amplitude are considered.

Three series of centrifuge model tests were conducted in saturated dense Toyoura sand. In the first series, three mo...[ Read more ]

Energy pile provides a mean to reduce energy consumption for space heating and cooling, while functioning as a support for superstructure. Despite of the environmental benefits of energy pile, some countries are still reluctant in implementing energy pile. This is because of knowledge gap on the influence of temperature cycles on energy pile ultimate and serviceability limit states. This research aims to fill the knowledge gap by conducting centrifuge and numerical modelling of floating energy piles embedded in saturated dense sand. Factors such as construction effects, pile length and radius, magnitude of working load, and thermal cycle amplitude are considered.

Three series of centrifuge model tests were conducted in saturated dense Toyoura sand. In the first series, three model energy piles made of three different materials were cooled 13 ºC below the ambient temperature, before being loaded to failure. For the second series, four model energy piles, also made of different materials, were subjected to seven thermal cycles, with amplitude of more than 7 ºC. The materials chosen had different coefficients of thermal expansion. It was intended to simulate energy piles with different equivalent diameter, as well as thermal cycle amplitude. By choosing invar, which has a coefficient of thermal expansion 8 times lower than concrete, as one of the material, the shearing, compression and extension induced from expansion and contraction of energy pile can be isolated from thermal-induced volumetric change of soil. In the third series, two model energy pile, one driven and one wished-in-place were subjected to five thermal cycles. This was to investigate the effects of pile driving on energy pile performance, which has yet to be considered in the current practice. The data from centrifuge tests were then back-analysed, and parametric studies were conducted. Hardening soil model with small strain stiffness was adopted for the numerical simulations. Results of parametric study results were used to produce design chart based on the ultimate and serviceability limit state criteria from Eurocode 7 (EC 7) design standard.

From the first series of tests, it was found that initial cooling caused a reduction in energy pile capacity, which was reflected by the pile settlement. The reduction in pile capacity was due to reduction in shaft resistance hence pile had to settle more to pick up more resistance from the pile toe. Cooling was also found to induce negative shaft resistance at the lower part of the pile, and positive shaft resistance at the upper part of the pile. The neutral plane was located around 2/3 of the pile depth. From the second and third series of tests, it was found that replacement energy piles underwent ratcheting settlement, but at a reducing rate, when subjected to thermal cycles. The magnitude of settlement is highly influenced by the magnitude of working load applied on the energy pile, as well as thermal cycle amplitudes. By reducing the working load by 25% (from 800kN to 600 kN), the settlement magnitude was reduced by 60% (from 4.4% to 1.6%). In contrast, displacement energy pile underwent ratcheting heave (0.4% D) instead of settlement. This is because of the densification effects of surrounding soil during pile driving, which made the soil to have dilative response when sheared (thermal-induced shearing). From the parametric study, it was found that when the thermal cycle amplitude is kept within ± 10 ⁰C, conventional factor of safety (FoS = 2.6) is sufficient to satisfy the criteria listed in EC 7. However, for higher thermal cycle amplitude (± 20 ⁰C), a higher factor of safety (FoS = 4.0) is needed.