There is an increasing interest and use of energy foundation worldwide. Heat transfer from/to energy piles may cause plastic contraction of soft clay with a low overconsolidation ratio (OCR) during heating and cooling. This can influence pile-soil interaction due to thermally induced excess pore water pressure and changes of lateral stress acting on the pile. These thermally induced phenomena cause additional plastic settlement of the soil and pile. The irreversible settlement may cause serviceability or even ultimate limit state problems for floating energy piles whose capacity is mainly derived from shaft resistance. Due to intended operational needs or by accident, cyclic non-symmetrical thermal loads may be applied to elevated floating energy pile groups. The thermomechanical intera...[ Read more ]

There is an increasing interest and use of energy foundation worldwide. Heat transfer from/to energy piles may cause plastic contraction of soft clay with a low overconsolidation ratio (OCR) during heating and cooling. This can influence pile-soil interaction due to thermally induced excess pore water pressure and changes of lateral stress acting on the pile. These thermally induced phenomena cause additional plastic settlement of the soil and pile. The irreversible settlement may cause serviceability or even ultimate limit state problems for floating energy piles whose capacity is mainly derived from shaft resistance. Due to intended operational needs or by accident, cyclic non-symmetrical thermal loads may be applied to elevated floating energy pile groups. The thermomechanical interaction among non-symmetrical loaded piles is not well understood. Full-scale tests, physical model tests at one Earth’s gravity and elevated gravity in a geotechnical centrifuge and numerical simulations have been widely reported in the literature. Almost all studies have focused on single energy piles (EPs) and pile groups subjected to symmetrical thermal loads, although it is not unusual to have energy pile groups stressed by non-symmetrical thermal loads.

In this study, four series of centrifuge model tests were conducted on the elevated floating energy pile groups subjected to cyclic non-symmetrical thermal loading. Saturated soft Kaolin clay was adopted with an overconsolidation ratio (OCR) of 1.7 at the pile toe since this kind of soil is much vulnerable to temperature changes. The performance of 2 x 2 energy pile groups was investigated under different pile spacing, thermal loading patterns, temperature change amplitudes, and working load levels. Two types of groups were studied with 3D and 5D (D denotes pile diameter) pile spacing to reveal the thermal interaction impact on the thermo-mechanical response of energy pile groups. The different numbers of energy piles representing non-symmetrical thermal loading (1EP, 2EPs, and 3EPs) were chosen in each group to investigate the influence of thermal loading patterns on the group. Since energy pile groups are under thermo-mechanical loading, the response of the energy pile groups was investigated at different thermal and mechanical loading level. For this aim, the groups were imposed by two temperature change amplitudes (ΔT = ±14˚C and ±20˚C) and three working load levels, i.e., the factor of safety (FS = 1.5, 2, 3). Furthermore, numerical modelling of elevated energy pile group was conducted to do back analysis of a centrifuge test and numerical parametric study.

The thermally induced irreversible settlement and tilting in the 3D-pile group with ΔT = ±14˚C exceeded serviceability and ultimate criteria. The thermally induced irreversible settlement and tilting in the 3D-pile group are 200% and 300% larger than those in the 5D one, respectively. Contrary to the 5D-pile group where the serviceability criterion (EN 1997-1, 2004) is satisfied, the settlement and tilting are 30% and 200% larger than the criteria in the 3D one, respectively. It can be recommended that an elevated floating energy pile group with 5D pile spacing should be used in soft clay instead of with 3D spacing at ΔT = ±14˚C. The thermally induced irreversible settlement of the groups with 3EPs is maximum among all the non-symmetrical thermally loaded groups. However, the group tilting with 2EPs is maximum. The temperature changes of ±20˚C, around 50% higher than the typical magnitude of temperature change (±14˚C), induces significantly larger settlement in the groups. The settlement of energy piles with higher temperature change in the group with 2EPs and 1EP exceeds the failure criterion at the 4^th and 6^th cycle, respectively. Similarly, the tilting of the higher thermally loaded energy pile group is more than three times that with the less thermal load. The tilting exceeds the serviceability criterion within the first thermal cycle in the 5D-pile group. The lower working load (higher FS) induces lesser irreversible settlement but larger displacement magnitude in the 5D-pile groups with 2EPs and 3EPs. Similarly, the lower working load (lower FS) induces lesser tilting in the non-symmetrical thermally loaded groups.