The energy pile is a combination of the borehole system and the traditional pile to use the shallow ground geothermal energy. Previous research shows that when subjected to thermal cycles single floating energy piles experience a ratchetting accumulation of irreversible settlement at a reducing rate. This phenomenon was further explored in this study via both finite element modelling and centrifuge modelling, considering the effect of the pile length. The study was then extended to that of floating energy pile groups subjected to non-symmetrical cyclic thermal loading using centrifuge modelling. Non-symmetrical cyclic thermal loading can potentially induce group tilting, and thus is more critical compared to symmetrical cyclic thermal loading. To quantify the group tilting, cent...[ Read more ]

The energy pile is a combination of the borehole system and the traditional pile to use the shallow ground geothermal energy. Previous research shows that when subjected to thermal cycles single floating energy piles experience a ratchetting accumulation of irreversible settlement at a reducing rate. This phenomenon was further explored in this study via both finite element modelling and centrifuge modelling, considering the effect of the pile length. The study was then extended to that of floating energy pile groups subjected to non-symmetrical cyclic thermal loading using centrifuge modelling. Non-symmetrical cyclic thermal loading can potentially induce group tilting, and thus is more critical compared to symmetrical cyclic thermal loading. To quantify the group tilting, centrifuge tests of 2 by 2 floating energy pile groups with a rigid pile cap were conducted in saturated Toyoura sand with a relative density around 65%. To create a condition of non-symmetrical thermal loading, only one or two piles of the groups were applied 15 two-way thermal cycles with an amplitude of about 10 °C. Throughout the test, a constant vertical working load was maintained. Effects of the raft-soil interaction and the working load level on the group tilting were studied.

The experimental results of single floating energy piles in sand show that the thermally-induced irreversible pile settlement decreases as the pile length increases. This is due to the effect of the finite soil depth. The numerical modelling can reproduce the ratcheting settlement behaviour of single floating energy piles under thermal cycles. While it predicts an opposite trend to the experimental results in terms of the effect of the pile length. Regarding the performance of floating energy pile groups under non-symmetrical cyclic thermal loading, the experimental results show that the group tilting also evolved in a ratcheting pattern. As the working load increases, the accumulated irreversible group tilting can exceed the design value for serviceability limit state suggested by EN 1997-1 (2004). Compared to the elevated pile groups, the soil-raft interaction helps to reduce the group tilting as the contact pressure provides additional restoring moment. Accompanying the group tiling, the axial load sustained by non-energy piles was observed to increase by about 40% for the test with one energy pile under the working load of 9500 kN (FoS ≈ 2.0). The induced extra bending moment was about 35% of the yield moment of the cross section. Both should be properly accounted for in practice.