Experimental and Numerical Study on the Behavior of Energy Piles Subjected to Thermal Cycles

Abstract

A laboratory-scale model test is conducted to improve the understanding of the effects of thermal cycles on the mechanical behavior of energy piles. The model pile is composed of cement mortar and dry sand with a relative density of 30% is used for the model ground. After applying the working load to the pile head, the pile is subjected to three thermal cycles with a magnitude of 15°C. The measured temperature response and mechanical behavior are analyzed and used to validate the proposed numerical approach. In the numerical analysis, the temperature variation due to thermal cycles is calculated using uncoupled heat transfer analysis. Then, the computed temperature field is used as the boundary condition in the sequence stress analysis. A series of numerical sensitivity analyses are carried out using the sequentially coupled method to investigate the long-term performance of energy piles under different soil and pile head restraint conditions. The numerical results suggest that the restraint condition at the pile head plays an important role in the mechanical response of energy piles. The ultimate pile resistance after thermal cycles does not decrease significantly. The accumulation of settlement of the free head pile and the reduction in the axial force of the restrained head pile should be considered in the design.

Numerical analysis of energy piles in a hypoplastic soft clay under cyclic thermal loading

“…The behaviour of the soil‐energy pile‐structure system under normal operating conditions—that is, subjected to thermal cycles—has been mainly investigated experimentally by laboratory or field tests on instrumented energy piles 22–35 …”

“…observed that a single energy pile subjected to heating and cooling cycles undergoes an accumulation of irreversible axial displacements at the pile head, progressing at an almost constant rate, while the soil experiences an increase and decrease of pore water pressures during heating and cooling stages, respectively, with progressively smaller increments of excess pore water pressure as the number of thermal cycles increases. These two studies represent the main experimental contributions concerning the cyclic behaviour of energy piles in soft clay in normal operating conditions, while there is a dearth of corresponding numerical studies 38,39 as the main contributions are mainly related to overconsolidated clay and sandy soils 27,40–43 …”

“…These two studies represent the main experimental contributions concerning the cyclic behaviour of energy piles in soft clay in normal operating conditions, while there is a dearth of corresponding numerical studies 38,39 as the main contributions are mainly related to overconsolidated clay and sandy soils. 27,[40][41][42][43] The present work is a numerical study of the main phenomena controlling the cyclic behaviour of energy piles installed in soft clays, with particular reference to the pile performance in terms of accumulated settlements. Non-linear, fully coupled thermo-hydro-mechanical Finite Element simulations were performed on a single ideal energy pile installed in a normally consolidated (NC) clay layer, whose mechanical behaviour was described using an advanced inelastic constitutive model capable of capturing the main aspects of the coupled thermo-mechanical behaviour of the soil under cyclic mechanical and thermal loading conditions.…”

“…21 The behaviour of the soil-energy pile-structure system under normal operating conditions-that is, subjected to thermal cycles-has been mainly investigated experimentally by laboratory or field tests on instrumented energy piles. [22][23][24][25][26][27][28][29][30][31][32][33][34][35] An experimental study conducted on small scale models tested in a geotechnical centrifuge at 40g 36 has shown that an energy pile installed in lightly overconsolidated (OC) clay accumulates larger irreversible settlements if compared to a pile in heavily overconsolidated clay. In a series of small-scale pile loading tests at 1 g, Wu et al 37 observed that a single energy pile subjected to heating and cooling cycles undergoes an accumulation of irreversible axial displacements at the pile head, progressing at an almost constant rate, while the soil experiences an increase and decrease of pore water pressures during heating and cooling stages, respectively, with progressively smaller increments of excess pore water pressure as the number of thermal cycles increases.…”

Small-scale cyclically thermally-activated pile under inclined mechanical loads

“…The works of Vitali et al [18] and Zhao et al [19] could be considered as the only available studies investigating the influence of monotonic heating and heating-cooling cycles on the flexural behavior of horizontally-loaded energy piles, respectively. Several studies have been performed to assess the behavior of axially-loaded energy piles during multiple thermal cycles through field scale tests [20][21][22][23][24][25], 1-g small-scale physical model tests [26][27][28][29][30][31][32][33][34][35][36] and centrifuge model tests [37][38][39][40][41]. Dry sand was the most common type of soil used in these small-scale physical model tests.…”