# A Numerical Study of the Behavior of Micropile Foundations under Cyclic Thermal Loading
###### Abstract
The observations undertaken by [17] regarding tension cracks forming in the grout body due to mechanical load-induced strain shed light on the significance of tension cracking. This concept was further reinforced by the work of [33], who demonstrated that tension cracks in the pile grout body can substantially mitigate the thermal axial load when subjected to tensile loading, resulting in minimal shaft restraint and limited impact on the pile-soil interface. In a different vein, the field tests conducted by [24] and [25] delved into the thermomechanical behavior of a group of steel-pipe micropiles, offering a distinct perspective on heat exchange efficiency. The field tests described by [24] and [25] monitored the thermomechanical response of a group of eight steel-pipe micropiles which were used to underpin an existing 600 mm diameter, 8 m long pile. Within the micropile group, two of the eight piles were instrumented and thermally activated. [PERSON] et al. (2020) [24] reported that the heat exchange efficiency of the steel-pipe TA micropiles used in the tests decreased as the number of thermal cycles increased. They also showed that the heat exchange efficiency under summer conditions was higher than in winter conditions, and confirmed that an intermittent mode of operation delivered better heat exchange efficiency than the continuous mode. Heat exchange per unit length was comparable to the values quoted earlier. The maximum axial thermal stress in the micropiles slightly increased with the number of cycles under the intermittent operation mode and a residual thermal stress was recorded when the temperature returned to its initial value. Both the reduction in heat exchange efficiency and the residual stress generation could perhaps be attributed to an accumulation of heating or cooling with time, i.e., the pile and soil were not returning to ambient conditions during the recovery periods. Building upon these findings, [25] validated a finite element model against experimental outcomes, shedding light on the intricate interplay between heat exchange potential and the configuration of thermally-activated micropile groups. As more piles within the group were thermally activated, the total heat exchange power increased, although the average heat exchange power per pile diminished due to thermal interference among adjacent piles. The specific problems addressed by this study pertain to the limited understanding surrounding the thermal and thermo-mechanical performance of micro-piles, thereby imposing potential constraints on their applicability in building developments, including refurbishments. This research aims to investigate these issues comprehensively through numerical analysis. The primary focus lies in scrutinizing the thermo-mechanical response of micropiles, both as single (isolated) installations and in group configurations, under compressive loading conditions. By undertaking this examination, the study seeks to illuminate any disparities in thermo-mechanical behavior compared with their larger-diameter counterparts, which were investigated in a parallel study [34].