# A Numerical Study of the Behavior of Micropile Foundations under Cyclic Thermal Loading
###### Abstract  
Micropiles are small-diameter foundation elements that are widely used in building refurbishment to reinforce existing foundations or provide new foundations where access for construction is difficult. Thermally-activated (TA) micropiles could be useful as an efficient means of providing cost-effective ground-coupling when shallow geothermal energy systems are considered in building rehabilitation. It is well-established that thermal activation of pile foundations results in thermomechanical interactions between the pile and the surrounding soil. These thermally-induced effects need to be examined to ensure that they do not adversely impact the load transfer function of the micropile. Numerical analysis is able to produce reliable predictions of thermo-mechanical behavior of TA piles, and this study applied this technique to examine the cyclic thermal behavior of micropiles, isolated and in groups. For the situations considered in this study, it is shown that during cyclic thermal activation, irrecoverable movements are unlikely to be significant in design terms, if the initial mobilization of the shaft resistance is low. Though stable, cyclic thermal movement amplitudes are large enough that they should be considered in design. The study highlights that large changes in thermal stress can develop and be locked-in to the response of long flexible piles, and that these should be verified in design. Further, as pile spacing reduces, thermal interference results in a loss of heat exchange capacity per pile, which has to be considered in the design of large groups of TA micropiles. Therefore, TA micropiles can offer an efficient and secure means of providing ground coupling in shallow geothermal energy systems. shallow geothermal energy; micropile; foundations; finite element analysis; heating and cooling; thermo-mechanical analysis Article  
A Numerical Study of the Behavior of Micropile Foundations under Cyclic Thermal Loading  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], D.  
concrete, driven steel tube and drilled TA micropile systems, respectively, and [7] evaluated various construction details for underpinning projects incorporating TA micropiles. Micropiles are defined as bored (replacement) piles with diameters less than 300 mm and driven (displacement) piles with diameters less than 150 mm [8]. Four basic types of bored micropile are used: Type A, where the growth filling the pile bore is placed under gravity alone; Type B, low-pressure grouted piles; Type C, high-pressure grouted piles; and Type D, high pressure repeatedly and selectively grouted piles [9]. In this study (and others discussed below), only micropile Types A and B have been considered. The construction of Type C and D micropiles involves high-pressure grouting which may preclude the possibility of thermally activating these pile types. Additionally, the expanded grout bodies and/or irregular grout permeation associated with high pressure grouting complicates the analysis of Type C and D bored micropiles. Due to the small dimensions of the equipment used in their construction, micropiles are particularly well suited for situations where access is difficult, e.g., when the foundations of an existing building require strengthening during rehabilitation. Although it is possible to install them to greater lengths, micropiles typically have lengths in the range of 15 to 30 m [9; 10]. Although considerable research has been undertaken to develop a better understanding of the mechanisms of behavior of TA piles to provide assurance regarding the efficacy and safety of the application in SGE systems [11; 12; 13; 14], only a few studies refer specifically to micropiles [15; 16]. Indeed, of 26 field tests reviewed, just 9 involved TA micropiles [17; 18; 19; 20; 21; 22; 23; 24; 25]. Of these studies, the thermo-mechanical response of TA micropiles was only reported by [17] (bored), and [24; 25] (driven), and in all three cases, the piles were installed in fine-grained soils.