# A Numerical Study of the Behavior of Micropile Foundations under Cyclic Thermal Loading
## 4 Conclusions  
There is a lot of interest in the potential for incorporating shallow geothermal energy systems within building retrofits, with one of the key concerns for this application being how to provide cost effective and efficient ground coupling. The work presented here addresses the possible use of thermally-activated micropiles for this purpose, and specifically, how cyclic thermal loading impacts on the pile load-transfer performance. In the work presented, a single micropile was analyzed with a simplified numerical model in terms of the ground profile and soil material behavior, and the effect of pile-soil-pile interaction within groups of micropiles was examined based on a unit cell approach (by reducing the radius of the analysis domain). Based on the work presented, the following observations and conclusions have been drawn:  
* At the applied service load, which was based on a factor of safety of 2 on shaft resistance, the mobilization factor, M, describing the proportion of shaft resistance mobilized relative to the maximum available shaft resistance, was around 50%. In the cases involving reduced pile spacing, the pile-soil-pile interactions led to M reducing further by about 5%. In an associated study examining large diameter piles, it was established that having M less than 80% was necessary to ensure the response to thermal loading was quasi thermo-elastic;
* Under cyclic thermal loading, an irrecoverable settlement developed in the first few cycles; the values were small, however, and are unlikely to be important in design terms. This initial settlement occurred in the first cooling cycles and was due to reductions in horizontal stress and, thus, available shaft resistance, as the pile contracted. The mobilized shaft and base resistances quickly adjusted to rebalance this effect and subsequent pile head displacements were thermo-elastic. The associated thermal stress changes also quickly stabilized having been modified by the initial alterations in the mobilized shaft and base resistance, discussed above. This stable response was due to the low initial mobilization (M < 50%) of the available shaft resistance;
* This study highlights another important issue related to the slenderness and axial stiffness of the TA pile; in this case, the initial response of the micropile to the thermal cycles resulted in large compression stresses being locked-in, which in the case of an isolated pile, led to stresses in excess of the value for perfect thermal restraint--in this case, about 60% higher. This effect should be carefully considered when long, slender TA piles are planned to be used;
* The results presented from this study illustrate the potential for using micropiles in SGE systems. It is clear that with a design based only on the shaft resistance and factors of safety typically used for micropile design (around 2.0), the thermo-mechanical response should be stable in the long term. Again, if long, slender piles are to be used, then attention should be paid to the thermal stress changes that could develop and, in all cases, especially in closely spaced groups, cyclic pile head movements should be verified against the allowable movement limits specified for the project. These findings are not applicable for micropiles used as settlement reducing type piles, with high initial mobilization of the available shaft resistance;
* The introduction of a surface temperature boundary condition representing an air-conditioned building supported by a pile foundation, resulted in an axial thermal tension load developing at the pile head and elsewhere within the pile body. This was due to the general warming of the ground near the surface which created an underlying tensile response, onto which was superimposed additional thermal tension due to cyclic thermal operation of the micropile.