According to my knowledge an example for 3U square geothermal piles does not exist. However, the general principles of spatially discretizing geothermal piles are the same as with any other spatial discretization. First of all, the geometrical features of interest must be represented within the finite element mesh. Moreover, domains where sharp contrasts of material properties and where strong gradients of process variables characterize the system need to be sufficiently resolved by the mesh. A smooth gradation between fine refined areas to coarse areas may contribute to a stable numeral solution. Last but not least, all process variables are solved at mesh nodes, while material properties are assigned on mesh elements. Accordingly, a fine spatial discretization within impermeable regions (concrete piles, aquitard) is required to account for its impervious nature. The FEFLOW book (chapter 16.6.4) addresses fully discretized BHE 3D models which could provide some additional hints on how to discretize geothermal installations.
