Time-dependent interaction coefficients to quantify the settlement of energy pile groups

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The use of energy piles is facing an exponential growth due to the increasing need of exploiting sustainable energy sources. Their design implies the estimation of the displacement induced by the combined thermo-mechanical interaction between piles belonging to the same group. In this regard, the work at hand derives sets of interaction coefficients for piles subjected to thermal load in not stationary conditions, using finite difference numerical analyses. These involved pile pairs embedded in different soil types at multiple spacing, namely 2, 4, 6, 8, 10 times the pile diameter. A new dimensionless parameter is also introduced to describe the evolution with time of the interaction coefficients accounting for pile-to-pile spacing, soil thermal diffusivity, pile-soil stiffness ratio and time. The numerical results demonstrate a strict time-dependence of the interaction coefficients that is worthy to be considered in the design practice. To this aim a practice-oriented approach is proposed allowing to evaluate the interaction effects and thereby derive the settlement of each pile in a straightforward manner. This requires as only ingredient the settlement of the isolated energy pile at the considered time instant. The simplified approach is successfully validated against full 3D thermo-mechanical numerical analyses on pile groups including conventional and energy piles with different layouts.