A practical method to design thermally stressed piles

The paper deals with the analysis and design of energy piles subjected to a cyclic temperature variation under constant mechanical axial load. The problem is first investigated through fully coupled thermo-hydro-mechanical analyses using the finite-element code ABAQUS. A single pile, whose head is either free or restrained, is embedded in a normally consolidated clay obeying different constitutive models. Numerical results are reported and discussed with emphasis on the role of the different constitutive assumptions on the development of the pile axial force due to the thermal load. Further, a design procedure is proposed to derive the maximum thermally induced axial load by using recently developed analytical solutions, worked out to obtain explicit formulae that can be readily employed in a spreadsheet for the direct computation of axial force in a multilayer soil. The method consists of employing these expressions after identifying some rules to: (a) select the soil stiffness profile as a function of the mechanical load and (b) establish the effect of the actual restraint condition at the pile head as a function of the stiffness ratio between the inactive and the active pile subgroups. The main advantage of the proposed procedure is that it requires only the load-settlement curve under the mechanical axial load, which is always assessed in pile design, through analytical, numerical or experimental means. After a comparison with the results of the finite-element analyses, the procedure is also applied to a number of field tests on groups of energy piles available in the literature, and provides a satisfactory match between measurements and predictions.