Pile Groups Under Long-Term Cyclic Thermal Loading-Paper

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The study uses three-dimensional nonlinear finite-element analyses to examine mixed pile groups containing six thermal piles and three mechanical piles under 50 cycles of heating and cooling. It evaluates two pile layouts, 4 m and 5 m spacing, and medium-dense Ottawa and Toyoura sands; the numerical model is checked against published field results.<br /><br />Repeated thermal expansion and contraction substantially alter load–displacement behavior. Thermal piles uplift, and this movement can also cause uplift in mechanical piles. Displacements and axial loads vary considerably among pile-cap connections, pile heads, and pile bases, with effects depending on pile layout, spacing, and soil type. By the 50th cycle, pile-base uplift increases and compressive base loads decrease. Axial stresses also become non-uniform along pile depth and change with successive cycles.<br /><br />Greater pile spacing generally reduces pile interaction and thermo-mechanical stresses, although it can increase pile-cap or pile-head uplift. Toyoura sand produces greater pile-cap uplift than Ottawa sand, while stress differences between the soils vary by pile type, layout, and cycle stage. Groups with mechanical piles on the diagonal show particularly uneven load distribution.<br /><br />The findings highlight that thermal and mechanical piles cannot be assessed independently: cyclic behavior in thermal piles affects the mechanical piles and the group’s overall load sharing. Reliable design should account for long-term cycling, pile-base response, layout, spacing, and soil conditions.

Keywords

thermal piles

mechanical piles

finite-element analysis

cyclic heating and cooling

pile group interaction

load-displacement behavior

pile spacing

Ottawa sand

Toyoura sand

axial stress distribution