Coupled Numerical Analysis of Variations in the Capacity of an Energy Pile in Clay Soil

摘要

Energy piles are an emerging alternative for the reduction of energy consumption to heat and cool buildings. The majority of the research to date focuses on thermodynamic properties or axial strain of piles. This paper concerns the effects of temperature fluctuation on the capacity of driven energy piles in clayey soils. Consolidation of clay (due to the dissipation of excess pore-water pressure in clay) surrounding piles affects the capacity of the pile (pile setup in clay). Reciprocally, it is reasonable to assume that heating and cooling periods will create or relax the excess pore-water pressure (EPWP) in clayey soils (due to the contraction and expansion of water) affecting the pile capacity. In the meantime, the thermal expansion and contraction of the pile also generates or relaxes the EPWP in the soil, which can be computed using the principle of cavity expansion theory. The resulting change in the EPWP due to the thermal cycle happens through day and night and is much faster than the drainage time required by the clay. This lack of time for dissipation of EPWP affects the effective stress and, hence, the capacity of the pile.Hence, there is a need for an analysis that couples: (i) consolidation of clay, (ii) thermal expansion/contraction of pile and, in turn, the resulting relaxed/generated EPWP in clay, and (iii) thermal expansion/contraction of soil and, in turn, the resulting generated/relaxed EPWP in clay computed through cavity expansion theory. Therefore, a numerical analysis model was developed to analyze the generation and dissipation of the EPWP resulting from all of these phenomena. This coupled analysis helps compute the variations of the generated EPWP and, in turn, its effect on the effective stress and, hence, pile capacity.The numerical model also supports the hypothesis that the heating effect can accelerate the pile setup in clay. This flexible model can be used to estimate the ultimate capacity of energy piles, or even evaluate the possibility of using heating through construction to expedite pile setup in clay.