Effects of soil thermo-plasticity on the performance of floating energy pile groups

Abstract

Energy pile is a green technology widely used for space cooling and heating in recent years. Significant temperature changes in clay, sand, and clay-sand mixtures often occur in energy pile foundations. The thesis aims to model and quantify the effects of soil thermo-plasticity on the performance of floating energy pile groups.

In this study, a unified thermo-mechanical bounding surface (UTMBS) model is proposed for saturated clay and sand. Based on the thermal effects on the isotropic compression line, the model proposes a new unified thermal softening relationship and a plastic modulus for clay and sand. The unified model considers the thermal effects on the critical state line, size and shape of the bounding surface, accounting for both drained and undrained shearing of clay and sand at different temperatures. In addition, a memory surface is used to account for the thermal cyclic behaviour of clay and sand. The non-linear elasticity relationship represents the hysteresis loops of the stress-strain relationship in the mechanical cycles. The constitutive model is implemented in finite element (FE) code to investigate the THM interactions of the floating energy pile group and pile raft. Computed results are compared with published centrifuge model test results in soft clay. Parametric studies are performed to study the effects of energy pile layout, pile spacing, and raft contribution to improve the design guideline.

It is revealed that the irreversible volumetric contraction of the soil adjacent to the energy piles accumulates with each thermal cycle, resulting in a decrease in the horizontal stress and hence shaft resistance of the floating piles. During thermal cycles, the stress states of the soil around the energy pile shaft and the soil beneath the pile toe approach the critical state line along different paths. The induced temperature in the soil adjacent to the non-energy pile (NEP) is 5℃ lower than that in the soil at the energy piles, Consequently, the induced excess pore pressure in the soil at the NEP is approximately 20% smaller than that in the soil at energy piles. The irreversible volumetric soil contraction at the NEP is about half of that at non-energy piles, resulting in approximately 45% less toe settlement. In addition, the layout of energy piles in the group has a large influence on the thermally induced load transfer. It is suggested the energy piles could be placed at the corner of the pile group. In this case, the transfer of thermally induced pile head load can be minimized. The ratcheting settlement of energy piles in soft clay can be reduced by using a wide-spacing energy pile group. By using a pile raft, the magnitude of thermally induced settlement and ratcheting group tilting can be reduced.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Charles Wang Wai NG (Supervisor)