Energy and geotechnical behaviour of energy piles for different design solutions

Numerical study of the response of a group of energy piles under different combinations of thermo-mechanical loads

Numerical modelling of energy piles in saturated sand subjected to thermo-mechanical loads

https://escholarship.org/content/qt658223s7/qt658223s7.pdf?t=p65ja1

Energy geotechnics: Advances in subsurface energy recovery, storage, exchange, and waste management

Energy piles, a new type of heat exchanger that serves dual purposes, have gained increasing attention due to the growing energy demand and corresponding carbon emissions. Depending on intended operational requirements or by accident, energy pile groups may be subjected to cyclic and variable thermal loadings. This study presents the results from a series of full-scale field tests of a 2×2 energy pile foundation. The energy pile group was operated partially or fully during the tests to investigate the potential effects of cyclic nonuniform thermal loadings. The results indicated that the cyclic thermal loadings could induce an increase in compressive stress of piles at the end of the experiments. Further comparative analysis showed that the residual compressive stress was attributed mainly to the drag-down effects of the surrounding soil. The compressive stress induced by drag-down effects will overlap with the thermally induced stress, and thus lead to a more significant effect on the energy piles. If the drag-down effects are presented, the stress change may exceed the theoretical upper bound of thermal stress and may be underestimated. In addition, cyclic nonsymmetrical thermal loadings could induce accumulation of differential settlement and group tilt in the energy pile group. It was shown that the group tilt caused by cyclic and variable thermal loadings can be acceptable for general engineering structures. The group tilt will not accumulate further when subjected to symmetrical thermal loadings. A reasonable arrangement of operational piles still can be used to avoid unwanted group tilt generation. 

Group Performance of Energy Piles under Cyclic and Variable Thermal Loading

Geothermal energy piles in sand subjected to cyclic thermal loading with fifty thermal cycles and constant mechanical axial load are analyzed numerically in the present work using the nonlinear transient finite element analysis method. The resulting soil–structure interaction is investigated through these analyses. The stress–strain response of sand is simulated herein using the Mohr–Coulomb constitutive model. Mechanical behavior of energy piles is simulated using the concrete damage plasticity model. Analyses are carried out for floating and end bearing piles in both loose and dense sands subjected to different constant axial load magnitudes at the pile head. Parametric sensitivity studies are performed for different amounts of temperature variation on the pile. It is observed from the results that the mechanical load governs the response of the piles when the piles are subjected to higher mechanical loads close to the limit load value on the pile. However, at low to moderate axial load magnitudes, thermal expansion of the pile causes uplift of the piles. Due to pile uplift, negative shear forces are generated at the pile–soil interface near pile head. The radial strains induced in the pile due to thermo-mechanical loading are observed to be higher than that induced due to only mechanical loading. The axial stress in the piles also increases when pile is subjected to heating.

Cyclic Thermo-Mechanical Analysis of Energy Piles in Sand

The thermomechanical response of geothermal energy pile groups in sand was studied in the present work by using a three-dimensional nonlinear finite-element analysis procedure. A raft was considered on the piles. The stress-strain responses of the piles and raft were considered linear elastic. The stress-strain response of sand was reproduced by using the state parameter-based constitutive clay and sand model (CASM). The CASM was implemented in finite-element software through a user-defined material subroutine. The geothermal energy pile group was analyzed for different combinations of thermal and nonthermal piles in the group in a single layer of sand. Analysis results were studied for the displacement at the base of the pile and the axial stresses in the pile. Parametric sensitivity studies were performed by varying the spacing between the piles. It can be observed from the results that the geothermal piles, when subjected to heating, exhibit higher displacement and axial stress than the piles u...

Thermomechanical Response of Geothermal Energy Pile Groups in Sand

The present work investigates the behaviour of geothermal energy piles in sand subjected to thermal loading and the resulting soil-structure interaction, numerically using the finite element software Abaqus and user-defined material subroutines for soil. The stress-strain response of sand has been simulated using CASM constitutive model based on critical-state soil mechanics. Detailed parametric sensitivity studies have been carried out to understand the effects of different end conditions of the pile, relative densities of the soil, coefficients of lateral earth pressure of the ground, lengths and diameters of the pile, thermal loads, coefficients of friction at the pile-soil interface, critical-state friction angles of soil, thermal conductivity of soil, specific heat of soil and thermal conductivity of the pile on the stress response of soil, deformation of the pile and soil, and strains in the pile. The results show that negative shear stress is generated in the soil at the pile-soil interface. In the...

Thermal analysis of energy piles in sand

Summary

A new method of analysis of piles in multi-layered elastic soil subjected to a torque at the head is developed. The differential equation governing the angle of twist in the pile is derived using the variational principles of mechanics. The method of initial parameters is used to obtain closed-form solutions of the angle of twist and torque in the pile as a function of depth. The inputs required for the analysis are shear moduli of pile and soil, pile geometry and thickness of soil layers. Copyright © 2013 John Wiley & Sons, Ltd.

Rajni Saggu

Papers

Cyclic Thermo-Mechanical Analysis of Energy Piles in Sand

Thermomechanical Response of Geothermal Energy Pile Groups in Sand

Thermal analysis of energy piles in sand

Analysis of pile subjected to torsion in multi-layered soil

Settlement Response of a Geothermal Energy Pile Group in Sand