Author: Cui, Sheqiang
Title: Thermo-mechanical behaviour of energy piles in unsaturated silt
Advisors: Zhou, Chao (CEE)
Yin, Jianhua (CEE)
Degree: Ph.D.
Year: 2023
Subject: Piling (Civil engineering)
Soil mechanics
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xxiii, 295 pages : color illustrations
Language: English
Abstract: Energy pile is an environment-friendly and efficient technology for harvesting shallow geothermal energy. It has received much attention during the past two decades. Most of the previous studies focused on the thermo-mechanical behaviour of energy piles in saturated soils. However, water tables are very deep in many regions. Energy piles in the unsaturated ground have been widely reported worldwide, such as Beijing in China, Melbourne in Australia and Colorado in the United States. An in-depth study is needed to understand the behaviour of energy piles in unsaturated soils.
Given this background, the major objectives of this study are to (i) reveal and model the coupled effects of soil moisture and stress state on the thermal conductivity of saturated and unsaturated soils, which is an important input in the analysis of pile thermal performance; (ii) investigate the coupled effects of temperature and suction on the soil-pile interface behaviour and develop an advanced constitutive model for it; (iii) study the thermo-mechanical performance of energy piles in different roughness and suction conditions through physical model tests. To meet these objectives, four interrelated investigations were carried out.
A new apparatus based on the thermal needle probe was developed for testing thermal conductivity. It was used to conduct a comprehensive test program for revealing the influence of porosity, degree of saturation, stress level and soil type. Particularly, the understanding of the influence of stress has been improved. With a stress increase from 0 to 1200 kPa, the thermal conductivity increases by 60% for kaolin clay, 25% for silt (completely decomposed granite) with 85% degree of compaction (DOC), 20% for the silt with 95% DOC, 10% for Toyoura sand with an initial void ratio of 0.76 and 7.5% for the sand with an initial void ratio of 0.60. The observed increase in thermal conductivity is attributed to different mechanisms, including a reduction of the void ratio and a change in inter-particle contact. These two mechanisms are relatively more important for clay and sand, respectively.
Based on the thermal conductivity results, a semi-empirical equation was newly proposed to model the thermal conductivity of saturated and unsaturated soils. It considers stress effects on the void ratio and inter-particle contacts. As compared to existing models, it has two major improvements: (i) it can well capture the influence of stress on thermal conductivities of various soils; (ii) it is able to capture the hysteresis of stress-thermal conductivity relation during the loading and unloading processes. It was then utilized in a finite element code to compute the heat exchange rate between the energy pile and the ground. The computed results indicate that the heat exchange rate is underestimated if stress effects on soil thermal conductivity are not considered. Taking energy pile of 0.6 m in diameter and 50 in aspect ratio as one example, the underestimation is up to 18%, 13% and 2% for the clay, silt and sand grounds, respectively.
To study the thermo-mechanical behaviour of the soil-pile interface, a new temperature- and suction-controlled direct shear device was developed. Two types of tests (i.e., constant-temperature shearing and constant-stress heating-cooling) were carried out at various temperatures, net normal stresses and suctions. The results show that temperature can have a minor impact on the friction angle, whose value at 42°C is smaller by about 2.2° than that at 8°C, likely because heating can increase the void ratio in the shear zone. More importantly, the interface strength increases nonlinearly with increasing suction, and the incremental rate is temperature-dependent. Heating the interface at a net normal stress of 50 kPa reduces this incremental rate due to surface tension reduction. In contrast, this incremental rate increases at a net normal stress of 150 kPa with the same temperature increment, probably because the heated specimen has more small-size pores due to thermal contraction and more menisci water lenses, whose influence outweighs the effects of surface tension. For the constant-stress heating and cooling tests, irreversible relative movement occurs during cooling. This is most likely due to the thermally induced contraction of soil particles, which could lead to the collapse of force chains.
A thermo-mechanical constitutive model was newly developed based on the bounding surface plasticity framework to predict the thermo-mechanical behaviour of saturated and unsaturated interfaces. Some new formulations were proposed to model the critical state void ratio and shear strength. Comparisons between measured and computed results suggest that this model can well capture the coupled effects of temperature, suction and net normal stress on the shear behaviour of interfaces at various suctions, stresses and temperatures.
Furthermore, a small-scale physical model was set up to investigate the thermo-mechanical behaviour of energy piles. Several series of tests were carried out with considering the effects of soil suction and interface roughness. It is observed that the bearing capacity of an energy pile increase with increasing suction and roughness but decreasing temperature. A suction increment can increase both shaft and toe resistance, while temperature and roughness mainly affect the shaft resistance. During cyclic heating and cooling, suction and roughness increment reduces the irreversible pile head settlement, due to the increment of shaft resistance. These findings are explained based on the results of thermal conductivity and interface shear behaviour, obtained from the above tests.
It is anticipated that the experimental and theoretical results from this study are useful for researchers and engineers to investigate the thermal efficiency and mechanical performance of energy piles in different ground conditions.
Rights: All rights reserved
Access: open access

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