PERFORMANCE OF GEOTHERMAL ENERGY PILES UNDER THERMO-

AXIAL LOADS

ADRIANA ERICA AMALUDIN

A thesis submitted in fulfillment of the

requirement for the award of the degree of

Master of Engineering (Civil - Geotechnics)

Faculty of Civil Engineering

Universiti Teknologi Malaysia

FEBRUARY 2015

iii

For

My Father, Amaludin Sagi and My Mother, Latifah Yacob

&

My Brothers, Hassanel Zachary Amaludin and Nazrein Adrian Amaludin

For their love, support, encouragement and prayers

iv

ACKNOWLEDGEMENT

Alhamdulillah, praise to Allah the Almighty, the Most Beneficent and the Most

Merciful. I would also like to express my deepest and sincerest gratitude to my

supervisor, Prof. Dr. Aminaton Marto who was always there to impart valuable advice,

constructive comments and to give useful guidance during the research and writing

process for this thesis.

I am thankful to the Ministry of Education and Universiti Malaysia Sabah for

being the benefactor of my Masters studies. Furthermore, I would like to thank the

Ministry of Science, Technology and Innovation, Malaysia for providing the

ScienceFund (No.: 03-01-06-SF1185) for the project entitled “Shallow Geothermal

Energy Pile Bearing Capacity in Clay”. Also, I am indebted to Universiti Teknologi

Malaysia for providing the facilities required to perform the experiments involved in

this study.

I would also like to extend my appreciation to my fellow researchers in the Soft

Soil Research Group (SSERG) of Civil Engineering Faculty, especially to Dr. Nima

Latifi, Mr. Mohsen Oghabi, and Mr. Tan Choy Soon. Additionally, I would like to

thank Mr. Zulkifly and Mr. Hadiyat of the UTM Geotechnical Engineering Laboratory,

who provided assistance during the preparation of the laboratory model tests.

Last but not least, I would like to express my gratitude to my parents and two

brothers who were always there for emotional support and for their prayers.

v

ABSTRACT

The geothermal energy pile system is a sustainable geostructure system

designed to meet the cooling demands of a building by storing excess heat from the

building into the soil, acting as a heat sink. Thermal loads stored in the soil will cause

thermally induced settlement, and this factor must be considered in the geotechnical

design process. This study aims to develop a preliminary geothermal energy pile

laboratory testing system in single gravity condition, to produce the load-settlement

curves generated by thermo-axial load tests. The model energy pile behaviour was

studied using a physical model consist of a soil container (450 mm height and 270 mm

diameter), model pile (15 mm diameter and 150 mm embedded length), and an axial

and thermal load control system provided by a pneumatic cylinder and a temperature

bath, respectively. Axial loads (100 N and 200 N), thermal loads (35˚C and 40˚C) and

combinations of both loads (thermo-axial loads) were applied to the model pile. The

axial load values were chosen based on the ultimate capacity of the model pile in soft

soil (about 0.5 and 0.25qu). The kaolin soil used during model testing was classified as

silt of intermediate plasticity (MI). The model soil was compacted at 80% and 90%

maximum dry density, which were classified as having soft and firm consistency. The

model pile behaviour was monitored with a linear variable displacement transducer,

load cell and wire thermocouple connected to a data logger to monitor the pile head

settlement, applied axial load and pile temperature. The pile response to thermo-axial

loads appears to be thermo-elastic and is attributed to soil consistency and magnitude

of thermal load applied to the pile. Thermal loads induced small settlement values,

whereby the highest value was about 1% of the pile diameter (0.15 mm). In general,

firm soils produce lower thermally induced settlement, due to higher restraint at the

pile head compared to soft soils. For 35˚C thermal loads, the resulting settlement did

not exceed the limiting settlement, which is defined as 10% of the model pile diameter

(1.5 mm). The highest thermo-axial settlement obtained was 1.66 mm for thermo-axial

load of 40C and 200 N (global factor of safety (FOS) of 1.9) in soft soil, and the

settlement at 100 N axial load (global FOS of 3.8) amounts to 1.59 mm. Consequently,

in firm soil and for thermo-axial load of 40˚C and 200 N (global FOS of 2.3), the

thermo-axial settlement is 1.54 mm. To ensure that the thermo-axial settlement does

not exceed the limiting settlement, the recommended global FOS used for soft soil and

firm soil should be more than 4.0 and 2.5, respectively. A laboratory scale model of

an energy pile system, specifically for cohesive soils in single gravity conditions has

been developed. This laboratory model is able to produce reliable thermo-axial load-

settlement curves in varying soil consistencies, initial axial loads and also varying

thermal loads.

vi

ABSTRAK

Sistem cerucuk tenaga geoterma adalah satu sistem geostruktur lestari yang

direka untuk penyejukan bangunan dengan menyimpan haba yang berlebihan dari

bangunan itu ke dalam tanah yang bertindak sebagai sinki haba. Beban haba yang

disimpan di dalam tanah akan menyebabkan enapan terma, dan faktor ini perlu

dipertimbangkan dalam proses reka bentuk geoteknik. Kajian ini bertujuan untuk

menghasilkan sistem pengujian makmal cerucuk tenaga geoterma peringkat

permulaan di dalam keadaan graviti tunggal, bagi menghasilkan lengkuk beban-

enapan yang diperolehi daripada ujikaji terma-paksi. Sifat model cerucuk tenaga telah

dikaji dengan menggunakan model fizikal yang terdiri daripada bekas tanah (450 mm

tinggi dan 270 mm diameter), model cerucuk (15 mm diameter dan 150 mm kedalaman

terbenam), serta sistem kawalan beban paksi dan suhu yang dihasilkan masing-masing

oleh silinder pneumatik dan kubang suhu. Beban paksi (100 N dan 200 N), beban terma

(35˚C dan 40˚C) dan gabungan kedua-dua beban (beban terma-paksi) telah dikenakan

kepada model cerucuk tersebut. Nilai beban paksi yang dipilih adalah berdasarkan nilai

kapasiti muktamad model cerucuk di dalam tanah lembut (kira-kira 0.5 dan 0.25qu).

Tanah kaolin yang digunakan untuk ujian model diklasifikasikan sebagai kelodak

berkeplastikan pertengahan. Tanah model dipadatkan pada tahap 80% dan 90%

ketumpatan kering maksimum dan diklasifikasikan sebagai mempunyai kekonsistenan

lembut dan kukuh. Kelakuan model cerucuk dipantau menggunakan transducer

anjakan linear boleh ubah, sel beban dan wayar termogandinganyang disambungkan

kepada alat pencatat data bagi memantau enapan kepala cerucuk, beban paksi dan suhu

cerucuk. Tindak balas cerucuk kepada beban terma-paksi adalah termo-elastik, yang

bergantung kepada kekonsistenan tanah serta magnitud beban terma yang dikenakan

kepada cerucuk. Beban terma menghasilkan nilai enapan yang kecil, di mana nilai

enapan terma tertinggi adalah kira-kira 1 % daripada nilai diameter cerucuk (0.15 mm).

Secara amnya, tanah kukuh akan menghasilkan enapan terma teraruh yang lebih

rendah daripada tanah lembut disebabkan oleh tahap kekangan yang tinggi di kepala

cerucuk. Bagi beban terma 35˚C, enapan terma dan terma-paksi tidak melebihi enapan

penghad yang ditetapkan sebagai 10% nilai diameter cerucuk (1.5 mm). Nilai enapan

terma-paksi tertinggi adalah 1.66 mm bagi beban terma-paksi 40C dan 200 N, (nilai

faktor keselamatan (FOS) global adalah 1.9) di dalam tanah lembut, manakala enapan

pada 100 N beban paksi (nilai FOS global adalah 3.8) memberi nilai sebanyak 1.59

mm. Selain itu, di dalam tanah kukuh, bagi beban terma-paksi 40˚C dan 200 N (nilai

FOS global sebanyak 2.3), nilai enapan terma-paksi adalah 1.54 mm. Untuk

memastikan bahawa nilai enapan paksi-terma tidak melebihi enapan penghad, nilai

FOS global yang disarankan bagi tanah lembut dan tanah kukuh masing-masing adalah

lebih besar daripada 4.0 dan 2.5. Sebuah model berskala makmal bagi sistem cerucuk

tenaga, khas untuk tanah jeleket di dalam keadaan graviti tunggal telah dihasilkan.

Model ujikaji ini mampu menghasilkan lengkuk beban-enapan bagi beban terma-paksi

di dalam kekonsistenan tanah, beban paksi awal, serta beban terma yang berbeza.

vii

TABLE OF CONTENTS

CHAPTER TOPIC

PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS viii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF APPENDICES xv

LIST OF ABBREVIATIONS xvi

LIST OF SYMBOLS xvii

1 INTRODUCTION 1

1.1 Background of Research 1

1.2 Problem Statement 3

1.3 Aim and Objectives of Research 4

1.4 Scope of Work 5

1.5 Significance of Research 6

1.6 Thesis Outline 7

2 LITERATURE REVIEW 9

2.1 Introduction 9

2.2 Ground Source Heat Pump System 10

2.2.1 Vertical Ground Source Heat Pump 11

2.2.2 Shallow Geothermal Energy Piles 14

viii

2.3 Drilled Shafts Foundation 16

2.4 Energy Pile Design Standards 18

2.5 Energy Pile System Models 18

2.5.1 Single Gravity Model Tests 19

2.5.2 Centrifuge Model Tests 21

2.5.3 Full-Scale Tests 22

2.5.4 Summary of Energy Pile Model Tests 28

2.6 Thermo-axial Response in Energy Pile System 32

3 METHODOLOGY 37

3.1 Introduction 37

3.2 Selection of Materials 40

3.3 Determination of Physical and Engineering

Properties of Soil

41

3.3.1 Sieve Test 42

3.3.2 Hydrometer Test 42

3.3.3 Particle Size Analysis 43

3.3.4 Atterberg Limits Test 45

3.3.5 Specific Gravity 45

3.3.6 Thermal Needle Test 47

3.4 Determination of Mechanical Properties 48

3.4.1 Standard Proctor Compaction Test 48

3.4.2 Unconfined Compression Strength

Test

50

3.4.3 Vane Shear Test 51

3.5 Physical Model of Energy Pile 53

4 PHYSICAL MODELLING 55

4.1 Introduction 55

4.2 Design of Experimental Setup 56

4.2.1 Soil Testing Chamber and Energy Pile

Model Design

56

4.2.2 Fabrication of Loading Frame 58

ix

4.3 Load Testing Assembly 59

4.3.1 Axial Load Control System 59

4.3.2 Thermal Load Control System 61

4.4 Data Measurement Methods 62

4.4.1 Linear Variable Displacement

Transducer

62

4.4.2 Thermocouple Strain Gauge 63

4.4.3 Thermocouple Wire 65

4.4.4. Thermocouple ProbeType K 65

4.4.5 Load Cell 66

4.4.6 Data Acquisition System 67

4.5 Energy Pile Model Testing 68

4.5.1 Testing Programme 68

4.5.2 Model Soil Preparation 69

4.5.3 Model Pile Installation 69

4.5.4 Energy Pile Model Testing Procedure 71

5 RESULTS AND DISCUSSION 73

5.1 Introduction 73

5.2 Physical Properties Of Soil 74

5.2.1 Particle Size Distribution 74

5.2.2 Atterberg Limits 75

5.2.3 Specific Gravity 76

5.2.4 Thermal Conductivity of Soil 77

5.2.5 Summary of Soil Physical Properties 77

5.3 Engineering Properties of Soil 78

5.3.1 Standard Proctor Compaction Test 78

5.3.2 Unconfined Compression Strength

Test

79

5.3.3 Vane Shear Test 81

5.3.4 Soil Consistency 82

5.4 Model Energy Pile Test 83

5.4.1 Bearing Capacity Test 83

x

5.4.2 Thermal Load Test 85

5.4.3 Thermo-Axial Load Test 95

6 CONCLUSIONS AND RECOMMENDATIONS 109

6.1 Conclusion 109

6.2 Future Research Recommendations 110

REFERENCES 112

Appendices A-F 118-128

xi

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Summary of tests to investigate the effect of thermal loads 28

on the axial response of single piles

3.1 Typical specifications of S300 kaolin 40

3.2 Summary of soil characteristic tests 41

4.1 Energy pile testing programme 68

5.1 Comparison of value of physical indices of silt 78

5.2 Unconfined compression strength test results for S300

kaolin at 0.9 MDD 80

5.3 Unconfined compression strength test results for S300

kaolin at 0.8 MDD 80

5.4 Vane shear test results for model soil compacted at

0.9 MDD 81

5.5 Vane shear test results for model soil compacted at

0.8 MDD 82

5.6 Summary of results for the undrained shear strength of

compacted kaolin 83

5.7 Factor of safety for chosen initial axial loads 85

5.8 Summary of results for time series of thermal load test 86

5.9 Thermal settlements with respect to the settlement

at ultimate load 92

5.10 Thermo-axial settlements with respect to settlement

at ultimate load 107

xii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Closed-loop ground-source heat pump system 10

2.2 Three types of closed loop ground-source heat pumps 11

2.3 Ground source heat exchanger system 12

2.4 A typical GSHP system used for heating purposes 13

2.5 Heat exchanger tubes integrated into a bored pile 14

2.6 The scheme of an energy pile system 15

2.7 Load-settlement curve for thermo-axial tests on model

piles in dry Fontainebleau sand 20

2.8 Comparison of load-settlement (t-z) curves for foundations

under cycles of heating, cooling and baseline test 21

2.9 Soil profile and instrumentation of the EPFL pile 23

2.10 Thermo-mechanical loading for different stages of

construction 24

2.11 Soil profile and pile geometry of the Lambeth College

energy pile 26

2.12 Observed and free thermal strain profiles due to

thermal loads: (a) Lambeth College energy pile, and

(b) EPFL energy pile 27

2.13 Stress and strain response in thermo-active foundations

during heating (a) Schematic of stress/strain changes

during heating; (b) Measurements of strains during heating 33

3.1 Flow chart of research methodology 39

3.2 Hydrometer Test 43

3.3 Particle size analysis device, CILAS 1180 44

3.4 Pycnometer bottles used in specific gravity test 46

xiii

3.5 Schematic diagram of the thermal needle test apparatus 48

3.6 Standard Proctor Compaction test equipment 49

3.7 Unconfined compression strength (UCS) test machine 51

3.8 Location of vane shear test (Plan view) 52

3.9 Vane shear test apparatus 52

3.10 Schematic of physical model of geothermal energy

pile system 53

3.11 Physical model of geothermal energy pile system 54

4.1 Soil and energy pile dimensions: (a) Prototype and

(b) Model 57

4.2 Soil testing chamber 58

4.3 Loading frame 59

4.4 Axial load control system 60

4.5 Temperature bath 61

4.6 Thermostat used for thermal load control 62

4.7 Linear Variable Displacement Transducer

for displacement measurement 63

4.8 Schematics of instrumented energy pile model 64

4.9 Instrumented energy pile model with pile cap 64

4.10 TSK Circular-type load cell 66

4.11 Portable data logger (UCAM-70A) 67

4.12 Soft soil preparation and model pile installation 70

4.13 Model pile behaviour monitoring equipment setup 72

5.1 Particle size distribution curve of S300 kaolin 74

5.2 Liquid limit value obtained via cone penetration test 75

5.3 Classification of S300 in the USCS plasticity chart 76

5.4 Specific gravity value of S300 kaolin obtained via

CILAS 1180 machine 77

5.5 Standard Proctor compaction curve for S300 kaolin 79

5.6 Bearing capacity of model pile for firm soil and soft soil 84

5.7 Time series of temperature change due to heating for

soft soil 86

5.8 Time series of temperature change due to heating for

firm soil 87

xiv

5.9 Time series of temperature change due to heating for

both types of soil and thermal loads 88

5.10 Pile head settlement due to thermal load at 35˚C and

40 ˚C for soft soil 90

5.11 Pile head settlement due to thermal load at 35 ˚C and

40˚C for firm soil 91

5.12 Thermal settlement values for tests conducted in firm

and soft soil 92

5.13 Pile head settlement due to thermal load at 35 ˚C for firm

and soft soil 93

5.14 Pile head settlement due to thermal load at 40 ˚C for firm

and soft soil 93

5.15 Thermo-axial load test at 35°C thermal load for soft soil 96

5.16 Thermo-axial load test at 40°C thermal load for soft soil 97

5.17 Thermo-axial load test at 100 N axial load for soft soil 98

5.18 Thermo-axial load test at 200 N axial load for soft soil 99

5.19 Thermo-axial load test at 100 N and 200 N axial load

for soft soil 99

5.20 Thermo-axial settlement in soft soil with respect to

ultimate load settlement 100

5.21 Thermo-axial load test at 35°C thermal load for firm soil 101

5.22 Thermo-axial load test at 40°C thermal load for firm soil 102

5.23 Thermo-axial load test at 100 N axial load for firm soil 103

5.24 Thermo-axial load test at 200 N axial load for firm soil 103

5.25 Thermo-axial load test at 100 N and 200 N axial load

for firm soil 104

5.26 Thermo-axial settlements in firm soil with respect to

settlement at ultimate load 105

5.27 Thermo-axial settlements in firm and soft soil 107

5.28 Thermo-axial settlements with respect to settlement

at ultimate load 108

xv

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Atterberg Limit Test

118

B Specific Gravity Test

119

C Particle Size Distribution

120

D Standard Proctor Compaction Test

121

E Thermal Loads Settlement

122

F Thermo-axial Loads Settlement

126

xvi

LIST OF ABBREVIATIONS

ASHP - Air-Source Heat Pump

ASTM - American Society for Testing and Materials

BS - British Standard

BSCS - British Soil Classification System

GSHP - Ground Source Heat Pump

HVAC - Heating, Ventilation and Air-Conditioning

IGSHPA - International Ground Source Heat Pump Association

LL - Liquid Limit

PI - Plasticity Index

PL - Plastic Limit

MDD - Maximum Dry Density

OMC - Optimum Moisture Content

SGEP - Shallow Geothermal Energy Pile

UCT - Unconfined Compression Test

USCS - Unified Soil Classification Soil

VDI - Verein Deutscher Ingenieure

xvii

LIST OF SYMBOLS

dmax = Maximum dry density of soil

fs = Ultimate skin friction

α = Adhesion factor of cohesive soil

β = Drained interface strength parameter

𝛿′ = Soil interface friction angle

IP = Plasticity Index

𝜎𝑣0′ = In-situ vertical effective stress (effective overburden pressure)

𝜏𝑖𝑛𝑡 = Soil interface shear strength

sf = Settlement at ultimate load

Su = Undrained shear strength

Gs = Specific Gravity

w = Moisture content

wL = Moisture content at liquid limit

wP = Moisture content at plastic limit

1

CHAPTER 1

INTRODUCTION

1.1 Background of Research

Malaysia is a tropical country located near the Equator with an average

temperature that varies from 20 to 32°C. Since the hot and humid weather is

experienced throughout the year, air conditioning systems are used to combat the effect

of heat, where it accounts for 16 to 50 percent of the total electricity consumption

(Saidur et al., 2007). Typically, office buildings consume about 21% of a country’s

total commercial energy use (Chirarattananon and Taweekun, 2003). Based on this

assumption, it is estimated that total energy used by Malaysian office buildings is about

6090 GWh in year 2007 (Saidur, 2009). Furthermore, in a comprehensive study

conducted by Saidur (2009), it was found that air-conditioning systems comprise of

one of the main electricity consumers in most of the buildings, consuming around 57%

of the electricity in office buildings.

In the near future, it is expected that there will be higher energy usage demands

in buildings due to various factors, e.g. a requirement of improved comfort levels, and

omore time spent inside buildings. Therefore, energy efficiency in buildings is a prime

objective for energy policy at regional, national, and international levels (Lombard et

al., 2008). As required in the green building technology initiative nowadays, existing

structures can be modified into energy efficient infrastructures by modifying the

existing structural elements within a building. A good example of such modification

2

is the shallow geothermal energy pile (SGEP) technology, a newly explored type of

sustainable geostructure.

The energy pile system is designed to achieve energy efficient space heating

and cooling for residential and commercial buildings of various sizes; while satisfying

load bearing requirements of the underlying foundation (Gao et al., 2008). It uses the

embedded structural elements e.g. piles in soil as a medium to transfer thermal energy

between the building and the soil directly underneath it. Heat exchanger piles transport

the ground thermal energy to buildings via heat exchanger fluids that circulate in heat

exchanger pipes embedded in the piles (Brandl, 2009).

Nowadays, geothermal energy pile systems have been implemented throughout

the world, for example in countries such as Austria, Switzerland, United Kingdom,

Japan, China and Hong Kong. However, the engineering community has addressed

concerns regarding the effect of thermal loads on the performance of energy piles

during its operation. Therefore, there is a need to gain a better understanding of the

effect of these thermal loads on the energy pile to justify its use on a large scale,

particularly in Malaysia. Recently, the behaviour of the geothermal energy piles under

the coupled axial and thermal loads (otherwise known as thermo-mechanical load) has

been the subject of research of European and Asian researchers alike. For instance,

full-scale in-situ tests on energy pile systems were carried out by Laloui et al. (2006).

in Switzerland and by Bourne-Webb et al. (2009) in United Kingdom.

On the other hand, numerical models of energy pile systems; specifically on

soil-structure interaction was developed by Knellwolf et al. (2011) and by Ouyang et

al. (2011) to gain a better understanding of the geothermal energy pile behaviour under

operational conditions. More recently, Shin et al. (2014) also conducted a numerical

study on ground heat exchange system, which pertains to the energy pile research.

Regardless, further work is needed to understand the mechanism for soil-pile

3

interaction behaviour at the soil-pile interface, in response to temperature changes

(Ouyang et al., 2011; Suryatriyastuti et al., 2012).

Even though geothermal energy pile systems have gained recognition and

acceptance in other countries, the technology has not been widely implemented in

other countries, including Malaysia. Very little is known about the performance of the

geothermal energy pile system in tropical countries, e.g. Malaysia, which will mostly

require the system to perform under cooling conditions. Nonetheless, cooling

dominated borehole heat exchanger systems are found in Hong Kong (Man et al.,

2011) and in Greece (Sagia et al., 2012). However, these researchers mainly focus on

the cooling and heating performances of the buildings, with very little emphasis on the

performance of the geothermal energy piles under thermo-mechanical loading.

In addition, there are two main concerns with regards to the implementation of

geothermal energy pile system. First is the uncertainty about the thermal effects to the

soil-structure interaction due to cyclic thermal loads which may affect the long-term

structural integrity and durability of the structural foundation (Rosenberg, 2010).

Secondly, there is currently no standard design method that considers the complex

interactions between thermal storage and the mechanical behaviour of these

geostructures (Knellwolf et al., 2011). Therefore, for years, the dimensioning of heat

exchanger piles has been based on empirical considerations (Boënnec, 2008).

Consequently, the safety factors that are usually employed for conventional pile design

are considerably increased.

1.2 Problem Statement

As previously stated, significant studies had been carried out through full-scale

experiment tests to examine the effects of both thermal and mechanical loading of

4

energy pile systems. However, the full-scale experimentations are usually expensive

and time consuming, and there is a need to develop new techniques to analyse and gain

a better understanding of the thermal effects on the soil-structure interaction in the

energy pile systems.

The geothermal energy foundation system is an element of “sustainable

structure” that has been developed in recent years; hence the published data on long-

term performance are scarce. Therefore, experimental investigations on geothermal

energy piles are able to provide significant information with regards to their long-term

performance. There are only four known laboratory experimental studies conducted to

observe the behaviour of these energy piles; where two studies on single gravity energy

piles embedded in sand, and two centrifuge study on energy piles in silt have been

carried out to date. Accordingly, additional testing and new analyses are required to

better understand the mechanisms of thermal soil-structure interaction in energy piles.

1.3 Aim and Objectives of Research

The aim of this research is to establish a preliminary geothermal energy pile

testing facility, to produce the load-settlement curves generated by thermo-axial load

tests. Since full-scale field testing is considered as an expensive research endeavour, a

laboratory physical modelling test with instrumentation approach is taken to achieve

this aim. A series of laboratory scale pile loading model tests were conducted in a

single-gravity energy pile model testing facility. These pile load tests were conducted

under firm and soft soil conditions with a combination of different axial loads, thermal

loads and thermo-axial loads. In order to achieve the aim of this research, the following

objectives have been identified:

5

i. To develop a reliable testing approach to characterize the load-settlement

behaviour of laboratory scale model energy pile foundations in single gravity

conditions.

ii. To evaluate the thermal settlement due to different thermal loads of model pile

embedded in soil with different strength values.

iii. To determine the impact of different thermo-axial loads on the bearing capacity

of the model pile embedded in soil with different strength values.

1.4 Scope of Work

In order to achieve the objectives of research, the following scope has been

determined. First and foremost, this research focuses on the closed-loop ground-source

heat pump (GSHP) system. In particular, this study focuses on the model foundation

and soil behaviour under varying thermo-axial loads. The materials used in this

research includes S300 kaolin (representing the model soil), and stainless steel (close-

ended steel tube representing the model energy foundation). Furthermore, the

experimental investigations were carried out in the UTM Geotechnical Laboratory.

The material classification was carried out via several tests, using standard

methods such as the British Standard and the American Society for Testing and

Materials (ASTM). Specifically, the clay material properties are determined via the

particle size distribution test, Atterberg Limits test, and specific gravity test for soil

classification purposes. Furthermore, soil strength was also carried out, namely the

unconfined compression strength test and shear vane test. Two types of reconstituted

soft kaolin soil were prepared through compaction to achieve the undrained shear

strength of about 25 kPa and 37 kPa.

6

Then, the small-scale single gravity physical model foundation design was

carried out and it consisted of two parts: the soil preparation and model test setup

design. For the model soil preparation, both the soil container and loading frame were

designed to facilitate the soil sample preparation process. Subsequently, the model test

design was carried out where the design of load testing assembly and instrumentation

takes place. Next, the setup of the experiment and the pilot tests are carried out. Once

the pilot tests are able to confirm the repeatability of the experiment, the main physical

modeling test stage commences thereafter.

1.5 Significance of Research

The constantly increasing energy demand, especially for the use of HVAC

systems in buildings requires local researchers to look for sustainable energy sources,

and a possible answer lies in the use of the geothermal energy pile system. Furthermore,

testing the geothermal energy pile system in Malaysia adds another dimension to the

energy pile system evaluation process, as this technology is inherently site specific.

Different weather, soil profiles and building cooling requirements makes this a unique

problem and is therefore essential to the contribution of knowledge to this newly

developed field of sustainable technology. Also, this could lead to a better

understanding of the limits of extractable energy in a given geothermal energy pile

system (Fragaszy et al., 2011). Ultimately, the findings of this research is hoped to

justify the funding and subsequent mobilization of a full-scale geothermal energy pile

test, first of its kind to be conducted in tropical conditions.

This research is expected to produce a reliable single gravity model test

approach of the geothermal energy pile that is expected to retain a degree of similitude

with the complex soil-structure interaction during its operational mode. Therefore, to

quantify the effect of thermal load on the energy piles, the development of a model

energy pile is proposed to study the phenomenon in a controlled environment. This

7

improved understanding can help the development of design methods and

identification of suitable factors of safety for energy piles systems. In a broader scope,

these results will be useful for GEP system designers to ensure optimal design and

operation of energy pile systems.

1.6 Thesis Outline

This thesis comprised of six chapters. As seen in this first chapter, an

introduction to shallow geothermal energy foundations was made to expound on the

motivation of the study. This is explained by discussing the research problem in terms

of the following research philosophies, namely the ‘problem statement’, ‘objectives of

study’, ‘scope of study’, and ‘significance of study’.

Chapter 2 of this thesis presents an in-depth review of ground source heat

pumps and shallow geothermal energy pile foundations. The review includes an

introduction to the components of a ground source heat pump system, and its

installation process. Furthermore, the load-transfer analysis of pile foundations is

discussed, followed by the discussion of physical modeling of energy pile systems that

were conducted in recent years. Based on the current state of knowledge on shallow

geothermal energy foundations, the research gap was determined in order to establish

the framework of the current research endeavour.

Meanwhile, Chapter 3 presents the research methodology of the study by

describing the laboratory work carried out to achieve the aim and objectives set in the

first chapter. In particular, the laboratory experiments that were carried out to

determine the soil’s physical, thermal and engineering properties based on the British

Standard were addressed in detail. Chapter 4 focuses on the laboratory model of the

energy pile system and the procedures adopted to carry out the physical modeling. The

8

elements of the physical model were presented via a detailed explanation that included

the design of the testing chamber, the axial load and thermal load control system, data

measurement and the instrumentation plan adopted for this study.

In Chapter 5, the results of the laboratory testing and physical model tests are

presented and discussed in a comprehensive manner. Results from the soil

classification and soil engineering properties tests are also presented. The discussion

includes several sub-topics such as bearing capacity, pile head settlement, effects of

thermal loads and effects of thermo-axial loads. Chapter 6 lists the main conclusions

obtained from this study to underscore the contribution of the work done to the existing

body of knowledge. In addition, the recommendations for future research work were

also specified and discussed.

112

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