HOLE EROSION TEST

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A MODIFIED HOLE EROSION TEST (HET-P)TO STUDY EROSION CHARACTERISTICS OF SOIL

by

MARCEL LUTHI

B.Eng., HSR University of Applied Sciences Rapperswil, 2005

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OF

MASTER OF APPLIED SCIENCE

in

The Faculty of Graduate Studies

(Civil Engineering)

THE UNIVERSITY OF BRITISH COLUMBIA

(Vancouver)

August 2011

Marcel Luthi, 2011


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Abstract

ii

Abstract

Todays increasing demand for energy and natural resources requires safe and reliable

infrastructure. This includes hydraulic earth structures like dikes, levees, or dams. Such structures are

susceptible to piping, a fundamental type of internal soil erosion. Piping is one of the principal causes

of failures and accidents affecting embankment dams. The Hole Erosion Test (HET) is based on soil

piping, and is used to determine the erodibility and critical shear stress of a soil. A soil specimen with a

preformed axial hole is subjected to a constant-head pressure flow, and the rate of enlargement of the

soil pipe is determined indirectly from flow rate and hydraulic gradient. This study presents a Modified

Hole Erosion Test (HET-P) that introduces a conventional Pitot-static tube to measure total energy

head and flow velocity of the exiting jet, which is correlated to a mean velocity within the axial hole. A

series of Modified Hole Erosion Tests (HET-P) was performed on non-erodible PVC specimens withaxial holes of constant, but different diameter, followed by HET-P tests on two types of soil, namely

glacial till material of a dam core and natural clay deposits from Ontario river banks. Results confirmed

that sidewall hydraulic head measurements to determine hydraulic gradients in the standard HET

overestimate the resulting axial wall shear stress by as much as an order of magnitude. Furthermore,

velocity measurements increase the confidence in test results as they allow for a more direct estimate

of the axial hole diameter at any time during a test. A Pitot-static tube used in the HET-P for velocity

and pressure measurement can easily be incorporated, and yields more transparent and reliable

results by eliminating or amending some of the limiting assumptions of the standard test. It is an easy,

fast, and economical approach that can be applied to soils in both constructed earth structuresincluding dams and embankments, and to natural river banks to determine their susceptibility to

internal and surface erosion.


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Table of contents

iii

Table of contents

Abstract ................................................................................................................................................... ii

Table of contents ................................................................................................................................... iii

List of tables ......................................................................................................................................... vii

List of figures ....................................................................................................................................... viii

List of symbols ..................................................................................................................................... xii

List of abbreviations ........................................................................................................................... xiv

Acknowledgements.............................................................................................................................. xv

Dedication ............................................................................................................................................ xvi

1. Introduction ..................................................................................................................................... 1

1.1. Background ................................................................................................................................ 1

1.2. Objective and scope .................................................................................................................. 3

1.3. Outline of the thesis ................................................................................................................... 4

2. Literature review ............................................................................................................................. 5

2.1. Erosion tests category 1: Flow over surface .............................................................................. 6

2.2. Erosion tests category 2: Rotating cylinder ............................................................................... 7

2.3. Erosion tests category 3: Jet impact .......................................................................................... 8

2.4. Erosion tests category 4: Flow through defect .......................................................................... 9

2.5. Hole Erosion Test (HET) .......................................................................................................... 10

2.5.1. UNSW Hole Erosion Test (HET) .................................................................................... 10

2.5.2. HET theory ..................................................................................................................... 13

2.5.3. HET data analysis .......................................................................................................... 15

2.5.4. Modifications to the HET ................................................................................................ 17

2.5.5. Main findings and conclusions ....................................................................................... 20

2.6. Summary and outline of the research program ....................................................................... 23

2.6.1. Outline of the research program .................................................................................... 24

3. Modified Hole Erosion Test (HET-P) ........................................................................................... 25

3.1. Hydraulic review of the HET .................................................................................................... 25


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Table of contents

iv

3.1.1. Friction head loss and hydraulic gradient ....................................................................... 25

3.1.2. Minor head losses and entrance length ......................................................................... 27

3.1.3. Flow conditions and axial hole velocity .......................................................................... 283.2. HET-P apparatus ..................................................................................................................... 30

3.2.1. Test cell .......................................................................................................................... 31

3.2.2. Pitot-static tube and differential pressure transducer system ........................................ 33

3.2.3. Water flow system .......................................................................................................... 35

3.3. Experimental setup and test procedure ................................................................................... 37

3.3.1. Calibration of measuring instruments ............................................................................. 37

3.3.2. Test preparation ............................................................................................................. 37

3.3.3. Test procedure ............................................................................................................... 38

3.4. Modified analysis ..................................................................................................................... 39

3.4.1. HET-P: based on energy gradient .................................................................................. 39

3.4.2. HET-P (V): based on energy gradient and flow velocity ................................................ 40

4. Non-erodible test specimens ....................................................................................................... 41

4.1. Research program ................................................................................................................... 42

4.2. Results and analysis ................................................................................................................ 43

4.2.1. Head ratio and shear stress ........................................................................................... 43

4.2.2. Flow coefficient and axial hole velocity .......................................................................... 45

4.3. Discussion................................................................................................................................ 47


4.3.2. Axial hole velocity and hydraulic roughness .................................................................. 48

4.3.3. Upstream total energy head ........................................................................................... 49

4.3.4. Flow conditions and minor losses .................................................................................. 50

4.3.5. Sources of errors ............................................................................................................ 51

4.3.6. Limitations ...................................................................................................................... 52

5. Erodible soil specimens ............................................................................................................... 53

5.1. Research program ................................................................................................................... 54

5.2. Results and analysis ................................................................................................................ 55


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Table of contents

v

5.2.1. Soil properties and description ....................................................................................... 55


5.2.3. Axial hole velocity and estimated diameter .................................................................... 605.2.4. Erosion rate and critical shear stress ............................................................................. 63

5.2.5. Dam core material .......................................................................................................... 64

5.2.6. Ontario clay samples ...................................................................................................... 65

5.3. Discussion................................................................................................................................ 68


5.3.2. Axial hole velocity and estimated diameter .................................................................... 69

5.3.3. Erosion rate and critical shear stress ............................................................................. 69

5.3.4. Test soils ........................................................................................................................ 70

5.3.5. Sources of errors ............................................................................................................ 71

5.3.6. Limitations ...................................................................................................................... 72

6. Conclusions and recommendations ........................................................................................... 73

6.1. Summary and conclusions ....................................................................................................... 73

6.2. Recommendations ................................................................................................................... 75

References ............................................................................................................................................ 77

Appendices ........................................................................................................................................... 83

Appendix A: Review of the standard Hole Erosion Test (HET) .......................................................... 84

Appendix B: Engineering drawings ..................................................................................................... 85

Appendix C: Calibration of measuring instruments ............................................................................ 88

Differential pressure transducers .................................................................................................... 88

Custom v-notch weir ....................................................................................................................... 89

Appendix D: HET-P test procedure .................................................................................................... 91

Soil preparation for reconstituted specimens .................................................................................. 91

Specimen preparation for reconstituted specimens........................................................................ 91

Specimen preparation for specimens from undisturbed (intact) soil samples ................................ 92

Test preparation .............................................................................................................................. 92

Test procedure ................................................................................................................................ 93


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Table of contents

vi

Post-test procedure ......................................................................................................................... 94

Appendix E: Analysis methods ........................................................................................................... 95

Appendix F: Experimental program .................................................................................................... 96Appendix G: Non-erodible test specimens ....................................................................................... 100

Appendix H: Soil properties .............................................................................................................. 103

Appendix I: Erodible soil specimens ................................................................................................. 105

Dam core material ......................................................................................................................... 105

Ontario clay samples .................................................................................................................... 109


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List of tables

vii

List of tables

Table 1.1: Embankment-dam engineering and safety evaluation ........................................................ 1

Table 2.1: Qualitative relation of representative erosion rate index and progression of internal

erosion (Wan and Fell 2002, 2004a, 2004b) ..................................................................... 12

Table 3.1: Specifications SETRA Model 230 Bidirectional Wet-to-Wet Pressure Transducers ......... 34

Table 4.1: Summary of head ratio values, H / h, obtained from the three non-erodible test

specimens for turbulent flow with Re> 5000 ..................................................................... 43

Table 4.2: Summary of flow coefficient values, K, obtained from the three non-erodible test

specimens for turbulent flow with Re> 5000 ..................................................................... 45

Table 4.3: Summary of estimated axial hole diameters HET-P (V), t, obtained from the three non-

erodible test specimens for turbulent flow with Re> 5000 ................................................. 46Table 5.1: Summary of velocity ratio values obtained from the Ontario clay specimens for turbulent

flow with Re> 2000 for HET-P, respectively Re> 5000 for HET-P (V) .............................. 61

Table 5.2: Summary of diameter ratio values obtained from the Ontario clay specimens for turbulent

flow with Re> 2000 for HET-P, respectively Re> 5000 for HET-P (V) .............................. 62

Table 5.3: Summary of soil properties for erodible soil specimens .................................................... 66

Table 5.4: Summary of test data and results for erodible soil specimens .......................................... 67

Table A.1: Known challenges and issues of the standard HET with suggested improvements ......... 84

Table E.1: Step by step analysis of test data for different methods .................................................... 95

Table F.1: Test number information .................................................................................................... 96Table F.2: Test program ..................................................................................................................... 97

Table G.1: Test data and results non-erodible test specimen D1-063 .............................................. 100




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List of figures

viii

List of figures

Figure 1.1: Internal erosion and piping process in earth dams .............................................................. 2

Figure 2.1: Schematic diagram flow over surface .................................................................................. 6

Figure 2.2: Schematic diagram rotating cylinder ................................................................................... 7

Figure 2.3: Schematic diagram jet impact.............................................................................................. 8

Figure 2.4: Schematic diagram flow through defect .............................................................................. 9

Figure 2.5: Schematic diagram Hole Erosion Test HET (adapted from Wan and Fell, 2004) ............. 11

Figure 2.6: Typical results Hole Erosion Test (schematic, no real test data) ...................................... 12

Figure 3.1: Flow pattern upstream and downstream of test specimen, including Energy Grade Line

(EGL), and Hydraulic Grade Line (HGL) at wall ................................................................ 27

Figure 3.2: Pitot-static tube with schematic turbulent velocity profile of jet exiting axial holeimmediately adjacent to test specimen .............................................................................. 29

Figure 3.3: Schematic diagram of HET-P apparatus ........................................................................... 30

Figure 3.4: Photograph of HET-P apparatus ....................................................................................... 31

Figure 3.5: a) Vertical HET-P test cell, b) Dismantled downstream part of HET-P test cell with bridge

element holding 6-mm wire mesh for soil specimen support, piezometer connection, and

Pitot-static tube .................................................................................................................. 32

Figure 3.6: Differential pressure transducer system, setup for high range differential pressures ....... 34

Figure 3.7: Schematic hydraulic profile of Modified Hole Erosion Test (HET-P) with Energy Grade

Line (EGL) for initial hole diameter o= 6 mm at medium flow rate .................................. 35Figure 3.8: Downstream constant head tank immediately after failure of soil specimen illustrating

performance of flow conditioning plate and v-notch weir at 20-25 l/min ........................... 36

Figure 3.9: Flowcharts describing test analysis of (a) HET, and (b) HET-P (V) (rectangle: measured

or deduced, parallelogram: assumed or affected by uncertainty) ..................................... 40

Figure 4.1: Non-erodible PVC specimens with axial holes of 6, 12, and 24 mm diameter .................. 42

Figure 4.2: Head ratio equals shear stress ratio versus flow rate for the three non-erodible test

specimens .......................................................................................................................... 44

Figure 4.3: Wall shear stress HET-P using Pitot-static tube data versus wall shear stress HET from

sidewall hydraulic heads for the three non-erodible test specimens ................................. 44

Figure 4.4: Flow coefficient, K, versus flow rate for the three non-erodible test specimens .............. 45

Figure 4.5: Mean flow velocity in axial hole using Pitot-static tube data versus mean flow velocity in

axial hole using continuity for the three non-erodible test specimens ............................... 46

Figure 4.6: Estimated axial hole diameters HET-P (V), back calculated from Pitot-static tube data

versus flow rate for the three non-erodible test specimens ............................................... 47


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List of figures

ix

Figure 5.1: Head ratio equals shear stress ratio versus estimated mean hole diameter for the Ontario

clay specimens .................................................................................................................. 58

Figure 5.2: Wall shear stress HET-P using Pitot-static tube data versus wall shear stress HET from

sidewall hydraulic heads for the Ontario clay specimens .................................................. 59

Figure 5.3: Mean flow velocity in axial hole from HET-P and HET-P (V) using Pitot-static tube data

versus mean flow velocity in axial hole from HET using continuity for the Ontario clay

specimens .......................................................................................................................... 61

Figure 5.4: Estimated axial hole diameter from HET-P and HET-P (V) back calculated from Pitot-

static tube data versus estimated axial hole diameter from HET for the Ontario clay

specimens .......................................................................................................................... 62

Figure 5.5: Erosion rate versus time for successively increased test heads (S3-993.01) ................... 63

Figure 5.6: Critical shear stress defined on flow rate versus shear stress diagram (S3-993.21) ........ 64

Figure C.1: H-U diagram differential pressure transducer #1 and #2 with fitted linear regression lines

to convert output voltage to differential pressure head ..................................................... 88

Figure C.2: H-Q diagram v-notch weir using Kindsvater-Shen relationship (curve fitting) ................... 89

Figure C.3: H-Q diagram v-notch weir using Kindsvater-Shen relationship (head readings) .............. 90

Figure H.1: Gradation curves dam core material (USCS) .................................................................. 103

Figure H.2: Standard compaction test Dam MV4-Altered material (ASTM D698 Method A) ............. 104

Figure H.3: Standard compaction test Dam MV4-Core material (ASTM D698 Method B) ................ 104

Figure I.1: Dam MV4-AlteredS1-003.01Measured flow rate & results at point of failure .......... 105

Figure I.2: Dam MV4-AlteredS1-003.01Dried specimen .......................................................... 105


Figure I.4: Dam MV4-AlteredS1-003.02Drained test cell US and detail with Pitot tube ........... 106


Figure I.6: Dam MV4-AlteredS1-553.01Drained test cell US and detail with Pitot tube ........... 107

Figure I.7: Dam MV4-CoreS2-553.01Measured flow rate & results at point of failure ............. 108

Figure I.8: Dam MV4-CoreS2-553.01Drained test cell US and detail with Pitot tube ............... 108

Figure I.9: Ontario ClayLittle CataraquiS3-993.01Prepared test specimen ......................... 109

Figure I.10: Ontario ClayLittle CataraquiS3-993.01Measured flow rate ................................. 109

Figure I.11: Ontario ClayLittle CataraquiS3-993.01Head difference and energy loss ............ 110

Figure I.12: Ontario ClayLittle CataraquiS3-993.01Head ratio ............................................... 110

Figure I.13: Ontario ClayLittle CataraquiS3-993.01Axial hole diameter ................................. 111

Figure I.14: Ontario ClayLittle CataraquiS3-993.01Wall shear stress .................................... 111

Figure I.15: Ontario ClayLittle CataraquiS3-993.01Erosion rate ............................................ 112

Figure I.16: Ontario ClayLittle CataraquiS3-993.01Flow rate versus shear stress ................ 112

Figure I.17: Ontario ClayLittle CataraquiS3-993.01Drained test cell US and plaster cast ..... 113

Figure I.18: Ontario ClayLittle CataraquiS3-993.01DS side of specimen pre- and post-test . 113


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List of figures

x

Figure I.19: Ontario ClayLittle CataraquiS3-993.02Prepared test specimen ......................... 114

Figure I.20: Ontario ClayLittle CataraquiS3-993.02Measured flow rate ................................. 114

Figure I.21: Ontario ClayLittle CataraquiS3-993.02Head difference and energy loss ............ 115

Figure I.22: Ontario ClayLittle CataraquiS3-993.02Head ratio ............................................... 115

Figure I.23: Ontario ClayLittle CataraquiS3-993.02Axial hole diameter ................................. 116

Figure I.24: Ontario ClayLittle CataraquiS3-993.02Wall shear stress .................................... 116

Figure I.25: Ontario ClayLittle CataraquiS3-993.02Erosion rate ............................................ 117

Figure I.26: Ontario ClayLittle CataraquiS3-993.02Flow rate versus shear stress ................ 117

Figure I.27: Ontario ClayLittle CataraquiS3-993.02Drained test cell US ................................ 118

Figure I.28: Ontario ClayLittle CataraquiS3-993.02Test specimen after testing (DS) ............ 118

Figure I.29: Ontario ClayBear BrookS3-993.12Prepared test specimen ................................ 119

Figure I.30: Ontario ClayBear BrookS3-993.12Measured flow rate ....................................... 119

Figure I.31: Ontario ClayBear BrookS3-993.12Head difference and energy loss ................... 120

Figure I.32: Ontario ClayBear BrookS3-993.12Head ratio ..................................................... 120

Figure I.33: Ontario ClayBear BrookS3-993.12Axial hole diameter ....................................... 121

Figure I.34: Ontario ClayBear BrookS3-993.12Wall shear stress .......................................... 121

Figure I.35: Ontario ClayBear BrookS3-993.12Erosion rate .................................................. 122

Figure I.36: Ontario ClayBear BrookS3-993.12Flow rate versus shear stress ....................... 122

Figure I.37: Ontario ClayBear BrookS3-993.12Drained test cell US and plaster cast ........... 123

Figure I.38: Ontario ClayBear BrookS3-993.12Extracted test specimen after testing ........... 123

Figure I.39: Ontario ClayWilton CreekS3-993.21Prepared test specimen ............................. 124

Figure I.40: Ontario ClayWilton CreekS3-993.21Measured flow rate ..................................... 124

Figure I.41: Ontario ClayWilton CreekS3-993.21Head difference and energy loss ............... 125

Figure I.42: Ontario ClayWilton CreekS3-993.21Head ratio ................................................... 125

Figure I.43: Ontario ClayWilton CreekS3-993.21Axial hole diameter ..................................... 126

Figure I.44: Ontario ClayWilton CreekS3-993.21Wall shear stress ........................................ 126

Figure I.45: Ontario ClayWilton CreekS3-993.21Erosion rate ................................................ 127

Figure I.46: Ontario ClayWilton CreekS3-993.21Flow rate versus shear stress .................... 127

Figure I.47: Ontario ClayWilton CreekS3-993.21Drained test cell US and plaster cast ......... 128

Figure I.48: Ontario ClayWilton CreekS3-993.21Test specimen after testing (DS) ............... 128

Figure I.49: Ontario ClayWilton CreekS3-993.22Prepared test specimen ............................. 129

Figure I.50: Ontario ClayWilton CreekS3-993.22Measured flow rate ..................................... 129

Figure I.51: Ontario ClayWilton CreekS3-993.22Head difference and energy loss ............... 130

Figure I.52: Ontario ClayWilton CreekS3-993.22Head ratio ................................................... 130

Figure I.53: Ontario ClayWilton CreekS3-993.22Axial hole diameter ..................................... 131

Figure I.54: Ontario ClayWilton CreekS3-993.22Wall shear stress ........................................ 131

Figure I.55: Ontario ClayWilton CreekS3-993.22Erosion rate ................................................ 132


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List of figures

xi

Figure I.56: Ontario ClayWilton CreekS3-993.22Flow rate versus shear stress .................... 132

Figure I.57: Ontario ClayWilton CreekS3-993.22Drained test cell US ................................... 133

Figure I.58: Ontario ClayWilton CreekS3-993.22Test specimen after testing (DS) ............... 133


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List of symbols

xii

List of symbols

= cross-sectional area of axial hole m

2

Ce = coefficient of soil erosion s/m

Cp = Pitot tube coefficient

Cv = velocity coefficient

D = diameter of flow chamber m

fL = friction factor for laminar flow conditions kg/m2s

fT = friction factor for turbulent flow conditions kg/m3

= forcing force N = retaining force Ng = acceleration of gravity m/s

2

Gs = specific gravity of soil

hd = measured downstream sidewall hydraulic head m

hf = friction head loss m

hu = measured upstream sidewall hydraulic head m

hv = centerline velocity head m

h = hydraulic head difference across test specimen m

Hd = measured downstream total energy head m

Hu = upstream total energy head m

H = energy head loss along test specimen m

i = H / L= energy gradient across the specimen in modified HET-P

= HET erosion rate index K = CpCv= flow coefficient

L = length of axial hole m

LT = transition length turbulent to laminar flow m

LL = liquid limit of soil %

p = fluid pressure N/m2

ps = static pressure (Pitot-static tube) N/m2

pt = total pressure (Pitot-static tube) N/m2

pv = dynamic pressure (Pitot-static tube) N/m2

p / g = pressure head m

P = = wetted perimeter circumference of circular cross section m

PL = plastic limit of soil %

Q = measured flow rate m3/s

Re = Reynolds number


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List of symbols

xiii

s = h / L= hydraulic gradient across the specimen in standard HET

Sr = degree of saturation %

t = elapsed time s

T = water temperature C

umax = centerline jet velocity m/s

V = mean flow velocity in circular hole or pipe m/s

Vd = downstream flow velocity (flow chamber) m/s

Vt = estimated mean flow velocity in axial hole m/s

Vu = upstream flow velocity (flow chamber) m/s

wf = final water content (post-test) %

wo = initial water content (pre-test) %

wopt = optimum water content %

= erosion rate per unit surface area kg/s/m2 = estimated erosion rate per unit surface area of the axial hole in HET kg/s/m2 = estimated erosion rate per unit surface area of the axial hole in HET-P kg/s/m2 = Darcy friction factor

= kinematic viscosity (water at 20C: = 1.004E-06 m2/s) m

2/s

d = dry density of soil kg/m3

d,max = standard maximum dry density kg/m

3

m = density of total (moist) soil specimen kg/m

3

w = density of water used as eroding fluid kg/m3

g = specific weight of eroding fluid N/m3

= hydraulic shear stress along wetted area N/m2

c = estimated critical shear stress for initiation of erosion N/m2

= estimated wall shear stress along axial hole in HET N/m2 = estimated wall shear stress along axial hole in HET-P N/m2o = initial shear stress N/m

2

= hole diameter m

f = measured final hole diameter m

o = initial diameter of preformed hole m

t = estimated mean diameter of axial hole m


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List of abbreviations

xiv

List of abbreviations

DHT = Drill Hole Test

EFA = Erosion Function Apparatus

EGL = Energy Grade Line

HET = Hole Erosion Test (Wan and Fell 2002, 2004a, 2004b)

HET-P = Modified Hole Erosion Test based on energy gradient

HET-P (V) = Modified Hole Erosion Test based on energy gradient and flow velocity

HGL = Hydraulic Grade Line

ICOLD = International Commission On Large Dams

JET = Jet Erosion Test

NEF = No Erosion Filter TestRCT = Rotating Cylinder Test

UBC = University of British Columbia, Vancouver, Canada

UNSW = University of New South Wales, Sydney, Australia

USBR = United States Bureau of Reclamation

USCS = Unified Soil Classification System


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Acknowledgements

xv

Acknowledgements

I offer my sincere gratitude to my research supervisors Dr. Robert G. Millar and Dr. R. Jonathan

Fannin for their patient guidance throughout this research. I am very grateful for the opportunity, and

their continued encouragement and support of this challenging interdisciplinary research project.

I would like to extend my thanks to the faculty and staff of the Department of Civil Engineering and

the School of Music for the opportunity to learn and teach at UBC. I would like to express my special

thanks to Bill Leung, engineering technician at the Civil Engineering workshop, for his valuable time

and help in building and setting up of lab equipment. His creative ideas and cheerful soul were always

very much appreciated. I thank my fellow students for their friendship inside and outside the class

room and laboratory. I always enjoyed the interesting vivid discussions and exchange of ideas.

I would like to express particular thanks to Dr. Colin D. Rennie of the University of Ottawa who

provided the Ontario clay samples tested as part of this study, and to Dr. Kerry A. Mazurek and Daniel

Cossette of the University of Saskatchewan for carrying out soil property tests and providing the soil

property data for the Ontario clay samples.

I owe particular thanks to my colleagues at my former employer, Basler & Hofmann, Consulting

Engineers AG, Switzerland, for their wide support during the course of this program, both financially

and by providing me with valuable information to complete my studies. I am very grateful for thegenerous financial support received from the Pestalozzi-Stiftung, Switzerland, with special thanks to

Kthi Schmidt and Barbara Schrmann. I am thankful for the financial assistance and support from

Swiss Engineering STV, the University of British Columbia Faculty of Graduate Studies, and Swiss

Association for Road and Transportation Experts (VSS).

My final and utmost thanks I owe to my family, especially my fiance, who never stopped believing

in me, and whose invaluable support and love inspired me each day and made sad days joyful.


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Dedication

xvi

Dedication

To Yvonne, my fiance

for her endless love and faith in me


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Chapter 1. Introduction

1

1. Introduction

1.1. Background

Hydraulic earth structures, which include dikes, levees, or dams, are used worldwide to restrict

standing or flowing water within an assigned area. But the presence of water in such structures may

cause severe damage that could lead to a failure of the structure, eventually resulting in the loss of

lives and catastrophic damage. The three main mechanisms causing substantial damage are

structural causes and slope instability, overtopping, and internal erosion (Table 1.1). The latter

appears to be a main cause of dam instabilities. According to statistics by ICOLD (1995) and Foster et

al. (2000a, 2000b), about 30-50% of failures and accidents affecting embankment dams are caused

by piping as one of the fundamental types of internal soil erosion.

Table 1.1: Embankment-dam engineering and safety evaluation

Primary causes of failures of embankments(ICOLD 1995)

Failure statistics for large dams(Foster et al. 2000a, 2000b)

Mode of failure % Mode of failure %

Internal erosion (Piping) 31 Internal erosion (Piping) 48

- through embankment 17 - through embankment 31

- through foundation 14 - through foundation 15

- embankment to foundation --- - embankment to foundation 2Overtopping 33 Overtopping 46

Structural causes 26 Slope instability 4

Other causes 11 Earthquake 2

The term internal erosion is used herein to describe conditions where seepage flow from the

reservoir erodes soil particles from within the structure and transports them downstream. Suffusion

and piping are the two fundamental types of internal erosion (e.g. Wan and Fell 2002) and describe

the manner in which eroded particles are transported downstream (Figure 1.1).

Suffusion, also referred to as internal instability, describes the selective erosion of fine particles

which are removed through the constrictions between the larger particles. This process leaves behind

an intact soil skeleton formed by the coarser particle fraction.

Piping involves the formation and development of a continuous tunnel or pipe within an earth

structure through erosion of the surrounding soil material driven by a hydraulic gradient. Piping failure


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2

may occur in one of three modes (e.g. Foster et al. 2000a, 2000b), namely piping through the

embankment, piping from the embankment to the foundation, and piping through the foundation

(Figure 1.1). Fell et al. (2003) divided the process of internal erosion and piping into four phases; i)

initiation of erosion, ii) continuation of erosion, iii) progression to form a pipe, and iv) formation of a

breach. Piping may be initiated by means of backward erosion or concentrated leaks (e.g. Sherard et

al. 1984; Sherard and Dunnigan 1985; Sherard 1986). Backward erosion usually commences at the

downstream side where the hydraulic gradient is high enough to cause a displacement of soil

particles, and progressively continues upstream. Concentrated leaks originate within the structure, and

are caused by differential settlement (crack through core), hydraulic fracturing (crack jacked open by

penetrating water), poor compaction (high permeable zone, e.g. around concrete structures or rock

foundations), or digging animals and vegetation. Whether or not initiated erosion progresses and a

final breach is formed depends on various factors (e.g. Fell et al. 2003), like hydraulic gradient (implied

flow velocity and shear stress), the ability of the soil to sustain an open pipe, upstream conditions to

provide crack filling material, downstream conditions to receive or stop eroded soil (filter design

criteria, free opening), and the rate of enlargement of the hole (erosion resistance).

Figure 1.1: Internal erosion and piping process in earth dams

Seepage

through foundation

embankment

to foundation

throughembankment

Suffusion Piping


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3

Internal erosion and piping are potentially extremely dangerous. There may be little or no external

evidence that piping erosion has developed. Common signs are sand boils that may be hidden under

water, or sinkholes at the crest of a dam. Sophisticated surveillance and monitoring pore pressures

and seepage can help to warn about potential problems. In most cases however, early stages of

internal erosion and piping are very hard to detect (Fell et al. 2003).

Internal soil erosion and piping are complex processes that are very difficult to describe by

theoretical analysis, and are influenced by many hardly quantifiable factors. For this reason, various

researchers have developed a number of tests to investigate internal erosion characteristics of soil,

one of which is the Hole Erosion Test (HET). The HET is an accepted laboratory index test method,

developed to study piping erosion in concentrated leaks in earth dams (Wan and Fell 2002, 2004a,

2004b). In the HET, a soil specimen with preformed axial hole is subjected to head-controlled flow,

and measured hydraulic gradient and flow rate are used to indirectly determine the rate of

enlargement of the idealized soil pipe.

The main advantages of the HET are that it is simple and straightforward to use, and that tests

can be performed in an economical manner without the requirement for large amounts of soil. Thus, it

has been applied in several research projects, and there is a growing data base for erosion

characteristics of many different types of soil that will help to understand the relationship between

basic engineering properties and erosion characteristics. However, the development of the HET is still

a work in progress. Studies revealed various challenges and issues affiliated to the HET, including

systematic differences between the HET and other commonly used erosion test methods (Lim 2006;

Bonelli et al. 2006; Farrar et al. 2007; Bonelli and Brivois 2008; Wahl et al. 2008, 2009; Marot et al.

2011).

1.2. Objective and scope

To tie in with U.S. Bureau of Reclamations goal to quantify the HET for standardization so

engineers now will have a readily available test for piping and internal erosion for risk analysis (Farrar

et al. 2007), the main objective of this study is to improve the test configuration and procedure, and to

enhance the science-based framework for the interpretation of the Hole Erosion Test. Emphasis is

placed on expanding the focus to fluid mechanics, which influences erosion mechanisms both within

the soil specimen and at the upstream and downstream soil-water interface. This will increase the

potential of obtaining more accurate results, implying possible applications of the HET beyond index

testing.


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4

Hydraulic effects and how they influence procedure and analysis of the HET will be investigated

by means of a hydraulic review of the standard HET to identify problems and possible improvements

that will guide the design and setup of a modified test apparatus. In question are in particular the

interpretation of the hydraulic gradient, so far believed to be responsible for erosion, and velocity

measurements with the potential to simplify the method of analysis. Laboratory tests not only on

erodible soil specimens but also using a novel method by means of non-erodible test specimens to

prove the applied changes will conclude this study.

1.3. Outline of the thesis

The structure of this report is based on the type of the different investigations as part of this study,

and is organized as follows:

Chapter 1: Introduction to the topic of internal soil erosion and piping, and description of the

objective and scope of this study.

Chapter 2: A review of research literature dealing with surface erosion as the principal

mechanism in piping with focus on the Hole Erosion Test (HET), as well as a

concluding summary and outline of the research program.

Chapter 3: Description of the Modified Hole Erosion Test (HET-P), including a hydraulic review

of the standard HET, followed by details about the modified design, setup, testing

procedure, and two new energy based methods of analysis of HET-P test data.

Chapter 4: Research program, results and analysis, and discussion of findings from HET-P

tests on three non-erodible test specimens as a novel approach to test the

applicability of the modified apparatus and suggested methods of analysis.

Chapter 5: Research program and main findings from HET-P tests on two types of erodible

soil specimens from four different origins, including a discussion of results and

description of the soil samples used in this study.

Chapter 6: Conclusions and recommendations for further research based on the two

completed series of laboratory tests presented in Chapter 4 and Chapter 5.


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Chapter 2. Literature review

5

2. Literature review

This chapter presents a review of research literature related to soil erosion characteristics with

respect to laboratory tests to quantify critical shear stress and erosion rate for surface erosion. These

types of tests can be grouped into the following categories:

1. Flow over surface;

2. Rotating cylinder;

3. Jet impact;

4. Flow through defect.

These four categories, and the related testing procedures, are summarized in the following

sections2.1,2.2,2.3,and2.4, respectively. The main focus of the review, however, is on the HoleErosion Test (HET), an erosion test of category 4, and its modifications, which is described

comprehensively in Section 2.5. A final summary and outline of the research program is given in

Section2.6.

Various other test methods had been developed and used by researchers to simulate other soil

erosion mechanisms, but were not meant to determine critical shear stress and erosion rate for

surface erosion. These include, but are not limited to, dispersivity tests like the Pinhole Test (Sherard

et al. 1976, ASTM D4647-06), the No Erosion Filter Test (NEF) in which dam core and filter materials

are tested at the same time (Sherard and Dunnigan 1985, 1989; Foster and Fell 2001; Soroush et al.2008, 2009), or other various methods of internal erosion tests by means of flow through intact soil

samples using permeameter cells. They had been summarized extensively by others, and were not

addressed in detail in this study.


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2.1. Erosion tests category 1: Flow over surface

- Gibbs (1962)

- Lyle and Smerdon (1965)

- Kandiah and Arulanandan (1974)

- Arulanandan and Perry (1983)

- Shaikh et al. (1988a, 1988b)

- Ghebreiyessus et al. (1994)

- Briaud et al. (1999, 2001)

- Ravens and Gschwend (1999)

- Zhang et al. (2003)

Figure 2.1: Schematic diagram flow over surface

This category comprises test methods for investigating surface erosion in river channels or unlined

canals, where water is flowing parallel to the soil surface at a certain speed and depth. This erosion

mechanism is modelled using hydraulic flume tests, where the specimens are placed on the flume bed

and subjected to open channel flow. Erosion rate is mostly only visually observed and described, while

hydraulic shear stress on the soil surface is deduced from flow velocity and water depth. While this

test is relatively simple, it requires large equipment and suffers from a lack of reproducibility because

of difficulties in controlling surface and soil properties.

Various criteria were developed to describe and evaluate erosion resistance of soil based on

Atterberg limits (Gibbs 1962; Lyle and Smerdon 1965), gradation (Gibbs 1962; Shaikh et al. 1988a),

void ratio (Lyle and Smerdon 1965), water content (Kandiah and Arulanandan 1974; Shaikh et al.

1988a), critical shear stress (Arulanandan and Perry 1983), chemistry (Kandiah and Arulanandan

1974; Shaikh et al. 1988b), or bulk density (Ghebreiyessus et al. 1994).

Flume tests have also been used to compare test results with other test methods like the Rotating

Cylinder Test (RCT). Kandiah and Arulanandan (1974) showed that the two tests produce similar

critical shear stresses, but different erosion rates. Others compared reconstituted and undisturbed soilsamples, showing that reconstituted specimens are less erosion resistant than undisturbed samples

(Zhang et al. 2003).

Special types of flow over surface type of tests are the Erosion Function Apparatus (EFA)

developed by Briaud et al. (1999, 2001) to test undisturbed thin wall tube specimens, or portable flume

tests for in-situ measurements (e.g. Ravens and Gschwend 1999).


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2.2. Erosion tests category 2: Rotating cylinder

- Moore and Masch (1962)

- Masch et al. (1963)

- Arulanandan et al. (1973, 1975)

- Kandiah and Arulanandan (1974)

- Sargunan (1977)

- Arulanandan and Perry (1983)

- Chapuis (1986a, 1986b)

- Chapuis and Gatien (1986)

- Lim (2006)

Figure 2.2: Schematic diagram rotating cylinder

The several designs of erosion test apparatus for measuring erosion resistance of cohesive soils

using a rotating cylindrical fluid chamber are known as Rotating Cylinder Test (RCT). A soil specimen

is submerged and suspended inside the rotating cylindrical chamber. The chamber is rotated relative

to the soil specimen, which induces a flow around the specimen that applies shear stress to the soil

surface. Shear stress is deduced from the measured torque applied to the stationary specimen. The

most recent methods also allow for measurement of erosion rates by removing the eroded material

from the rotating chamber, and weighing the oven-dried mass. The RCT is difficult to perform, and the

apparatus is costly, but it provides accurate measures of erosion parameters (Lim 2006). However,

test results are influenced by the way the specimen is prepared as different sample preparation

methods may yield a different surface roughness of the specimen (Chapuis 1986a, 1986b).

The RCT was originally developed by Moore and Masch (1962) and Masch et al. (1963), and

similar designs have been used by others (see above). Chapuis (1986a, 1986b) and Chapuis and

Gatien (1986) developed a more advanced design that allowed testing both reconstituted as well as

undisturbed soil specimens, and more accurate measurement of erosion rates. Most recent, Lim

(2006) designed and used a more elaborate apparatus, which allowed for accurate shear stress

measurement, and easier control of the testing process. Lim (2006) also compared RCT results withHET results, which is described in Section2.5.5 below.


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2.3. Erosion tests category 3: Jet impact

- Dunn (1959)

- Moore and Masch (1962)

- Hanson (1991, 1992)

- Hanson and Robinson (1993)

- Hanson and Simon (2001)

- Hanson and Cook (2004)

- Hanson and Hunt (2006)

- Wahl et al. (2008, 2009)

- Marot et al. (2011)

Figure 2.3: Schematic diagram jet impact

Jet erosion tests are primarily designed to simulate erosion of spillway channels from a

submerged jet. The Jet Erosion Test (JET)also called Submerged Jet Erosion Test uses a nozzle

positioned above the center of the submerged specimen to produce a jet perpendicular to the soil

surface. The jet, driven by a constant water head, applies a shear stress to the soil surface, which in

turn experiences scouring. The measured geometry of the produced scour is used to determine both

shear stress and erosion rate, which are used for qualitative classification of the erodibility of soils. The

JET is relatively easy and straight forward to perform, and can be carried out on a wide range of

cohesive soils.

Dunn (1959) first proposed the use of the JET before Moore and Masch (1962) used the JET and

RCT in their research on cohesive soils. Hanson and his companion researchers further developed the

JET, which has become an ASTM standard (ASTM D5852-00). Hanson et al. (see above) developed

advanced and simplified designs to carry out in-situ and laboratory tests using reconstituted or

undisturbed tube samples.

Wahl et al. (2008, 2009) used the JET in their research at the United States Bureau of

Reclamation (USBR), which is developing tools for risk evaluation of piping and internal erosion(Farrar et al. 2007). They have compared the JET with the Hole Erosion Test (HET), showing that the

two tests yield different results for shear stress and soil erodibility classification. Based on this work,

Marot et al. (2011) developed a new energy based method to obtain a unique soil erosion

classification for HET and JET.

Head

ASTM D5852-00


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2.4. Erosion tests category 4: Flow through defect

- Christensen and Das (1973)

- Hjeldnesa and Lavania (1980)

- Lefebvre et al. (1985, 1986)

- Rohan et al. (1986)

- Wan and Fell (2002, 2004a, 2004b)

Figure 2.4: Schematic diagram flow through defect

Flow through defect erosion tests were categorized as internal erosion tests, and were set out to

simulate conditions along a crack or any other flow path in an earth structure. These types of tests

involve an undisturbed or reconstituted soil specimen in which a preformed defect is introduced prior

to testing. The prepared specimen is then subjected to head or flow rate controlled pressure flow,

while the hydraulic conditions are monitored in order to determine shear stress and erosion rate.

Various methods have been suggested to model the defect and monitor hydraulic conditions.

Christensen and Das (1973) used a 3 mm thick soil lining inside a brass tube to investigate the relation

between erosion rate and critical shear stress. Another method was developed by Hjeldnesa and

Lavania (1980), who formed a crack by applying tension to the soil specimen. But the use of this

method was very limited since the dimensions of the crack were not determined.

Lefebvre et al. (1985) and Rohan et al. (1986) presented a more sophisticated device, the Drill

Hole Test (DHT). They applied a flow rate controlled pressure flow to a cylindrical clay sample with a

predrilled axial hole. The friction head loss along the specimen was measured to determine shear

stress, and any eroded material was collected in a downstream sedimentation tank to calculate

erosion rate. Using the DHT, Lefebvre et al. (1986) showed that naturally structured clay was highlyerosion resistant and considerably less erodible than de-structured and reconstituted specimens.

Wan and Fell (2002, 2004a, 2004b) developed two head controlled devices, the Slot Erosion Test

(SET) and Hole Erosion Test (HET). The two tests are based on the same concepts, which are

comprehensively explained for the HET in Section2.5.The SET used a larger 1 m long reconstituted

soil specimen with preformed slot along the side rather than in the center of the specimen.

Head


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2.5. Hole Erosion Test (HET)

The Hole Erosion Test (HET) was developed at the University of New South Wales (UNSW) as an

index test to model piping erosion in concentrated leaks in earth dams (Wan and Fell 2002, 2004a,

2004b). It is generally a faster and more economical alternative to the Slot Erosion Test (SET)

mentioned above with the goal to study relationships between basic engineering properties and

erosion characteristics of different types of soil. It allows testing of a much smaller soil specimen at

lower water heads. In a HET, a reconstituted soil specimen with predrilled axial hole is subjected to a

constant-head pressure flow. Erosion rate and shear stress are determined using measured flow rate,

hydraulic gradient, and final hole diameter. With most emphasis on the rate of erosion rather than

critical shear stress, test soils are characterized by a soil group number, which typically ranges from 1

(extremely rapid erosion) to 6 (extremely slow erosion) (Wan and Fell 2002, 2004a, 2004b).

Further work at UNSW (Lim 2006) and the United States Bureau of Reclamation USBR (Farrar et

al. 2007; Wahl et al. 2008, 2009) yielded technical improvements to the test apparatus and procedure,

and interpretation of test data. French researchers also developed a non-dimensional numerical model

for the interpretation of HET data, which simplifies post-test measurements and data analysis (Bonelli

et al. 2006; Bonelli and Brivois 2008). Most recently, Marot et al. (2011) developed an energy based

model for providing a unique erodibility ranking for different erosion tests.

2.5.1. UNSW Hole Erosion Test (HET)

In a standard HET, a soil specimen is reconstituted by compaction inside a standard compaction

mold at 95% maximum dry density and optimum water content. These properties are standardized

because test results are strongly influenced by the degree of compaction and water content (Wan and

Fell 2002, 2004a, 2004b). An axial hole with diameter o= 6 mm is drilled prior to testing. The

specimen is subjected to a constant-head pressure flow (Figure 2.5). The hydraulic head difference,

h, over the length of the soil specimen, L, describes the hydraulic gradient, s, which is increased

stepwise until progressive erosion (enlargement) of the hole is produced. Ideally, the upstream head is

initially set to a height where progressive erosion immediately starts. But choosing an appropriate

upstream head requires a few trial test runs. Once progressive erosion is produced, the upstream

head remains constant until the end of the test. A test is stopped before the eroded hole expands to

the side of the mold, or the maximum available flow rate is reached. During a standard HET, the

hydraulic gradient across the specimen, s , and flow rate, Q , are measured at select time intervals.

After the test, the size and shape of the eroded hole is carefully recorded, and a final hole diameter,

f, representative of the entire length of the eroded axial hole is defined.


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Figure 2.5: Schematic diagram Hole Erosion Test HET (adapted from Wan and Fell, 2004)

Measured hydraulic gradient across the soil specimen and flow rate are used to indirectly

determine the erosion rate per unit surface area, , and wall shear stress along the axial hole, , at any time during the test. A plot of versus (Figure 2.6) is then used to graphicallydetermine critical shear stress, c, coefficient of soil erosion, Ce, and erosion rate index, ,presuming:

Equation (2.1a) expresses soil erodibility in terms of erosion rate when the applied shear stress

exceeds the critical value. The rate of mass removal per unit surface area to represent the erosion rate

is considered most appropriate because porosity and density of the soil material is taken into account.

The coefficient of soil erosion is defined as the slope of the linear best-fit line where both and increase. Critical shear stress describes the ease of erosion and has the physical meaning of thevalue of shear stress at which erosion starts. It is defined as the x-intercept of the extrapolated linear

best-fit line as illustrated inFigure 2.6.

(2.1)

Q

L

Eroding

fluid

circulation

system

Hydraulichead

difference,

h

=

hu

-hd

(

-

)

Upstream flow

chamber filled with

20-mm gravel

Downstream

flow chamber,

empty

Compacted soil

specimen with 6 mm

axial hole

Two vertical

piezometer tubes,

connected to sidewall

of flow chambers,

providing hydraulic

head upstream, hu,

and downstream, hd

Constant head tank,

adjustable in height

50 mm

diameter

pipe


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13

2.5.2. HET theory

The underlying theory to derive shear stress is based on the force equilibrium on the body of

eroding fluid along a differential length of an axial circular hole:

where:

= retaining force, N = forcing force, N = hydraulic shear stress along wetted area, N/m

2

P = = wetted perimeter circumference of circular cross section, m

= hole diameter, m

L = length of axial hole, m

= cross-sectional area of axial hole, m2p = fluid pressure, N/m

2

In a uniform circular cross section with fully developed flow, the differential pressure along can be expressed by the energy head loss due to wall friction, or friction head loss, :

where:

g = specific weight of eroding fluid, N/m3

Combining equations(2.2) and(2.3) gives

The term in equation (2.4) is called hydraulic radius, and is equal to for acircular cross section. Equation (2.4) is integrated over the total length of the soil specimen by

introducing the following assumptions:

1. Flow through the soil matrix is negligible;

2. Only the soil surface along the preformed hole provides shear resistance;

3. Energy losses due to the eroding fluid entering and exiting the preformed hole are negligible;

4. Uniform circular cross section along the length of the soil specimen;

5. Fully developed flow profile in the preformed hole throughout the length of the soil specimen;

6. Hydraulic head difference across the soil specimen, h ,equals total friction head loss, hf.

(2.2)

(2.3)

(2.4)


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For a given diameter, the shear stress is then directly proportional to the friction head loss along

the length of the soil specimen, which is represented by the hydraulic gradient across the soil

specimen:

where:

HET = estimated wall shear stress along axial hole, N/m2

w = density of water used as eroding fluid, kg/m

3

g = acceleration of gravity, m/s2

t = estimated mean diameter of axial hole, m

L = length of axial hole, m

s = hydraulic gradient across the soil specimen in standard HET

hu = measured upstream sidewall hydraulic head, N/m2

hd = measured downstream sidewall hydraulic head, N/m2

Further, the erosion rate per unit surface area is given by:

where:

= erosion rate per unit surface area, kg/s/m2d = dry density of the soil, kg/m

3

= change in cross-sectional area with time, m2/s = change in diameter with time, m/s = change in diameter t over a short time interval t , m/s

Equation(2.7) makes use of the following two additional approximations:

7. The change in radius , which yields and thus ;8. The derivative can be approximated by ;

(2.5)

(2.6)

(2.7)


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2.5.3. HET data analysis

Both shear stress, , and erosion rate, , depend on the diameter of the preformed anderoded axial hole,

t, which cannot be measured directly during a HET, and has to be indirectly

estimated from the measured flow rate, Q, and hydraulic gradient, s. It is assumed that the change in

diameter correlates to the change in assumed friction factors, fL and fT, which are based on continuity

and known common relations between shear stress, , and velocity, V, for laminar and turbulent flow

conditions:

Equations(2.5),(2.8), (2.9),and(2.10) can be combined to:

Since diameter and flow conditions are known for both the beginning and the end of a test, theinitial and final friction factors can be determined, and interpolated over time to estimate diameter at

any particular time tduring a HET:

The flow conditions to determine whether Equation (2.12a) or(2.12b) will be used, are governed

by the Reynolds number, Re:

where:

Vt = estimated mean flow velocity in axial hole, m/s

= kinematic viscosity, m2/s (water at 20C: = 1.004E-06 m

2/s)

(2.8)

(2.9)

(2.10)

(2.11)

(2.12)

(2.13)


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Flow is laminar below a critical value of Re, defined below. For the use in HET, flow is considered

turbulent if the Reynolds number exceeds this critical value.

The above analytical solution to analyze HET data requires the following additional assumptions

and approximations:

9. Shear stress is proportional to V2in turbulent flow conditions;

10. Pure laminar or turbulent flow throughout the entire test;

11. Friction factor fLor fTis linearly interpolated between its initial (t = 0) and final value (t = tf);

12. Flow conditions are turbulent if Reynolds number Re> 5000;

13. The measured final hole diameter, f, is representative of the entire length of the eroded hole.

The complete analysis of HET data involves the following steps:

i) Define initial and final flow conditions using Equations (2.8) and (2.13), and identify a

representative flow condition (laminar or turbulent) to be used for this test;

ii) Estimate the initial friction factor fL,o or fT,o based on the initial diameter of the preformed hole,

o= 6 mm, using Equation(2.11);

iii) Estimate the final friction factor fL,f or fT,f based on the measured final diameter of the eroded

hole, f, using Equation(2.11);

iv) Interpolate the friction factor fL or fT linearly between its initial (t = 0) and final value (t = tf);

v) Estimate the diameter of the axial hole, t, at any time during the test using Equation(2.12);

vi) Plot a curve of estimated diameter, t, against time, t;

vii) Estimate the slope , if appropriate approximated by ;viii) Estimate wall shear stress, HET, using Equation(2.5);

ix) Estimate erosion rate ,, using Equation(2.7);x) Plot against HET, and fit a linear straight line through the rising part of the curve;xi) Determine coefficient of soil erosion, Ce, and erosion rate index, IHET, using Equation(2.1);

xii) Graphically obtain critical shear stress, c, as illustrated inFigure 2.6.


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2.5.4. Modifications to the HET

Lim (2006), University of New South Wales UNSW

Lim (2006) noted some problems with the Hole Erosion Test during his PhD work at UNSW. The

preformed hole was introduced by drilling. It was observed that this method can lead to smearing and

remoulding of the surrounding soil, which lead to a denser surface layer and a higher critical shear

stress. Compaction of the soil into the test mold introduced inhomogeneity that could lead to vertical

layering with different densities. Both of these problems eventually lead to delayed erosion that

affected test results. Another problem was the effects of slaking, where soil particles detached from

the specimen due to the presence of water (hydrostatic conditions), rather than the applied shear

stress. This lead to a reduced hole length and difficulties defining a representative final hole diameter.

To avoid these problems, Lim (2006) proposed modifications to:

1. the estimation of friction factors fL and fT;2. the specimen preparation;

3. the interpretation if a test was affected by slaking.

It was found that the friction factors fL and fTdo not vary linearly with time, and a better correlation

was found between the friction factor and diameter. It was assumed that the friction factors are linearly

proportional to the diameter of the eroded hole. However, this implied practical difficulties because the

diameter is not known until the analysis is complete. An iterative predictor approach was thus adopted,

introducing new assumptions and complexity to the analysis. It is further proposed herein that a better

correlation between friction factors and diameter can be assumed since diameter is used to calculatefriction factors (Equation(2.11)).

With respect to specimen preparation, it was recommended to increase the number of compaction

layers to produce a more homogeneous test specimen. To reduce the effect of remoulding during

drilling of the preformed hole, it was further recommended to use a slowly penetrating and sharp auger

drill. Emphasis was also placed on measuring the final hole diameter as precisely as possible, since

this measurement is crucial for the estimation of shear stress.

The effect of slaking and the resulting reduced hole length was investigated in terms of estimated

wall shear stress. It was found that small reductions of the hole length up to about 20 mm can be

ignored. Ignoring slaking up to about 40 mm resulted in minor errors of less than 10% for the

estimated wall shear stress, and negligible errors for the HET erosion rate index. Lim (2006) proposed

a method to correct for slaking if the amount was more than 40 mm. However, it is questionable how

representative a HET would be if almost half of the soil specimen is lost by failing mechanisms under

hydrostatic conditions.


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Farrar et al. (2007), Wahl et al. (2008, 2009), the United States Bureau of Reclamation USBR

The Hole Erosion Test was further studied at the United States Bureau of Reclamation USBR

(Farrar et al. 2007; Wahl et al. 2008, 2009). During the research program at USBR, the apparatus and

data collection procedures proposed by Wan and Fell (2002, 2004a, 2004b) were considerably

improved:

- Flow rate was measured by a custom 10 v-notch weir at the downstream end of the setup;

- Automated head measurements upstream and downstream of the soil specimen, as well as at

the v-notch weir using pressure transducers and computerized data acquisition system with

recording intervals of 5 s throughout a test;

- High-head HET setup to produce test heads of up to 5400 mm;

- Successively doubled test head during a HET until progressive erosion was observed, starting

at a low test head of usually 50 mm.

The USBR further defined 5 major issues affecting the HET interpretation:

1. Identification of erosion regimes;

2. Curve fitting procedures;

3. Laminar versus turbulent flow;

4. Variation of friction factors;

5. Determination of final hole diameter and length.

Progressive erosion was indicated by an accelerating flow rate at a constant test head. The USBR

emphasized that only data collected during the period of progressive erosion should be considered in

the data analysis to determine erosion parameters. Data collected prior to progressive erosion

described the removal of disturbed and loose material, and were not useful for defining erosion

characteristics of the intact soil.

The development of the hole diameter, t , during progressive erosion was assumed to follow a

polynomial function, whose time derivative defines the erosion rate, , as shown in Equation(2.7).The degree of the fitted polynomial curves varied between different types of soils. To find the

coefficient of soil erosion, Ce, from the

plot, another linear curve fitting was necessary.

This need for data smoothening and adjustment adds complexity to the analysis procedure.

The USBR reported difficulties in justifying the use of the virtual friction factors fL and fT

introduced by Wan and Fell (2002, 2004a, 2004b) in regards of flow conditions. They also questioned

the high critical Reynolds number of Re,crit= 5000 used at the UNSW. They declared flow conditions as

turbulent if Reynolds number Re> 2000, as it is widely recognized in traditional fluid mechanics.


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Friction factors were assumed to vary linearly with time. This could cause erroneous results,

especially if a test was started at a low head. In such cases, erosion was computed in the early stage

of a test, even though there was no observable sign of erosion. Also irregularities in computed hole

diameter suggested that the friction factors were incorrectly modeled. An independent investigation of

this problem revealed a better relationship between the friction factor and estimated hole diameter,

similar to the findings of Lim (2006). But the iterative solution method used by the USBR to determine

friction factors and hole diameter showed problems obtaining convergence. Thus, this method was

simplified by assuming that friction factors vary in proportion to for laminar flow and for turbulent flow. These terms are substitutes for the hole diameter, t.

Also the USBR experienced difficulties in measuring the final hole diameter due to irregularities of

the eroded hole. These were observed especially at the entrance and exit of the soil specimen. These

points are prone to slaking and scouring due to eddies at the soil interface. To reduce these problems,

they introduced end plates with an orifice opening of 15 mm for stronger soils or 25 mm for weaker

soils. Unfortunately this method was less effective than expected. They also investigated several

methods to measure the final hole diameter, including measurements from plaster castings of the

eroded hole. To account for the changing length of the eroded hole in some tests, they assumed that

the length of the eroded hole varies linearly with time.

Bonelli et al. (2006), Bonelli and Brivois (2008), Cemagref, France

French researchers around Stphane Bonelli developed a universal erosion model for permanent

flow over erodible soil. They adapted their model for piping erosion to be applicable to the Hole

Erosion Test by spatial integration of the system over a cylindrical volume representing the axial hole.

It provides an alternative method for analysing HET data, which allows the determination of the

estimated erosion characteristics without the need of measuring and interpolating the hole diameter.

This non-dimensional numerical model correlates a dimensionless hole radius with critical shear

stress, hydraulic gradient, and a dimensionless test time. It has only two unknown parameters that can

be solved with a simple non-linear numerical solver. The model is promising, but the list of

assumptions and limitations restricts its applicability to tests with (not conclusive):- constant pressure drop (constant test head and hydraulic gradient);

- turbulent flow conditions (large Reynolds numbers);

- no variation of the friction factor;

- constant length and uniform diameter of the eroded axial hole;

- test heads exceeding critical shear stress, c;

- data only collected during progressive erosion period.


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Marot et al. (2011), Universit de Nantes and the United States Bureau of Reclamation USBR

The Universit de Nantes in collaboration with the USBR developed a new method to analyze

HET and Jet Erosion Test (JET) data based on fluid energy dissipation and measurement of eroded

mass to provide a unique erodibility classification of test soils. In HET, energy dissipated by erosion

(friction head loss) is correlated with measured hydraulic gradient and flow rate. An energy balance

equation for the fluid was applied between the upstream and downstream point where the hydraulic

heads are measured at the sidewall of the flow chamber (Figure 2.5). It takes into account minor head

losses due to fluid entering (flow contraction) and exiting (flow expansion) the axial hole, and assumes

same average velocities in both measuring sections. It was thus assumed that the hydraulic head

difference across the soil specimen, h ,equals total energy head loss, which is the sum of friction

and minor energy head losses. Based on a series of HET on a non-erodible poly-acrylic model of the

specimen with predrilled 6-mm hole, an empirical constant was determined to isolate unknown friction

head losses responsible for erosion. It was found that, for this particular hole diameter, only about 25%

of the measured hydraulic head difference across the soil specimen, h, is due to wall friction.

2.5.5. Main findings and conclusions

Wan and Fell (2002, 2004a, 2004b), University of New South Wales UNSW

Wan and Fell (2002, 2004a, 2004b) did extensive tests on 13 different soil samples covering a

range of soil properties and origins. Most soils were from borrow areas used for dam construction in

Australia, but also included one each from the USA and New Zealand. The utility of the HET was

demonstrated by a total of 225 HET s carried out in this research program. It was found to be fast,

simple, and easy to reproduce under identical test conditions.

They found that fine-grained and some plastic soils are more erosion resistant than coarse-

grained non-plastic soils and soils with low plasticity. Most of the soils showed higher erosion

resistance if they were compacted to the wet side of optimum water content and a higher dry density.

Results further suggested that the mineralogy, especially iron oxides content, plays an important role

regarding erosion resistance of a soil.

They suggested using an initial shear stress, o , rather than critical shear stress, c, in order to

describe initiation of erosion. Results of critical shear stress were mostly scattered, possibly due to

extrapolation of data, observed non-linearity in the coefficient of soil erosion, and other simplifications

and assumptions in the process of data analysis. The initial shear stress, o, refers to the shear stress

corresponding to the minimum water head at which erosion is first initiated. It is obtained

experimentally by multiple HET trial runs on identical test specimens.


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Wan and Fell (2002, 2004a) further proposed predictive equations to estimate the HET erosion

rate index, IHET, based on multiple linear regression models obtained from statistical analysis of HET

data. They are not presented herein because it was recommended to use them with great caution as

they are based on a limited number of soils, and may not imply any strong relationship between the

erosion rate and the predictor variables.

In summary, the HET was considered a simple and rapid index test, but one that does not intend

to provide accurate quantitative measurements since all derived erosion parameters are indirectly

estimated from a few measured hydraulic parameters.

Lim (2006), University of New South Wales UNSW

Lim (2006) conducted a total of 139 HETsat the UNSW on 9 natural clay soils and 5 engineered

artificial soil mixtures. The main findings were similar to those by Wan and Fell (2002, 2004a, 2004b).

A distinction was made between dispersive and non-dispersive erosion. Dispersive erosion, indicated

by dirty or cloudy outflow, was characterized by immediate erosion and fast enlargement of the hole,

implying considerable erosion at small shear stress. These soils were rarely affected by effects of

slaking, and erosion occurred evenly over the surface of the axial hole. Non-dispersive soils, on the

other hand, experienced severe slaking and reduction of the hole length, and the shape of the eroded

hole was usually irregular. Lim (2006) also claimed that the HET provides limited information about the

erosion behaviour of clay soils due to the reported deficiencies. A comparison with Rotating Cylinder

Test (RCT) results further revealed that the erosion rate indices from HET were significantly higher

than those from the RCT for non-dispersive soils, indicating slower erosion and higher shear stresses.

Farrar et al. (2007), Wahl et al. (2008, 2009), the United States Bureau of Reclamation USBR

The USBR tested a total of 10 soil samples. Test specimens were either reconstituted or

undisturbed. They claimed the HET to be difficult and less reproducible, especially for weak and very

strong soils, while it was good for intermediate soils. Collapsing and scouring were the main problems

in weak soils, while clogging of the axial hole and insufficient test head limited the success rate in

testing strong soils. These problems often lead to subjective interpretations and combinations of

analysis methods. They introduced a subjectivity index to quantify the level of uncertainty in test

results. It ranges from 0 to 3, with low values indicating high confidence in test results. They further

compared HET with Jet Erosion Test (JET) results. The JET indicated lower erosion resistance for all

test soils, including critical shear stresses that were as much as an order of magnitude smaller than

those from HET. With reference to Lim (2006), it was noted that differences between HET and JET are

of a similar order of magnitude as between HET and RCT.


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Bonelli et al. (2006), Bonelli and Brivois (2008), Cemagref, France

The numerical model developed by Bonelli et al. (2006) was tested on 17 (18 in Bonelli and

Brivois, 2008) HET data sets from 9 different soil samples collected by Wan and Fell at the University

of New South Wales. Despite the many simplifying assumptions, they showed good agreement

between the analytical solution proposed by Wan and Fell (2002, 2004a, 2004b) and the new

numerical solution. This method was also successfully applied by the USBR (Farrar et al. 2007; Wahl

et al. 2008, 2009) and Marot et al. (2011), solely or in combination with the analytical solution.

Recognizing that the model is based on even more restrictive assumptions than the analytical solution,

and that it only considers processes within the cylindrical volume of the eroded hole, it deals with

similar issues as the analytical solution.

Marot et al. (2011), Universit de Nantes and the United States Bureau of Reclamation USBR

The motivation for the new energy-based method developed by Marot et al. (2011) to obtain a

unique soil erosion classification for HET and JET was based on observed differences between the

two tests. Consistent with previously reported data by the USBR (Farrar et al. 2007; Wahl et al. 2008),

results from 17 paired HET and JET tests on seven different soil samples with a large range of

erodibility showed that erosion rate indices from HET were consistently larger than those from JET,

and that critical shear stresses from JET were on average about 50 times smaller than those from

HET. Thus, the two tests yielded different soil classifications. Based on the new method, Marot et al.

(2011) introduced an erosion resistance index,

, used for soil erodibility classification similar to the

classification system proposed by Wan and Fell (Table 2.1). Test soils are classified into six categories

of soil erodibility, ranging from highly erodible for , to highly resistant for .They showedthat the new energy based method applied to the 17 paired tests yields the same soil classification for

both the HET and JET. However, it is important to note that this method only provides an erosion

resistance index for soil erodibility classification, and no absolute values of erosion rate and, more

importantly, critical shear stress.


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2.6. Summary and outline of the research program

After the development of various field and laboratory tests to investigate different mechanisms of

soil erosion, the Hole Erosion Test (HET) was developed as a simple and economical laboratory test

to simulate soil erosion in a crack or pipe within an earth dam. Understanding the mechanisms

involved in the HET is a work-in-progress. After the development and first improvements of the HET at

the UNSW, the USBR automated and further improved the test, and also applied it in several cases.

Bonelli et al. added a mathematical background to the more pragmatic approach, while Marot et al.

introduced an energy-based method for providing a unique soil erosion ranking to be comparable with

JET. Extensive testing had been done using the HET with various attempts to improve s

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HOLE EROSION TEST