Performance Characterization of Rock Bolts through In Situ Pull

Tests

by

Paolo L Tomasone

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Graduate Department of Civil Engineering

University of Toronto

© Copyright by Paolo L Tomasone 2018

ii

Performance Characterization of Rock Bolts through In Situ Pull Tests

Paolo L Tomasone

Master of Applied Science

Graduate Department of Civil Engineering

University of Toronto

2018

Abstract

In Ontario underground hard rock mines rock bolts are the principal reinforcement element of ground

support systems. This thesis investigates rock bolt response to in situ pull test conditions. A database of

1,924 pull tests from 18 mines across Ontario was assembled. The performance of rock bolts was quantified,

identifying the influence of installation, rock bolt and rock mass parameters on the resulting in situ behavior.

A statistical analysis provided the means to construct representative behavior. Furthermore, the behavior of

resin grouted rebar was analyzed using numerical models. A series of numerical simulations of resin

grouted rebar pull tests were undertaken to illustrate bolt response in underground conditions. These

investigations provide insights in understanding the behavior of resin grouted bolts in explicit models and

indicated how this information can be used in stability analysis.

iii

Acknowledgments

I thank my supervisor, Professor John Hadjigeorgiou, for the opportunity to study again. It has been both

educational and fulfilling. I appreciate the technical discussions, the excellent learning environment and

the friendship.

This project was funded by the Ministry of Labour Research Opportunities Program. Throughout this

process the members of the Ground Control Committee of Workplace Safety North provided feedback

and useful suggestions. A number of suppliers have also shared bolt data and information.

Thanks are due to the contributing mines and helpful mine personnel. I am sincerely thankful to them for

providing access to both mine sites and technical data.

Thank you to my research colleagues Luke Nicholson, Navid Bahrani and Efstratios Karampinos for their

technical help and conversation.

Lastly, I would like to thank my family for their support and patience.

iv

Table of Contents

Acknowledgments ....................................................................................................................................... iii

Table of Contents ......................................................................................................................................... iv

List of Tables ................................................................................................................................................. x

List of Figures ............................................................................................................................................ xiv

Chapter 1 ....................................................................................................................................................... 1

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

1.1 Problem Definition ........................................................................................................................... 1

1.2 Objectives ......................................................................................................................................... 1

1.3 Methodology .................................................................................................................................... 2

1.4 Thesis Outline................................................................................................................................... 2

Chapter 2 ....................................................................................................................................................... 4

Background on In Situ Rock Bolt Testing ............................................................................................... 4

2.1 Introduction ...................................................................................................................................... 4

2.2 Rock Bolts and Ground Support ...................................................................................................... 4

2.3 In Situ Testing .................................................................................................................................. 5

2.3.1 ASTM, ISRM Pull Tests and Differences to Common Practice ......................................... 7

2.4 Laboratory and In Situ Testing ......................................................................................................... 8

2.5 Data Collected in a Pull Test .......................................................................................................... 10

2.5.1 Working Capacity and Measuring Load............................................................................ 10

2.5.2 Recording Displacement ................................................................................................... 13

2.5.3 Stiffness ............................................................................................................................. 16

2.5.4 Limitations of the Rock Bolt Pull Test Metrics ................................................................. 17

2.6 Conclusion ...................................................................................................................................... 18

Chapter 3 ..................................................................................................................................................... 19

v

Pull Test Database .................................................................................................................................. 19

3.1 Introduction .................................................................................................................................... 19

3.2 Constructing the Database .............................................................................................................. 19

3.3 Participating Mines......................................................................................................................... 19

3.4 Database Composition .................................................................................................................... 22

3.4.1 Responsibility for Pull Testing .......................................................................................... 23

3.4.2 Measurement devices ........................................................................................................ 24

3.5 Conclusions .................................................................................................................................... 25

Chapter 4 ..................................................................................................................................................... 26

Statistical Analysis and Interpretation of Pull Test Data ....................................................................... 26

4.1 Introduction .................................................................................................................................... 26

4.2 Summary Statistic Techniques ....................................................................................................... 26

4.2.1 Summary Statistics ............................................................................................................ 26

4.2.2 Statistical Techniques ........................................................................................................ 27

4.3 Resin Grouted Rebar ...................................................................................................................... 30

4.3.1 Theoretical Behavior of Resin Grouted Rebar .................................................................. 30

4.3.2 Observed Behavior of Resin Grouted Rebar ..................................................................... 32

4.3.3 Characterization of Resin Grouted Rebar.......................................................................... 33

4.4 Friction Rock Stabilizers ................................................................................................................ 37

4.4.1 Theoretical Behavior of Friction Rock Stabilizers ............................................................ 37

4.4.2 Observed Behavior of Friction Rock Stabilizers ............................................................... 37

4.4.3 Characterization of Friction Rock Stabilizer Performance ................................................ 38

4.5 Mechanically Anchored Bolts ........................................................................................................ 44

4.5.1 Theoretical Behavior of Mechanically Anchored Bolts .................................................... 44

4.5.2 Observed Behavior of Mechanically Anchored Bolts ....................................................... 45

4.5.3 Characterization of Mechanically Anchored Bolt Performance ........................................ 46

4.6 Modified Cone Bolts ...................................................................................................................... 48

vi

4.6.1 Theoretical Behavior of Modified Cone Bolts .................................................................. 48

4.6.2 Observed Behavior of Modified Cone Bolts ..................................................................... 49

4.6.3 Characterization of Modified Cone Bolt Performance ...................................................... 51

4.7 D-Bolts ........................................................................................................................................... 53

4.7.1 Theoretical Behavior of D-Bolts ....................................................................................... 53

4.7.2 Observed Behavior of D-Bolts .......................................................................................... 55

4.7.3 Characterization of D-Bolt Performance ........................................................................... 56

4.8 Expandable Bolts ............................................................................................................................ 58

4.8.1 Theoretical Behavior of Expandable Bolts ........................................................................ 58

4.8.2 Observed Behavior of Expandable Bolts .......................................................................... 59

4.8.3 Characterization of Expandable Bolt Performance ........................................................... 60

4.9 Cable Bolts ..................................................................................................................................... 63

4.9.1 Theoretical Behavior of Cable Bolts ................................................................................. 63

4.9.2 Observed Behavior of Cable Bolts .................................................................................... 63

4.9.3 Characterization of Cable Bolt Performance ..................................................................... 64

4.10 Conclusions .................................................................................................................................... 66

Chapter 5 ..................................................................................................................................................... 69

Factors Controlling Rock Bolt Performance Under Pull Test Conditions ............................................. 69

5.1 Introduction .................................................................................................................................... 69

5.2 Resin Grouted Rebar ...................................................................................................................... 69

5.2.1 Rebar Length ..................................................................................................................... 69

5.2.2 Encapsulation Length ........................................................................................................ 73

5.2.3 Spin Time .......................................................................................................................... 75

5.2.4 Residence Time ................................................................................................................. 77

5.2.5 Rock Mass Quality ............................................................................................................ 79

5.2.6 Summary of Investigations into Resin Grouted Rebar Rock Bolt Influences ................... 82

5.3 Friction Rock Stabilizers ................................................................................................................ 83

vii

5.3.1 Length of Bolt (Anchorage Length) .................................................................................. 83

5.3.2 Installation Method ........................................................................................................... 86

5.3.3 Impact of Drive Time ........................................................................................................ 88

5.3.4 Impact of Drill Bit Diameter ............................................................................................. 89

5.3.5 Influence of Bolt Diameter ................................................................................................ 91

5.3.6 Impact of Borehole Medium ............................................................................................. 92

5.3.7 Impact of Rock Mass Quality ............................................................................................ 95

5.3.8 Summary of Friction Rock Stabilizers Investigations ....................................................... 97

5.4 Mechanically Anchored Bolts ........................................................................................................ 99

5.4.1 Impact of Bolt Length ....................................................................................................... 99

5.4.2 Impact of Rock Mass Quality .......................................................................................... 101

5.4.3 Drill Bit Diameter ............................................................................................................ 103

5.4.4 Summary of Investigations into Mechanically Anchored Bolts ...................................... 104

5.5 Modified Cone Bolts .................................................................................................................... 105

5.5.1 Impact of Residence Time ............................................................................................... 105

5.5.2 Impact of Rock Mass Quality .......................................................................................... 107

5.5.3 Impact of Debonding Agent ............................................................................................ 108

5.5.4 Intervariable Relationships .............................................................................................. 110

5.5.5 Summary of Investigations into Modified Cone Bolts .................................................... 110

5.6 Conclusions .................................................................................................................................. 111

Chapter 6 ................................................................................................................................................... 114

Characterization of Rock Bolt Performance ........................................................................................ 114

6.1 Introduction .................................................................................................................................. 114

6.2 Characterizing Bolt Response using Laboratory Testing ............................................................. 114

6.3 Resin Grouted Rebar .................................................................................................................... 115

6.3.1 Characterization of Resin Grouted Rebar Performance .................................................. 115

6.3.2 Characterization of Resin Grouted Rebar Behavior ........................................................ 117

viii

6.4 Friction Rock Stabilizers .............................................................................................................. 123

6.4.1 Characterization of Friction Rock Stabilizer Performance .............................................. 123

6.4.2 Characterization of Friction Rock Stabilizer Behavior ................................................... 125

6.5 Mechanically Anchored Bolts ...................................................................................................... 127

6.5.1 Characterization of Mechanically Anchored Bolt Performance ...................................... 127

6.5.2 Characterization of Mechanically Anchored Bolt Behavior ........................................... 128

6.6 Modified Cone Bolts .................................................................................................................... 130

6.6.1 Characterization of Modified Cone Bolt Performance .................................................... 131

6.6.2 Characterization of Modified Cone Bolt Behavior ......................................................... 132

6.7 D-Bolts ......................................................................................................................................... 135

6.7.1 Characterization of D-Bolt Performance ......................................................................... 135

6.7.2 Characterization of D-Bolt Behavior .............................................................................. 136

6.8 Expandable Bolts .......................................................................................................................... 139

6.8.1 Characterization of Expandable Bolt Performance ......................................................... 139

6.8.2 Characterization of Expandable Bolt Behavior ............................................................... 140

6.9 Cable Bolts ................................................................................................................................... 144

6.9.1 Characterization of Cable Bolt Performance ................................................................... 144

6.9.2 Characterization of Cable Bolt Behavior ........................................................................ 145

6.10 Conclusions .................................................................................................................................. 147

Chapter 7 ................................................................................................................................................... 150

Numerical Modelling Methods for the Simulation of Pull Tests ......................................................... 150

7.1 Introduction .................................................................................................................................. 150

7.2 Numerical Modelling Methods for Tests on Rock Bolts .............................................................. 150

7.2.1 Continuum and Discontinuum Approaches..................................................................... 150

7.2.2 Implicit and Explicit Numerical Models ......................................................................... 151

7.2.3 Rock Bolt Formulation .................................................................................................... 152

7.2.4 2D vs 3D Models ............................................................................................................. 152

ix

7.2.5 Model Selection ............................................................................................................... 152

7.3 Universal Distinct Element Code (UDEC) ................................................................................... 152

7.3.1 Representation of Reinforcement Elements in UDEC .................................................... 152

7.3.2 The ‘Rockbolt’ Reinforcement Element in UDEC ......................................................... 153

7.4 Literature Review of Numerical Modelling of Pull Tests ............................................................ 156

7.4.1 Failure of the Rock Bolt Grout Interface ......................................................................... 156

7.4.2 Failure of the Steel Dowel ............................................................................................... 156

7.4.3 Related Phenomena ......................................................................................................... 157

7.5 Conclusions .................................................................................................................................. 158

Chapter 8 ................................................................................................................................................... 159

Numerical Modelling of Pull Tests ...................................................................................................... 159

8.1 Introduction .................................................................................................................................. 159

8.2 Selection of Pull Tests for Numerical Simulations ...................................................................... 159

8.3 Model Geometry and Boundary Conditions ................................................................................. 162

8.4 Model calibration and results ....................................................................................................... 163

8.5 Comparison to Numerical Modelling of Laboratory Testing ....................................................... 167

8.6 Conclusions .................................................................................................................................. 168

Chapter 9 ................................................................................................................................................... 170

Conclusions .......................................................................................................................................... 170

9.1 Contributions ................................................................................................................................ 170

9.2 Characterization of Rock Bolt Performance ................................................................................. 171

9.3 Limitations ................................................................................................................................... 172

9.4 Recommendations ........................................................................................................................ 173

9.5 Future work .................................................................................................................................. 173

References ............................................................................................................................................ 175

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List of Tables

Table 3-1 Number of tests by mine, with precious/base metal distinction. .................................................21

Table 3-2 Summary of database composition, by bolt type. .......................................................................22

Table 4-1 Description of ANOVA for J sets containing I entries, NIST/SEMATECH (2013). .................29

Table 4-2 Summary statistics for working capacity of 20 mm resin grouted rebar.....................................34

Table 4-3 Summary statistics of 20 mm rebar by encapsulation level, excluding outliers. ........................34

Table 4-4 Summary statistics for 20 mm resin grouted rebar. ....................................................................36

Table 4-5 Summary statistics for working capacity of FRSs, separated by supplier (when available) and

nominal diameter. ........................................................................................................................................40

Table 4-6 Summary of working capacity of FRSs normalized to 1.8 m (6') long bolts. ............................41

Table 4-7 Comparison of ultimate capacities of FRSs by different studies. ...............................................41

Table 4-8 Summary statistics of first drop and secant stiffness for FRS bolts. ...........................................44

Table 4-9 Summary statistics of working capacity for mechanically anchored bolts. ................................46

Table 4-10 Summary statistics for stiffness measures of mechanical bolts. ...............................................47

Table 4-11 Summary statistics for load metrics of MCB33s. .....................................................................52

Table 4-12 Summary statistics for stiffness measures of MCB33s. ............................................................53

Table 4-13 Summary statistics of working capacity of 20 and 22 mm D-Bolts. .........................................57

Table 4-14 Stiffness measures for 20 mm and 22 mm D-Bolts. .................................................................58

Table 4-15 Summary statistics of yield controlled working capacity of expandable bolts. ........................61

Table 4-16 Cable bolt pull tests with load displacement information grouped by outcome. ......................63

Table 4-17 Summary of bonding strengths for cable bolts which experienced pull out failure. .................64

Table 4-18 Summary statistics of cable bolt working capacity, with failure mode identified. ...................65

xi

Table 4-19 Summary statistics of stiffness measures of cable bolts. ..........................................................65

Table 4-20 Summary of working capacity statistics. ..................................................................................68

Table 5-1 Summary of ANOVA results comparing performance metrics of differing lengths of 20 mm

resin grouted rebar. ......................................................................................................................................71

Table 5-2 Summary of ANOVA results comparing secant stiffness of the three most common lengths of

partially encapsulated resin grouted rebar, with an outlier removed. ..........................................................72

Table 5-3 Summary of t-test results comparing secant stiffness for the two most common lengths of fully

encapsulated 20 mm resin grouted rebar. ....................................................................................................73

Table 5-4 Summary of t-test results comparing performance metrics of resin grouted rebar between partial

and full encapsulation, with outliers removed. ............................................................................................75

Table 5-5 Summary of t-test results comparing the influence of residence time on performance metrics of

fully encapsulated resin grouted rebar. ........................................................................................................78

Table 5-6 Summary of ANOVA results comparing fully encapsulated 20 mm resin grouted rebar in Good,

Fair and Poor quality rock masses. ..............................................................................................................80

Table 5-7 Summary of ANOVA results comparing partially encapsulated 20 mm resin grouted rebar in

Good, Fair and Poor quality rock masses. ...................................................................................................81

Table 5-8 Summary of ANOVA results comparing ultimate capacities of FS35s with different lengths...84

Table 5-9 Summary of ANOVA results comparing ultimate capacities of FS39s with different lengths...85

Table 5-10 Summary of ANOVA results comparing ultimate capacities of FS46s with differing lengths.

.....................................................................................................................................................................86

Table 5-11 Results of t-tests comparing the influence of installation method for ultimate capacities of

FRSs. ...........................................................................................................................................................87

Table 5-12 Summary of ANOVA results comparing ultimate capacities of FRSs with differing nominal

diameters. ....................................................................................................................................................92

Table 5-13 Summary of t-test results comparing ultimate capacities of mechanically installed FS35s in

sulphides and hard rock. ..............................................................................................................................93

xii

Table 5-14 Summary of ANOVA results of comparing ultimate capacities of mechanically installed

FS39s and FS46s in differing host media. ...................................................................................................95

Table 5-15 Summary of t-test results comparing ultimate capacities of FRSs installed in ‘Good’ or ‘Poor’

rock mass conditions. ..................................................................................................................................97

Table 5-16 Summary of influences on ultimate capacity of FRSs with comparison to Tomory et al. (1998)

and Nicholson and Hadjigeorgiou (2017). ..................................................................................................99

Table 5-17 Summary of ANOVA results comparing performance metrics of mechanical bolts of different

length. ........................................................................................................................................................101

Table 5-18 Summary of ANOVA results comparing performance metrics of mechanically anchored bolts

in different rock mass conditions. .............................................................................................................103

Table 5-19 Summary of t-tests comparing the effect of same day and previous installation on various

performance metrics of MCBs. .................................................................................................................106

Table 5-20 Summary of ANOVA results comparing performance metrics of MCBs in differing rock mass

conditions. .................................................................................................................................................108

Table 5-21 Summary of t-tests comparing performance metrics of MCBs by debonding agents. ............109

Table 5-22 Summary of investigations into factors influencing rock bolt performance during pull tests for

resin grouted rebar and FRSs. ...................................................................................................................111

Table 5-23 Summary of investigations into factors influencing rock bolt performance during pull tests for

mechanically anchored bolts and MCBs. ..................................................................................................112

Table 6-1 Performance metrics of resin grouted rebar. .............................................................................116

Table 6-2 Comparison of observed working capacities to supplier specifications....................................123

Table 6-3 Distribution of performance metrics of FRSs. ..........................................................................124

Table 6-4 Performance metrics of mechanically anchored rock bolts. .....................................................128

Table 6-5 Performance metrics of 1.5 m - 1.8 m (5’-6') long MCB33 type rock bolts. ............................131

Table 6-6 Comparison of supplier specified thread yield loads to observations. ......................................134

xiii

Table 6-7 Performance metrics of D-Bolts................................................................................................135

Table 6-8 Supplier specified minimum and typical yield loads for D-Bolts. ............................................138

Table 6-9 Performance metrics of expandable bolts, separated by wall thickness. ...................................139

Table 6-10 Comparison of observed working capacity to supplier specifications for yield of ‘standard’

expandable bolts. .......................................................................................................................................142

Table 6-11 Performance metrics of all cable bolts in the database. ..........................................................145

Table 6-12 Summary of results for similar bolts tested by Stillborg (1993) and Stjern (1995). ...............149

Table 8-1 Summary of in situ and laboratory pull tests of fully encapsulated resin grouted rebar. ..........161

Table 8-2 Input parameters for calibration of 'rockbolt' element. .............................................................166

Table 8-3 Input parameters of numerical simulation of Stjern (1995) laboratory tests after Bahrani and

Hadjigeorgiou (2017). ...............................................................................................................................168

xiv

List of Figures

Figure 2-1 Examples of rock bolts Hadjigeorgiou and Charette (2001). ......................................................5

Figure 2-2 Example of typical pull test set up. The bar protruding from the hydraulic ram is not the rock

bolt being tested, but is rather a pull rod connected to installed bolt via an adapter. ....................................6

Figure 2-3 Diagram of pull test apparatus as described in ASTM D4435-13. ..............................................7

Figure 2-4 Schematic of experimental apparatus for replicating rock bolts being loaded across a joint

Stillborg (1993). ............................................................................................................................................9

Figure 2-5 Picture and diagram of experimental apparatus for replicating rock bolts being loaded across a

joint, with shear loads Chen and Li (2015). ..................................................................................................9

Figure 2-6 Static (a) and dynamic (b) testing of 22 mm D-Bolts, after Li (2012) and Li and Doucet (2012)

respectively. .................................................................................................................................................12

Figure 2-7 ASTM E6 09b description identifying yield; a) upper and lower yield strength as the stress

which results in plastic deformation of the material. b) a pre-defined strain offset is used to identify a

yield stress. ..................................................................................................................................................12

Figure 2-8 Defining working capacity from pull out test. ...........................................................................13

Figure 2-9 Illustration of pull test with generic bolt, after Nicholson (2016). ............................................14

Figure 2-10 The interactions between components of a generic rock bolt system, after Thompson et al.

(2012). .........................................................................................................................................................15

Figure 2-11 Total displacement of axially loaded point anchored bolt Nicholson (2016). .........................15

Figure 2-12 Influence of support stiffness and installation time, after Brady and Brown (2004). ..............16

Figure 2-13 Tangential and secant stiffness as determined for a manually recorded load displacement

curve. ...........................................................................................................................................................17

Figure 3-1 Map of Ontario with all mines (2016) labelled, after Ontario Prospectors Association (2016).

.....................................................................................................................................................................20

Figure 3-2 Frequency of pull tests separated by bolt type and target metal for the mine. ...........................21

xv

Figure 3-3 Histogram of number of pull tests over time for the database. ..................................................23

Figure 3-4 Pull tests sorted by testing personnel, distinguished by year of installation. .............................24

Figure 3-5 Measurement device used in pull tests. .....................................................................................24

Figure 4-1 Shear stress distribution along a fully grouted rebar bolt both before (a) and after (b)

decoupling occurs, after Li and Stillborg (1999). ........................................................................................30

Figure 4-2 Shear stress and tensile stress distribution along a fully grouted rebar bolt which has

experienced a degree of decoupling, after Li and Stillborg (1999). ............................................................31

Figure 4-3 A pull test of a fully encapsulated 20 mm rebar showing little eccentric behavior. ..................32

Figure 4-4 A fully encapsulated 20 mm resin grouted rebar pull test with eccentric behavior with

tangential stiffness highlighted in red. .........................................................................................................33

Figure 4-5 Working capacity of 20 mm rebar by encapsulation level (a) and supplier (b). ........................33

Figure 4-6 Tangential stiffness of both fully encapsulated (a) and partially encapsulated (b) 20 mm rebar.

.....................................................................................................................................................................35

Figure 4-7 Secant stiffness of both fully encapsulated (a) and partially encapsulated 20 mm rebar. .........35

Figure 4-8 Shear stress along a frictionally coupled bolt subjected to axial load, after Li and Stillborg

(1999). .........................................................................................................................................................37

Figure 4-9 Load displacement curve of a 39 mm FRS pull test with discrete loading stages labelled. ......38

Figure 4-10 Ultimate capacities of FRSs separated by supplier and nominal diameter. .............................39

Figure 4-11 Stiffness measures for FRSs. ...................................................................................................42

Figure 4-12 First drop and secant stiffness for FS35, FS39 and all FRS. ...................................................43

Figure 4-13 Axial and shear stress plots of a mechanically anchored rock bolt after Li et al (2014). ........45

Figure 4-14 Mechanical bolt pull test with well-defined elastic and yielding phases. ................................45

Figure 4-15 Working capacities of mechanically anchored bolts. ..............................................................46

Figure 4-16 Tangential (a) and secant (b) stiffness for mechanical bolts after length normalization. ........47

xvi

Figure 4-17 Pull test result from laboratory, after Simser et al. (2006).......................................................48

Figure 4-18 Conceptual load displacement of a cone bolt subject to quasi-static loading conditions, after

Simser et al. (2006). ....................................................................................................................................48

Figure 4-19 Proposed loading phases of MCB. ...........................................................................................49

Figure 4-20 Load displacement curve of MCB which shows progressive softening of pre-yield response,

rather than two distinct linear responses......................................................................................................50

Figure 4-21 An MCB pull test load displacement curve showing the five metrics used to characterize

MCB performance. ......................................................................................................................................51

Figure 4-22 Plough load (a) and yield load (b) for MCB33s. .....................................................................52

Figure 4-23 Bolt and plough stiffness distributions for MCB33 type bolts. ...............................................52

Figure 4-24 Secant stiffness distributions for MCB33 type bolts. ..............................................................53

Figure 4-25 D-Bolt static test set up and conceptual diagram, after Li and Doucet (2012). .......................54

Figure 4-26 D-Bolt response to static loading, after Li and Doucet (2012). ...............................................54

Figure 4-27 Comparison 22 mm D-Bolt field data to theoretical result of axially loaded equivalent metal

bars with lengths equal to anchor point. ......................................................................................................55

Figure 4-28 D-Bolt subjected to pull loading with axial stress plotted over length of bolt after Li et al.

(2014). .........................................................................................................................................................56

Figure 4-29 Distributions of working capacity for 20 mm and 22 mm D-Bolts. ........................................56

Figure 4-30 Tangential and secant stiffness of 20 mm D-Bolts. .................................................................57

Figure 4-31 Tangential and secant stiffness for 22 mm D-Bolts. ................................................................58

Figure 4-32 Tensile and shear stresses of an expandable bolt subjected to axial load, Li and Stillborg

(1999). .........................................................................................................................................................59

Figure 4-33 Working capacities of ‘standard’ (a) and ‘super’ (b) expandable bolts which began yielding.

.....................................................................................................................................................................60

xvii

Figure 4-34 Slip controlled working capacities (anchorage capacity) of ‘standard’ (a) and ‘super (b)

expandable bolts. .........................................................................................................................................61

Figure 4-35 Slip controlled working capacity (anchorage) of all expandable bolts as distinguished by host

medium. .......................................................................................................................................................61

Figure 4-36 Tangential (a) and secant (b) stiffness of ‘standard’ expandable bolts. ...................................62

Figure 4-37 Distributions of tangential and secant stiffness of ‘super’ expandable bolts. ..........................63

Figure 4-38 Load displacement curves of four recorded behaviors of cable bolts. .....................................64

Figure 4-39 Distribution of tangential and secant stiffness for cable bolts. ................................................65

Figure 5-1 Performance metrics of 20 mm resin grouted rebar vs. rebar length, with encapsulation level

displayed. .....................................................................................................................................................70

Figure 5-2 Working capacity and secant stiffness of 20 mm rebar vs. resin coverage. ..............................73

Figure 5-3 Tangential stiffness of 20 mm resin grouted rebar vs. resin coverage. ......................................74

Figure 5-4 Distribution of performance metrics of 20 mm resin grouted rebar grouped by full or partial

encapsulation. ..............................................................................................................................................74

Figure 5-5 Performance metrics of 20 mm resin grouted rebar vs. spin time. ............................................76

Figure 5-6 Performance metrics of 20 mm resin grouted rebar vs. spin time with data points distinguished

by encapsulation level. ................................................................................................................................77

Figure 5-7 Performance metrics of fully encapsulated 20 mm resin grouted rebar vs. same day or previous

installation. ..................................................................................................................................................78

Figure 5-8 Performance metrics of fully encapsulated 20 mm resin grouted rebar bolts in differing rock

mass qualities. .............................................................................................................................................79

Figure 5-9 Working capacity and secant stiffness of 20 mm rebar installed in different rock mass

qualities. ......................................................................................................................................................80

Figure 5-10 Tangential stiffness of partially encapsulated 20 mm resin grouted rebar bolts in differing

rock mass qualities. .....................................................................................................................................81

xviii

Figure 5-11 Relationship between anchorage length and ultimate load for FS35 bolts. .............................83

Figure 5-12 Relationship between anchorage length and ultimate load for FS39 bolts. .............................84

Figure 5-13 Relationship between anchorage length and ultimate loads for FS46 bolts. ...........................85

Figure 5-14 Relationship between ultimate capacity and installation method of FS35 and FS39 bolts. ....87

Figure 5-15 Comparison of ultimate capacity vs. drive time for three diameters of FRS. ..........................88

Figure 5-16 Comparison of ultimate capacity vs. drill bit diameter for three diameters of FRS. ...............89

Figure 5-17 Comparison of ultimate capacity vs. drill bit diameter of FS35 bolts while distinguishing

between installation equipment. ..................................................................................................................90

Figure 5-18 Comparison of ultimate capacity vs. drill bit diameter of FS39 bolts while distinguishing

between installation equipment. ..................................................................................................................90

Figure 5-19 Distribution of length normalized ultimate capacities of FRSs distinguished by nominal bolt

diameter. ......................................................................................................................................................91

Figure 5-20 Comparison of ultimate capacities (kN/m) of mechanically installed FS35 bolts in different

rock types. ...................................................................................................................................................93

Figure 5-21 Length normalized ultimate capacities (kN/m) vs. borehole medium for a) FS39 and b) FS46

bolts. ............................................................................................................................................................94

Figure 5-22 Comparison of ultimate capacities (kN/m) of FRSs installed in ‘Good’ and ‘Poor’ ground

conditions. ...................................................................................................................................................96

Figure 5-23 Relationship of bolt length and a) working capacity, b) secant stiffness and c) tangential

stiffness. .....................................................................................................................................................100

Figure 5-24 Rock mass conditions vs. performance metrics of mechanically anchored bolts. .................102

Figure 5-25 Relationship of drill bit diameter to a) working capacity, b) secant stiffness, and c) tangential

stiffness. .....................................................................................................................................................104

Figure 5-26 Initial stiffness and Plough point of MCBs installed the day of testing or previously. .........105

Figure 5-27 Plough stiffness and secant stiffness of MCBs installed the day of testing or previously. ....106

xix

Figure 5-28 Performance metrics of MCBs in differing ground conditions. ............................................107

Figure 5-29 Performance metrics of MCBs sorted by debonding agent, namely grease or a plastic sleeve.

...................................................................................................................................................................109

Figure 6-1 Composite of Stillborg (1993) results by Hoek et al. (1995). ..................................................115

Figure 6-2 Working capacity and displacements for 20 mm rebar, with percentiles of working capacity

and stiffness for all 20 mm rebar. ..............................................................................................................116

Figure 6-3 Load displacement curves of all 20 mm diameter rebar in the database. ................................117

Figure 6-4 Load displacement curves of fully encapsulated 20 mm resin grouted rebar, distinguished by

pre-load. .....................................................................................................................................................118

Figure 6-5 Load displacement curves of fully encapsulated 20 mm resin grouted rebar, normalized to zero

pre-load. .....................................................................................................................................................118

Figure 6-6 Distribution of displacement measured for all fully encapsulated 20 mm resin grouted rebar.

...................................................................................................................................................................119

Figure 6-7 Conceptualized distribution of responses of fully encapsulated 20 mm resin grouted rebar

bolts, with working capacities of all 20 mm rebar. ...................................................................................120

Figure 6-8 Load displacement curves for all 22 mm diameter resin grouted rebar bolts. .........................121

Figure 6-9 Load displacement curves of all fully encapsulated 22 mm resin grouted rebar, normalized to

have no pre-load. .......................................................................................................................................121

Figure 6-10 Distribution of responses of 22 mm fully encapsulated rebar. ..............................................122

Figure 6-11 Distributions of max load and displacement at max load of FRSs. .......................................124

Figure 6-12 Load displacement curves of FRS in the database.................................................................125

Figure 6-13 Load displacement distributions of FRS. ...............................................................................126

Figure 6-14 Conceptualized distribution of FRS response, showing perfectly plastic behavior. ..............126

Figure 6-15 Working capacity and displacement at working capacity for mechanical bolts in the database,

with percentiles of working capacity and secant stiffness. ........................................................................127

xx

Figure 6-16 Load displacement curves of all mechanically anchored rock bolts in the database. ............128

Figure 6-17 Load displacement curve of all mechanically anchored bolts in the database, length

normalized to 1.8 m, with pre-load corrected. ...........................................................................................129

Figure 6-18 Distributions of mechanically anchored bolt responses, normalized to a length of 1.8 m. ...129

Figure 6-19 Conceptual distribution of pull test responses of mechanically anchored bolts. ...................130

Figure 6-20 Yield load and displacement at yield for 5'-6' MCB33s. .......................................................131

Figure 6-21 Load displacement curves of all modified cone bolts between 1.5 m and 1.8 m (5'-6') in the

database. ....................................................................................................................................................132

Figure 6-22 Load deformation curves of 5'-6' MCB33s with pre-load corrected to zero. .........................133

Figure 6-23 Distributions of 5'-6' MCB33s responses. .............................................................................133

Figure 6-24 Conceptual distribution of 5'-6' MCB33 responses. ..............................................................134

Figure 6-25 Working capacity and displacement at working capacity for 20 mm. ...................................136

Figure 6-26 Load displacement curves of 20 mm D-Bolts and ‘similar’ bolts. ........................................137

Figure 6-27 Distributions of 20 mm D-Bolts and ‘similar’ from Supplier B. ...........................................137

Figure 6-28 Distributions of 20 mm D-Bolts from Supplier D. ................................................................138

Figure 6-29 Working capacity and displacement at working capacity for ‘standard’ expandable bolts. ..140

Figure 6-30 Load displacement curves of ‘standard’ expandable bolts. ...................................................141

Figure 6-31 Distributions of ‘‘standard’’ expandable bolts. .....................................................................141

Figure 6-32 Conceptual distribution of ‘standard’ expandable bolt responses. ........................................142

Figure 6-33 Load displacement curves of ‘super’ expandable bolts in the database. ...............................143

Figure 6-34 Distributions of 'super' expandable bolt responses to pull tests. ............................................144

Figure 6-35 Load displacement curves of cable bolts in the database. .....................................................145

xxi

Figure 6-36 Distribution of load displacement curves of cable bolts. .......................................................146

Figure 6-37 Conceptualized distribution of responses of cable bolts. .......................................................146

Figure 6-38 The median response of several bolt types to pull test conditions. ........................................147

Figure 7-1 Diagram of formulation of 'rockbolt' element from UDEC, Itasca Consulting Group Inc.

(2014b). .....................................................................................................................................................154

Figure 7-2 Mechanical behavior of the grout interface of ‘rockbolt’ element: a) displacement in response

to load and b) influence of confining stress on grout cohesion, Itasca Consulting Group Inc. (2014b). ..155

Figure 8-1 Pull tests of fully encapsulated 20 mm resin grouted rebar installed in Good, Fair and Poor

ground conditions. .....................................................................................................................................160

Figure 8-2 Fully encapsulated resin grouted rebar pull test data, Stjern (1995). .......................................161

Figure 8-3 In situ pull tests in differing ground conditions supplemented with ultimate tensile loads and

post yield displacements. ...........................................................................................................................162

Figure 8-4 a) Diagram of drift in rock b) diagram of a manually operated pull test c) boundary conditions

and geometry of the numerical model, and d) a closer view of the numerical model which clearly shows

the nodes and segemnts of the 'rockbolt' element. .....................................................................................163

Figure 8-5 results of calibration with calibration targets. ..........................................................................165

Figure 8-6 Cropped view of numerical model showing rupture of 'rockbolt' element. .............................166

Figure 8-7 Shear spring stiffness influence on ‘rockbolt’ response. .........................................................166

Figure 8-8 Boundary conditions (a) and calibrated numerical result of 'rockbolt' element (b) after Bahrani

and Hadjigeorgiou (2017)..........................................................................................................................167

1

Chapter 1

Introduction

1.1 Problem Definition

Rock bolts are the primary means of rock mass reinforcement in underground excavations, Hadjigeorgiou

and Charette (2001). To safely design a ground support system using rock bolts, an understanding of the

expected loads and capacities on rock bolts are essential. Practical considerations dictate whether installing

a specific system is feasible in a specific ground conditions.

Quality control is an important element of a ground control program. The frequency and number of bolt

testing is defined by the company and must comply with the regulatory requirements of the specific

jurisdiction. In situ pull tests of rock bolts are part of all ground control quality control and quality assurance

programs. In particular, pull tests are used for:

• Measuring the in situ response of bolts to axial loads, thereby exposing unanticipated interactions

between the rock mass and bolt;

• Demonstrating rock bolts can (or cannot) be installed in specific working conditions;

• When repeated regularly and in sufficient number, alerting testing personnel to changes bolt in

performance.

In situ pull testing of rock bolts involves applying an axial load to an installed rock bolt while measuring

or observing the response. Typically, a hydraulic ram is used to apply the axial load while a caliper or

electronic transducer is used to measure extension of the ram. In Ontario underground mines pull tests are

generally performed to test new installation equipment, new suppliers, new products or as a quality control

measure. This thesis presents the results of collecting and analyzing a comprehensive database of rock bolt

pull test results across Ontario.

1.2 Objectives

The thesis aims to provide useful information for the design of underground mines using in situ pull test

data. There are three principal objectives of this thesis:

• To improve our understanding of the in situ behavior of different rock bolt types under pull test

conditions;

• To characterise the performance of rock bolts under pull conditions in underground hard rock mines

and provide typical design values for working capacity and deformation; and

2

• To reproduce the observed field performance of rock bolts in a stress analysis model and provide

further guidelines on how this information can be used in stability analysis.

1.3 Methodology

Following a literature review the methodology used in the thesis had three components:

• Data acquisition: A comprehensive database was developed from pull tests performed between

1998 and 2016 in 18 Ontario underground hard rock mines. The data was reviewed for

completeness and quality.

• Analysis and interpretation of data: A phenomenological interpretation of the behaviour of different

bolt types was used along with statistical analysis methods to characterize the performance of

different reinforcement elements under in situ pull test conditions. Linear regression and analysis

of variance (ANOVA) were used to indicate the impact of bolt installation conditions, bolt

geometry, rock mass quality on the rock bolt performance. Performance envelopes were developed

for seven different rock bolt types based on the analysis of load displacement pull test data.

• Numerical modelling investigation: The distinct element method (DEM) was used to simulate the

behavior of rock bolts under pull test conditions. The developed numerical models provided means

to further investigate the interaction between critical rock mass parameters and rock bolt properties.

The numerical models reproduced the performance of rock bolts in pull condition as observed in

the field and provided calibrated material properties for a range of rock mass conditions.

1.4 Thesis Outline

The thesis structure is as follows:

Chapter 1: Introduction - The importance of in situ rock bolt data is presented. The methodology and outline

of the thesis are presented.

Chapter 2: Background on in situ rock bolt testing - Presents background information of commonly used

rock bolts and in situ testing of rock bolts and reviews of prior work into in situ rock bolt pull tests.

Chapter 3: In situ pull test database formation - Details of the composition of the database are presented. A

general description of database composition follows, with discussion of broad trends of testing apparatus,

personnel and methodology.

Chapter 4: Statistical analysis and interpretation of the pull test data - The behavior of rock bolts is analyzed

using both phenomenological and statistical approaches. Methods of analysis are described as well as

theoretical models used to interpret results.

3

Chapter 5: Factors controlling rock bolt performance under pull test condition - Factors influencing the

response of resin grouted rebar bolts, friction rock stabilizers, modified cone bolts and mechanical bolts are

investigated using statistical and qualitative comparisons.

Chapter 6: Characterization of rock bolt performance - The behavior of rock bolts under pull test condition

is characterized by presenting distributions of bolt responses and comparing observed behavior to supplier

specifications and other bolt types.

Chapter 7: Numerical modelling methods for the simulation of pull tests - Following a literature review of

numerical modelling of pull tests the approach used in this thesis is presented.

Chapter 8: Numerical modelling of pull tests - The results from the numerical simulations are presented.

The calibrated material properties are related to the impact of rock mass quality to the rock bolt

performance. Calibrated material properties are presented for different rock mass conditions considering

the impact of rock mass quality to bolt behavior.

Chapter 9: Conclusions - The main conclusions are presented, and the contributions and limitations of this

thesis are discussed. Finally, this thesis provides recommendations for further work.

4

Chapter 2

Background on In Situ Rock Bolt Testing

2.1 Introduction

This chapter provides a concise introduction on rock bolts as ground support and a description of in situ

testing of rock bolts. The variations between in situ and laboratory rock bolt pull testing techniques are

discussed.

2.2 Rock Bolts and Ground Support

Rock bolts are the primary means of support of underground mining excavations, Hadjigeorgiou and

Charette (2001). Figure 2-1 illustrates some examples of rock bolts used in underground mines. There are

a variety of mechanisms for transferring load from the rock mass to the rock bolt. These can be broadly

grouped as fully grouted bolts, two point anchored bolts, and frictionally coupled bolts, Li et al. (2014) and

Li and Stillborg (1999). These categories are analogous to the continuously mechanically coupled,

discretely mechanically or frictionally coupled, and continuously frictionally coupled categories suggested

by Thompson et al. (2012).

Continuously mechanically coupled bolts typically use a cement, or resin based grout medium or grout, to

connect the bolt tendon to the borehole. The most commonly used medium in Ontario hard rock mines is a

mixture activated resin encased in thin plastic film cartridges. Examples include resin grouted rebar bolts,

or 'spin cable bolts' (pre-coiled lengths of cable with terminal attachments to function as a rock bolt).

A common type of discretely mechanically coupled bolt is the mechanical bolt, which uses an expansion

shell, activated by torqueing the bolt, to anchor the bolt at the toe of the borehole. Another variant is to

partially grout a continuously mechanically coupled bolt, e.g. rebar, such that it is anchored at a discrete

point.

Friction bolts are thin walled steel tubes. Anchorage is achieved by the frictional resistance at the contact

between the steel tube and borehole. These friction bolts can be subdivided into 'friction rock stabilizers’

(FRS) and expandable bolts. As described by Hadjigeorgiou and Charette (2001), FRSs are slotted tubes

with a tapered end and a rolled or welded collar. Expandable bolts are folded metal bladders, allowing

expansion in the borehole with the use of a water pump.

5

Figure 2-1 Examples of rock bolts Hadjigeorgiou and Charette (2001).

A specific class of bolts are designed to resist the dynamic loads of mine seismicity or the large

deformations associated with squeezing ground. These types of bolts are referred to as 'yielding' or 'energy

absorbing' bolts. An early example of yielding bolts is the modified cone bolt, with a shank that is decoupled

from resin by grease or a thin plastic coating and a cone shaped plow which can be pulled through the resin

column, Simser et al., (2006). Another example is the D-bolt, with a smooth high strength steel column that

is anchored to the borehole at discrete points. This allows the smooth shank to decouple from the resin and

stretch while the deformed portion maintains contact with the resin column. Li et al. (2014) provides a

comprehensive review of energy absorbing bolts.

2.3 In Situ Testing

A typical in situ pull out test in an Ontario underground hard rock mine involves attaching a hydraulic ram

to an installed bolt which then exerts an axial load on the bolt when expanded against the rock surface,

Figure 2-2. Depending on bolt type the hydraulic ram can be attached using a pull rod connecting to the

threading of a rock bolt or attached with a 'claw' adapter which grips the bolt. If deformation is measured,

a common approach is to quantify it using a Vernier caliper attached to the ram. This can then be manually

read out at set load increments. A better method is the use of a linear displacement transducer with a load

cell, which produces a digital output to a tethered computer.

6

If the seating of the ram on the rock is uneven, faceplates, or similarly shaped metal pieces, are used as a

wedge. Bolts are typically loaded up to initiation of yield, pull out, or to a minimum value of the required

strength of the bolt. Cyclic loading is not common. In practice, testing the bolt to failure is avoided for most

bolt types due to the safety concerns of testing materials or equipment being released from the rock surface.

Pre-load, tension applied to the bolt before zeroing displacement readings, may or may not be applied.

When used, a typical pre-load amount is 2 tons.

Figure 2-2 Example of typical pull test set up. The bar protruding from the hydraulic ram is not the

rock bolt being tested, but is rather a pull rod connected to installed bolt via an adapter.

Most rock bolt pull tests at Ontario mine sites are performed by personnel of ground support suppliers.

These tests are most commonly performed as part of an annual quality assurance check where multiple bolt

types are tested at the same location. Typically, five to ten bolts of each type would be tested. A second

common motivator for pull tests would be the introduction of a new bolt type or change in supplier or

component of a bolt. Finally, testing is also conducted following a fall of ground due to the failure of ground

support.

7

2.3.1 ASTM, ISRM Pull Tests and Differences to Common Practice

The ASTM has a standard for in situ pull tests of rock bolts, ASTM D4435-13. Similarly, the International

Society for Rock Mechanics has suggested methods for in situ pull tests of rock bolts, ISRM (1981). These

procedures are not as comprehensive as ASTM. Common practice in Ontario hard rock mines draws from

both procedures but does not fully comply with either. However, this is true for most mining jurisdictions.

The 'ASTM D4435-13: Standard Test Method for Rock Bolt Anchor Pull Test' requires testing 10-12 bolts

per expected scenario (rock unit, bolt type, orientation to underlying structure) and that at least five of these

bolts be loaded cyclically to ¼, ½ and ¾ of the expected failure load. The loading system should be a hollow

center hydraulic ram with a loading frame 'usable against uneven rock surfaces'. An electronic load

transducer to directly measure the load on the bolt is recommended, though measurement of pressure

supplied to the hydraulic ram is acceptable if friction losses and the like are accounted for. Measurement

of deformation should be performed by an instrument anchored to the rock wall or floor by at least 0.9 m

away. The end of bolt or pulling rod should have a dimple approximately 6 mm in diameter to accommodate

the tip of the dial gauge or measuring instrument. A diagram of an ASTM test is shown in Figure 2-3, from

ASTM International (2013).

Figure 2-3 Diagram of pull test apparatus as described in ASTM D4435-13.

The ISRM recommends testing 5 bolts per bolt type for expected rock and installation conditions using a

hydraulic ram capable of extending 50 mm. A dial gauge mounted to 'firm rock' is the recommended method

of recording bolt head displacement. The pre-load should not exceed 5 kN as per ISRM (1981).

8

The majority of pull tests in the database are neither compliant with the ASTM standard nor do they follow

the ISRM recommendations. Field tests generally measure ram travel as a proxy for bolt displacement rather

than the direct measurement of bolt head movement compared to a stable portion of the rock mass.

Compression of the rock mass near the hydraulic ram footing is therefore not accounted for and this may

introduce errors into the deformation measurement of the pull test. Similarly, the ASTM D4435-13 requires

the use of a loading frame "useable against uneven rock surfaces". If this is not the case it may cause uneven,

non-axial, loading of the bolt. A number of pull tests did not measure deformation of the rock bolt but relied

on qualitative descriptions of visibly observable movement and what was considered adequate bolt

behavior. The number of pull tests, per bolt, is generally lower than required by ASTM although it meets

or exceeds ISRM values; testing of bolts on multiple rock types or structural conditions is not attested to in

the database. The ASTM does not require annual testing, though this practice is widespread. Common pre-

loads exceed the ISRM recommended maximum of 5 kN.

2.4 Laboratory and In Situ Testing

Laboratory pull testing of rock bolts can aim to replicate in situ pull tests, or try to simulate the loading

conditions of a rock bolt crossing a discontinuity in the rock mass, for example Stillborg (1993).

Korzeniowski et al. (2015) describe a laboratory testing apparatus with a similar loading mechanism to in

situ pull tests. The authors constructed a testing apparatus which allows for loading of full length (

9

embedment length of resin rebar in high strength cement blocks. Chapters 4, 5 and 6 of this thesis will show

resin grouted rebar consistently yield with minimal embedment for in situ tests.

There has been a variety of work done on simulating the loading conditions of a rock bolt crossing a

discontinuity. Figure 2-4 shows a common approach which involves embedding one or more bolts through

two high strength concrete blocks which can then be loaded axially. This general set up can be extended to

applying non-axial loads to the rock bolts; that is a shearing motion across a joint or discontinuity.

Typically, this involves applying a load orthogonally to the axial pulling action, shown in Figure 2-5. These

approaches were used by Stillborg (1993), Li et al. (2014) and Chen and Li (2015).

Figure 2-4 Schematic of experimental apparatus for replicating rock bolts being loaded across a joint

Stillborg (1993).

Figure 2-5 Picture and diagram of experimental apparatus for replicating rock bolts being loaded

across a joint, with shear loads Chen and Li (2015).

10

Stillborg (1993) used the concrete block configuration in investigating the pull strength of several types of

rock bolt. A similar approach was used by Li et al. (2014), extended to include pure shear response. These

comparisons are useful in that they provide a reference point of relative bolt performance. Changes in

support strategies and technology, such as the D-Bolt (Li 2010) demonstrate the importance of these

laboratory tests.

While testing the bond strength of resin, or cement grouted, rebar bolts an alternate loading mechanism is

the push test. In a push test a small length of rebar has a compressive load applied axially to the bolt. An

example has been provided by Ma et al. (2014) where push test results were used to calibrate numerical

models of fully grouted rebar bolts.

One method of reproducing field conditions in a laboratory setting is to recover a rock bolt by overcoring

and removing the installed rock bolt with surrounding rock. This assemblage can then be tested with

laboratory equipment. This approach was used by Villaescusa et al. (2008) to investigate some rock bolts

used in Australian mines, with a focus on resin grouted rebar.

Barnard et al. (2016) compared pull out test results of expandable rock bolts in different ground conditions

as determined by the RMR76 system described by Bieniawski (1976). The authors constructed a database

of expandable bolt pull tests from goldmines in the Nevada area. The authors note that the use of RMR76

for rock mass classification is common practice in Nevada gold mines. A weak but positive correlation

between RMR76 and bond strength was found. This was attributed to the observed association between the

RMR76 and the borehole diameter, with lower quality rock masses producing wider holes with same size

drill bits. It is worth noting that although this was a large database of results, the majority (>80%) of bolts

were not loaded to slippage which leads to conservative bond strength estimate. Most pull test results also

lacked geotechnical information, with a portion of those with information having Q values which were

subsequently converted to RMR76. This introduced a further degree of uncertainty in the analysis.

2.5 Data Collected in a Pull Test

During a pull test, load is applied to the bolt and the resultant displacement is recorded. These measurements

allow the properties of the bolt-anchor-rock mass system to be calculated, or inferred, and the bolt

performance to be quantified. This thesis focuses primarily on the working capacity and stiffness of the

rock bolts under in situ pull test conditions. These are defined in the subsequent section.

2.5.1 Working Capacity and Measuring Load

The ultimate capacity of a reinforcement element is in many ways the most pertinent metric of bolt behavior

for ground support design. However, due to safety concerns over the sudden release of elastic energy stored

11

in a loaded rock bolt, pull tests in Ontario hard rock mines are rarely carried on to failure of the bolt.

Working capacity offers a safer alternative. It refers to the point when large permanent deformations have

occurred, and would continue with relatively little increase in load, ultimately leading to failure. It should

be noted that safety concerns do not apply to FRS and as such they are typically loaded to (slip) failure.

A ground support system can be considered successful when it contributes to safe, stable, excavation

ensuring its continued operation. A bolt reaching its working capacity begins (at least partially) to deform

and would experience large deformations with relatively little increase in load. This in turn may lead to

unravelling of the rock mass or closure of the drift. Both cases having the potential to interfere with

production or lead to progressive rock mass failure. Such a design can be said to have failed, despite the

absence of actual failure of ground support elements or mesh, in that rehabilitation or abandonment of the

drift may be required. Dynamic loading is similarly tied to reaching the working capacity (yield) of ground

support elements.

Differences in the loading rate prevent the direct comparison of results of static and dynamic tests as

material properties of support elements are strain rate dependent. Figure 2-6 shows the results of both a

quasi-static laboratory pull test and a dynamic laboratory impact test of 22 mm D-Bolts. The dynamically

loaded bolt had both a higher yield load and total deformation despite being shorter than the quasi-statically

loaded bolt. An indirect comparison of these results, shows how the working capacity from quasi-static

testing can be used as a design parameter for static conditions to maximize a D-Bolts dynamic capacity, Li

et al. (2014). The energy capacity of a rock bolt is defined as the area under the load displacement curve.

A D-Bolt which has undergone elastic deformation under load less than its working capacity retains most

of its energy absorbing capacity. Conversely, a D-Bolt which has begun deforming plastically, while

supporting a static load, has substantially decreased its ‘available’ energy capacity. This degradation results

in a reduction from its design capacity to support dynamic loads.

Most pull test reports refer to working capacity as ‘yield’ for all bolt types. The ASTM defines yield as “the

engineering stress at which, by convention, it is considered that plastic elongation of the material has

commenced” ASTM (2015b). Figure 2-7 shows two methods of determining yield as per ASTM E6-09b.

12

Figure 2-6 Static (a) and dynamic (b) testing of 22 mm D-Bolts, after Li (2012) and Li and Doucet

(2012) respectively.

Figure 2-7 ASTM E6 09b description identifying yield; a) upper and lower yield strength as the stress

which results in plastic deformation of the material. b) a pre-defined strain offset is used to identify

a yield stress.

Figure 2-7 shows the yield strength, Sy (UYS and LYS are upper and lower yield strength respectively),

determined by either a perfectly plastic response, where deformation continues with the same load, or at a

predefined offset to the elastic response. In most cases, the resolution of load data recorded in pull test

reports is quite poor. Manually recorded tests typically followed increments of 1 short ton (8.9 kN).

Digitally recorded tests are frequency based, with a variable resolution dependent on the sampling

frequency and loading rate. The plastic behavior is not typically captured when the displacement is recorded

at set load increments. A pre-defined strain offset to determine yield is not practical due to the high

variability of many bolts. Therefore, the methods presented in Figure 2-7 have severe practical limitations.

13

For the purposes of this thesis the working capacity was defined as the highest recorded load before the bolt

response softens as attributed to either plastic deformation of rock bolt or movement of the entire bolt (slip

failure). Figure 2-8 shows an example of a working capacity estimate using a load deformation curve of a

pull test with manually recorded displacement.

Figure 2-8 Defining working capacity from pull out test.

For most bolt types, this working capacity is probably a lower bound of the yield strength of the bolt. It is

noted that for some bolt types the working or ultimate capacity occurs at loads lower than the supplier

specified yield loads. This was consistently observed for recently installed FRSs. The bolts regularly

experience a pull out ‘slip’ failure rather than yield of the steel tube. For FRSs, the ultimate capacity is

defined as the maximum load the bolt sustains without slipping. Working capacity and ultimate capacity

are the primary load metrics used in this thesis to assess the performance of rock bolts..

2.5.2 Recording Displacement

Bolt head displacement during a pull test can be attributed to multiple mechanisms: elastic (or plastic)

deformation of the bolt, compression of the rock mass (when not recording displacement from a stable

datum), movement of the entire bolt (slip), movement of the loading frame and others. Figure 2-9.

14

Figure 2-9 Illustration of pull test with generic bolt, after Nicholson (2016).

The total apparent displacement (da) is a combination of actual bolt head displacement (db) and

displacement of the rock surface (ds). The actual displacement of the bolt head results from bolt deformation

and/or movement as a response to load. Displacement of the rock surface can result as a combination of

compression and fracturing/moving of the rock mass. It can also occur due to compression of any surface

support and compression of the hydraulic ram. This total apparent displacement is the recorded parameter

when displacement is measured using the approaches (ram mounted caliper or linear transducer) used in

Ontario hard rock mines. Recording bolt head displacement from an independent datum, as per ISRM

recommended practices and ASTM standards, would minimize the influence of displacement of the rock

surface.

Recording apparent displacement may not accurately reflect the behavior of the bolt alone. It may yet be

representative of certain loading scenarios. Figure 2-10 is a conceptual representation of the interactions

between the components of a generic rock bolt system.

15

Figure 2-10 The interactions between components of a generic rock bolt system, after Thompson et

al. (2012).

Recording bolt head displacement from an independent datum captures two of the interaction types

illustrated in Figure 2-10: between the element and internal fixture, and between the internal fixture and

rock mass. By recording displacement as a hydraulic ram extension, a third interaction type is

approximated, the interaction between the external fixture and the rock mass.

Figure 2-11 shows a discretely anchored bolt, such as a mechanical bolt, which is axially loaded while

supporting a structurally defined rock wedge. The total apparent displacement (da) results from the

deformation of the bolt (db) and compression of the rock wedge around the bolts external fastener or

faceplate (ds). In this way recording the hydraulic ram extension may approximate the loading condition of

an axially loaded point anchored bolt. For continuously coupled bolts, such as a resin grouted rebar, this

would depend more on where the bolt is loaded and how load attenuates