-
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
-
x
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