Hartman W & Hebblewhite B, 2003.

in Proceedings 1st Australasian Ground

Control in Mining Conference. Sydney November 2003.

Understanding the Performance of Rock

Reinforcement Elements under Shear
Loading through Laboratory Testing: A
30-year History
By W Hartman 1 & B Hebblewhite 2

Abstract
This paper outlines the history of the developments over the last 30 years in understanding the
performance of rock reinforcement elements under shear loading through laboratory testing and
where the research stands today. Shear testing of rock bolts was first conducted at the Swedish Rock
Mechanics Research Foundation in 1974 in hard rock reinforced by rock bolts and was followed by a
series of other research attempts around the world over the last 30 years. The factors looked into
included the size (length and diameter) and number of bolts, the inclination of the bolts, the relative
displacements in joints, joint roughness, the effect of compression, relative strength of rock and grout
and elastic modulus of rock and grout. Analytical and numerical solutions were also proposed based
on these experiments.

The paper takes the reader through these developments and critically analyses their achievements
and shortcomings. It highlights the current understanding and its shortcomings, and identifies the
need for further research. It then introduces the state of the art facility being established at The
Mining Research Centre at UNSW for shear testing of reinforcement elements and the anticipated
outcomes from this experimentation exercise.

Introduction The difference between support and reinforcement was also

understood as support being the application of a reactive force
Although the use of rock bolts can be traced as far back as the at the face of the excavation and reinforcement as the
Roman Empire when slot and wedge type rock bolts were in improvement of the overall rock mass performance from within
use, the more recent use in mining and tunnelling dates back to the rock mass by techniques such as rock bolts, cable bolts and
the late nineteenth century. Up until the 1950’s however, it was ground anchors.
generally held that the purpose of rock bolts was to pin surface
rock (either individual blocks or bedded strata as encountered The mechanism of bolt performance in reinforcing the rock
in underground coal mining) to more stable rock some distance mass under field loading conditions, although widely studied,
from the surface of the excavation. still remains to be established in the form of universally
recognised and accepted code of practice. There is a
At the time, mechanically anchored (slot and wedge or requirement by industry (i.e. end users, consultants, educators
expansion shell) bolts were used. The next two decades saw the and manufacturers) to actively identify the key parameters,
advent of chemically anchored bolts, full column grout bolts which govern safe and cost effective excavation practices,
and resin bolts being used in underground mines including in which include the following:
Australia. By the 1980’s it was accepted that the term rock
bolting was “generally to indicate any form of mechanical • Reinforcement installation in the form of cable bolts
support that is inserted into the rock mass with the primary and splitsets in the footwall of long hole open stopes
objective of increasing its stiffness and/or strength with is used to prevent slide out failure on pre-existing
respect to tensile and/or shear loads”, Gerard (1983). This geological structures where the reinforcement
included the application of compression (by active or passive element acts as a dowel to prevent shear failure
tensioning) to improve resistance to shear and tension (referred • In underground coalmines with roadway roofs
to as the “friction effect” by Panek (1964)). consisting of laminated shale and sandstone, solid
chemical bonded rock reinforcement elements are the
preferred method to prevent large displacements and
act as shear resistant dowels.
1. Senior Geotechnical Engineer, AMC Consultants Pty Ltd, 19/114 • Rock reinforcement elements are also used in hard
William Street, Melbourne Vic 3000.
E-mail: whartman@amcconsultants.com.au
rock open cut mines with slope stability problems
above ramps where large blocks are prevented from
2. Professor of Rock Mechanics Head of School and Research Director,
School of Mining Engineering, The University of New South Wales
sliding out and equilibrium is established through
dowel action.
Understanding the Performance of Rock Reinforcement Elements under Shear Loading through Laboratory Testing:
A 30-year History

• Rock reinforcement elements have been installed in Influencing parameters

roadside cuts to secure blocks of rock with potential
The influencing parameters, which have been studied through
to slide out or topple over.
laboratory experiments thus far maybe, grouped under the
• In the tunnelling community, installation of following sub headings:
reinforcement elements is the accepted practice in
stabilising the tunnels. i) Rock Mass
• Joint opening/ aperture
The Mine Safety and Health Administration (MSHA), in the
United States, statistics show that more than 400 miners are • Joint surface roughness
injured in roof and rib falls annually; in the 5 years from 1994 • Joint strength
through 1998, 53 miners died in such accidents (McHugh &
Signer, 1999). • Dilatancy during shearing
• Deformability of host rock
In general, sagging of a mine roof and yielding of pillars results • Rock deformability vs. bolt deformability
in vertical and horizontal stresses in a mine roof, imparting both
axial and shear forces on roof bolts. Combined tensile and shear
forces are at times sufficient to cause failure of the bolts. ii) The reinforcement element system
• Hole size
Currently the rock reinforcement requirements for a site are
estimated still based on the assumption that the bolt will pin the • Hole roughness
loose block of rock to a stronger horizon of rock. The • Bolt diameter
contribution of the bolt in increasing the shear strength across • Type of grout
the discontinuous surface (block interface see Figure 1),
although recognised, is still not quantifiable. • Thickness of grout collar
• Grouted vs. un-grouted bolt
Figure 1 - Reinforcement actions at opening and • Fully bonded vs. point anchored bolt
shearing discontinuities (from Windsor and Thompson,
• Bolt material and its strength and deformability
1992:3)
• Bolt deformability vs. rock deformability
• Inclination of the bolt
• Tension force in the bolt
• Deformed length of the bolt

iii) Loading conditions

• Pre-loading of bolts (tensioning and torque)
• Normal stress on the shear surface
• Shear displacement induced tension
• Dilatancy induce tension during shearing
• Magnitude of shear displacement
• Deformed length of the bolt

A summary of the past research results is presented below

Numerous attempts have been made at laboratory testing to showing the chronological development in the understanding of
determine both the tensional and shear strength contribution of the performance of bolted rock joints unde shear loading.
bolts across joints. This research has evolved over the years and
is at a stage where a final comprehensive testing exercise would
Experimental set-up and results
reveal and state the principles involved.
a) The earliest traceable report of shear testing of rock bolts is
Historical Review of Testing by Sten Bjurström in 1974. He recognised the fact that the
capacity of bolted joints to transfer shear forces is an important
Methodologies but often unknown factor when estimating stability and
deformational behaviour of bolted jointed rock mass.
Starting from simple direct shear experiments focusing on the
mechanism of shearing in a reinforced jointed rock mass, it He dealt specifically with shear tests on fully cement-bonded
quickly became quite apparent that the problem was a complex rock bolts embedded in blocks of granite. He considered the
one and the number of parameters which could potentially following four aspects of the bolt effect:
influence the performance of rock reinforcement elements in a
jointed rock mass subjected to shear loading was large. Thus • Tension force in the bolt
more sophisticated ways of conducting the tests were designed.
What is intriguing is the fact that over the years researchers • Friction at the shear surface as a consequence of
have reported quite contradictory results with regards to some increased normal stress
of these parameters. • Dowel effect of the bolts

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Understanding the Performance of Rock Reinforcement Elements under Shear Loading through Laboratory Testing:
A 30-year History

• Bolt inclination with respect to shear surface perpendicular to the joint and about 90% for inclined
bolts
He concluded that inclining the bolts resulted in stiffening the • The friction characteristics of the joint do not
shear and in an increase of the shear strength at smaller influence the contribution of the bolt
displacements. • Perpendicular bolts do not experience considerable
tension stress
b) Haas et al. in 1976 conducted experiments using various
bolt types and anchors in blocks of limestone and shale. They • For a given shear displacement, dilatancy increases
found that resistance to shear stress was increased about 3.7 the resistance of the bolted joint (this is in direct
times when fully grouted bolts were used to secure a natural contradiction to his second conclusion stated above).
fracture. The loading frame was orientated so that the interface
between the blocks of rock was vertical. A compressive force d) Haas et al (1981) repeated their earlier experiments, this
was exerted normal to the plane form the right and left sides time applying uniform normal pressure on the shear surface.
(Figure 2). They used different types of bolts, grouts and drill holes for
their experiments. Their main conclusions were that:
Figure 2 - Schematic sketch of loading geometry
showing section of rock, fracture surface, and grouted • No positive effect of pre-tensioning could be
bolt (Haas, 1976). observed
• Bolts with full bond were much stiffer than point
anchored bolts
• Inclined bolts were stiffer and contributed more to the
shear strength of the bolted blocks than perpendicular
ones
• The normal stress on the shear surface did not
influence the shear resistance of a bolt (contradicting
their earlier conclusion from 1976)
• Effects of dilatancy contributed to the stiffness of the
bolted joint.

e) In 1981 Hibino and Motjima published their results from

shear tests with un-grouted bolts (both fully bonded and point
anchored) in concrete blocks. Their conclusions were:

• For given displacements the shear resistance of fully-

bonded bolts was considerably higher than that of
point anchored ones
• The inclination of a bolt did not really increase its
shear resistance. This conclusion was contrary to all
other researchers’ findings regarding the influence of
A torque was applied to the bolts after the normal load was bolt inclination.
applied to the block. The objective of applying this torque is not • “Pre-tensioning of the bolts reduced the shear
clear and appears that this was used to apparently lock in the displacements but did not influence the shear
applied normal loads. The effectiveness of this process is it self, resistance”
doubtful. They concluded that very low normal pressure values
(of, σn = 25psi) provided a significant increase in the shear f) Egger and Fernandez from the Institute of Swiss Federal
resistance of a bolted rock joint. An increase in shear resistance Institute of Technology, Lausanne, Switzerland, commenced
is also noted at the higher normal pressure of 250 psi; however, the reporting of their research in this area in 1983 (Egger et.
the increase is small compare to corresponding shear resistance Al., 1990). They tested bolted samples of concrete blocks in a
without a bolt. Haas’s conclusions with regards to bolt high capacity press. They found that
inclination were that when the bolt is orientated between 0 and
+450 degrees (measured from normal to the shear surface and in • The optimum angle of bolt inclination with respect to
the direction of applied shear force) the shear resistance the joint was 30o to 60o
increases significantly, whereas at -450 no shear resistance is • Bolts perpendicular to the shear plane furnished the
added as the bolt lost tension at very small shear displacement. lowest shear resistance
• Shear displacements at failure were minimal for bolt
c) Azuar in 1977 reported laboratory tests with resin-grouted
inclinations between 40o and 50o
bolts embedded in concrete (Haas et. al., 1981) He concluded
that:
g) In 1983 Ludvig presented results from tests, which were
• The maximum contribution of a rock bolt to the shear conducted using the same rig as that used by Bjurström in 1976.
resistance of a joint is 60% to 80% of the ultimate He applied normal loads in the range of 0.2 MPa to 10 MPa. He
tension load of the bolt in the case of the bolt being reported that a 8mm steel bolt installed in slate could bear up to

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Understanding the Performance of Rock Reinforcement Elements under Shear Loading through Laboratory Testing:
A 30-year History

18mm of shear displacement due to the large normal stress and was more easily crushed near the exit point. In addition, higher
relatively weak rock. reinforcement resistance was obtained in weaker blocks of rock
because the cable aligned itself with the load around the
Bolts were tested at 45o to the shear surfaces. The shear crushed borehole.
resistance varied between 490 MPa to 780MPa, which is larger
than the shear strength observed for bolts installed The shear resistance of the bolted joint consists of its proper
perpendicular to the shear surface. This was found to be true for shear strength τ = σ n • tan θ and of the contribution of the
fibreglass bolts as well. However, the shear displacement
before failure was smaller than those observed for bolt. This latter is a result of the elastic responses of the bolt,
perpendicular installed steel bolts. All inclined steel bolts, apart the mortar and the rock and depends, therefore, on the Young’s
from the test conducted in slate, failed after a displacement, moduli of these materials as well as on the dimensions of the
which was smaller than the bolt diameter. His conclusions bolt and the mortar cylinder. The stresses in the three materials
were: are compressive on the side behind the bolt (C in Figure 3, and
tensile on its front side (T Figure 3).
• Shear strength of the rock bolts is strongly dependent
on the material used for construction of the bolt and Figure 3 - Initial state of a bolted joint (Spang & Egger,
also whether the bolt is solid or hollow 1990:205)
• Massive steel bolts have the largest shear strength
• For a given diameter the shear strength of fibreglass
bolts is about one half to shear strength of steel bolts

h) Dight in 1982 used various different materials including

gypsum, basalt and steel and examined the shear resistance of
bolted joints in them. He mainly found that:

• “The normal stress acting on the joint had no

influence on the shear resistance”
• “Joints with inclined bolts were stiffer than those with
perpendicular ones”
• “The influence of dilatancy had the same effect as
that of a bolt inclination” but did not conclude which
direction angle had the same effect as dilatancy
• The deformed length of the bolt was related to the
deformability of the rock
• The dowel effect was defined as the difference The tension stresses in the mortar and the rock disappear
between the shear resistance of a fully bounded and because of the very low adhesive strength between steel and the
that of a point anchored bolt mortar giving rise to a gap on the tension side. The maximum
contribution of the bolt (To) to the total shear strength of the
(i) Schubert (1984) conducted shear tests on bolted concrete joint is a function of various parameters which is shown in an
and limestone blocks. The main results of his research work empirical expression below:
are:
To=Tu[1.55+0.011σc1.07Sin2[β+i]
• The deformability of the surrounding rock is . σc0.14(0.85+0.45tanφ)
important for the bolt reaction
Where,
• Bolts embedded in harder rock require smaller
displacements for attaining a given resistance than Tu is the ultimate axial bolt load (tensile strength)
those in softer rock
β is the angle between the bolt and joint surface
• “Soft steel improved the deformability of the bolted
system in soft rock” i is the angle of dilatancy

j) Turner (1987) reported that appreciable opening of fractures

φ is the angle of friction along the shear plane
prior to shear movements resulted in double bends and failure
σc is the compressive strength of the host rock
of rock bolts by tension. Bolts across unopened fractures had
been actually cut (guillotined) during rock bursts (Spang &
They further stated that one of the most important parameters is
egger, 1990).
the stiffness of the rock and the mortar, which depends, to a
k) Spang and Egger (1990) studied the shear resistance of certain degree, on the compressive strength σ c
different blocks reinforced with 8mm bolts. In these tests, the
deformability of the rock was shown to be one of the most l) Egger and Zabuski (1991) reported that bolts work as an
important parameters for the shear resistance of bolted joints. additional resistance against shear failure along joints, hence
They differentiated between elastic, yield and plastic stages of the entire rock mass becomes stronger and deforms less. They
the shear stress vs. shear displacement curve and pointed out used a standard apparatus for direct shear testing of rock
that the response was softer in weaker blocks since the rock samples. The tests were conducted without an external normal

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Understanding the Performance of Rock Reinforcement Elements under Shear Loading through Laboratory Testing:
A 30-year History

force. They explained that with no external force, the bolt was smooth versus rough joint surfaces, grouted versus un-grouted
getting stressed because vertical displacements developed due cables and small, large and over size drill holes.
to the sliding of the sample on the joint asperities. Therefore Pouring concrete around a rod and extracting it without any
additional frictional shear resistance appears, proportional to drilling simulated the drill holes. The increase in shear stress
the normal force. was attributed to shear resistance of the cable and to the fact
that tensile force along the cable was transferred to the shear
Small displacements were accompanied by a great increase of blocks, which resulted in an increase in the normal force on the
the shear force. During this stage, local resisting forces were joint.
mobilised in the concrete, which limit bending of the bolt until
the UCS of the concrete is reached. At that moment the o) Roberts (1995) reported shear test results for smooth bars
concrete was destroyed in those zones and the bolt was able to and cone bolts and compared his results to Spang and Egger’s
move more freely. The failure surface showed signs of shear theory. Although his results agreed with the theory he pointed
and tension failure. The bolt was deformed only in closest out that the grout and rock have a significant contribution to the
vicinity of the joint. Inside the sample, at a maximum depth of overall shear strength of the bolted rock joint and had not been
about 10mm from the joint surface, there was no evidence of included in the theoretical treatment of predicting shear
the bolt deformation or traces of lost contact between the steel resistance. He also compared results of shearing an element at
and concrete or of slip between them. two interfaces (double shear) to a single interface shear and
found that the former was not simply double of the latter as true
m) Pellet et al. (1996) related theoretical and experimental symmetry did not exist in the case of double shear. Shear
analysis of rock bolt shear strength and found that -‘bolts failure would occur at one interface first and subsequently
installed perpendicular to a joint plane allowed the greatest resulted in failure of the other interface (GAP 335. 1995).
displacement along the joint before failure”-, but that
displacement at failure decreased rapidly as the angle between p) Ed McHugh and Steve Signer (1999) reported cases from
the bolt and joint plane decreased. They also found that harder the US of shear loading contributing significantly to failure of
rock led to bolt failure at smaller displacements. bolts used for rock reinforcement in coalmine roofs. They
conducted a series of tests to study the behaviour of roof bolts
n) Goris and Lewis (1996) conducted shear tests on concrete subjected to shear loading over a range of axial bolt loads and
blocks having joint surfaces ranging from rough to smooth, measured the distribution of axial and bending strain along the
with and without reinforcement. Three variables were studied; length of the bolts.

Figure 4 - Scheme of the experimental set up - 3 bolted blocks (60·60·100cm) with two bolts each joint (Grasselli et
al., 1999).

Tv: Vertical
Force

60cm Vertical load cell

Bolt

100 cm N: Confinement
force

Horizontal Load
Cell

There main findings included; i) that axial loading has little actual strength of the bolt iii) the hardness of host rocks plays a
effect on a joint‘s resistance to shear loading (Recent analytical role in how a bolt responds to shear loading iv) the nut and
research in the performance of pre-tensioned bolts suggests that plate anchors at the ends of the test bolts ensured that grout
only about 10% of the torque applied to a bolt remains as the failure would not be a factor in load and displacement profiles.
axial tension in the bolt, the remaining 90% goes towards
overcoming thread and bearing friction, Fernando, 2001) ii)
joint shear strength at yield averaged about 76% of expected

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Understanding the Performance of Rock Reinforcement Elements under Shear Loading through Laboratory Testing:
A 30-year History

q) Grasselli, Kharchafi and Egger (1999) did an analysis of the two plastic hinges. A great displacement (≈8mm) is
the results obtained from large scale (1:1) laboratory tests of associated with the formation of the plastic hinges and with the
bolt-reinforced rock with fully grouted rods and hollow tubes. progressive breaking up of the grout around the steel rod. The
resistance contribution T* reaches its maximum value (≈0.9
Apart from the influencing parameters already reported in this Fmax). However Roberts (1995) had already reported the
paper Grasselli et. al. pointed out that the 3D aspect of this limitation of this testing method involving double shear and
system, makes it hard to analyse and simulate. thus the results cannot be accepted as representative of
performance of a bolt along a single interface.
An experimental set-up was developed as shown in Figure 4.
The set-up simulated double shearing of one or two strain Current Understanding From Past
gauged bolts installed through three large blocks. During a test,
a controlled jack pushed progressively down the central block. Research

A numerical relation was developed giving the contribution of There seems to be a general agreement among researchers and
each bolt T* as: practitioners regarding the influence of some of the parameters
involved e.g. bolt inclination increases resistance and decreases
Tv − 2 ⋅ N ⋅ tgφi shear displacements. There is a general lack in understanding of
T* =
2 ⋅ Fmax the actual reinforcement installation method and the associated
loading mechanisms that may cause failure. Especially of
concern is the effectiveness of pre-tensioning, torqueing, axial
Where, tensioning and or normal loading. Experimental set-ups in the
Tv is the vertical force applied on the central block, past have used one or more of these loading systems either to
clamp the model or simulate the pre-tensioning practice, which
N is the confinement force, is very popular in the field. Their research remains inconclusive
to date. In view of the fact that pre-tensioning / torqueing is
φi is the friction angle of the block surface, and considered vital to the reinforcement methodology being
practiced over the last two decades and that installation rigs
Fmax is the ultimate tensile load of the bolt. have specifically been designed to incorporate this process
active research is called for to look exclusively at the
effectiveness of this process before any more time and money is
Figure 5 shows the load shear displacement curve for a full spent in advancing this practice for which limited
steel bolt. understanding exists.

Similar to the findings of earlier researchers the curve exhibits: In particular the following questions still remain un-answered
in our quest of understanding the performance of reinforcement
1. Linear behaviour, with small displacements and large elements across a jointed interface.
increase of load
2. A non linear behaviour corresponding to the • Is pre-tensioning of the reinforcement element across
plastification of the materials a jointed surface maintained until loading of the
3. Plastic behaviour - nearly free deformation of the rockmass occurs?
bolt, until its failure • Is actual torque reducing the yielding strength of a
reinforcement element and subsequently the shear
Figure 5 - Full Steel Bolt Behaviour (Grasselli et al., resistance?
1999). • What role does the size of the bearing plate play in
the performance of the reinforcement element in
shear loading?
• Will it be beneficial in upgrading the elastic moduli
of grout (including resin)?
• Is the stiffness of the reinforcing element assisting or
destroying the resistance to shear loading within the
reinforcing system?

What effect has shear loading on the surrounding rockmass?

Will an Australian standard or guideline assist in proper design
for expected shear loading conditions?

New Testing Facility Being Developed at

The Mining Research Centre, UNSW
(ACARP Project No. C12010)
A laboratory test facility for full scale testing of shear
performance of installed reinforcement elements is being
According to Grasselli et al. (1999) the bolt failure is
principally caused by the traction load concentrated between

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Understanding the Performance of Rock Reinforcement Elements under Shear Loading through Laboratory Testing:
A 30-year History

Figure 6 - Schematic diagram of test rig facility at Mining Research Centre, UNSW

developed at the Mining Research Centre, School of Mining 4) To conduct parallel theoretical, mechanistic and
Engineering at UNSW as part of an ACARP funded project. computational studies in support of above experimental
program.
A schematic diagram of the testing facility is shown in Figure
6. 5) To prepare a set of industry guidelines for the application of
reinforcement systems in discontinuous materials, with respect
Objectives to their performance in shear resistance, and comparison with
axial loading performance subject to similar variables.
The main objectives of this project include:
Expected outcomes and benefits
1) To research the current understanding of the performance of
reinforcement elements in shear. • A significant improvement in the level of
understanding of the performance of reinforcement
2) To design and develop a testing rig which meets the need of elements under shear – a role that is fundamental to
the required testing. good roof control in Australian coal mines, open
stopes, open cut ramps wall stability and underground
3) To conduct a series of controlled laboratory experiments tunnels.
using the facility, to study the effect of the following variables • The availability of a controlled environment,
on the performance of reinforcement elements in both direct laboratory test facility for future evaluation of
shear resistance, and indirect shear resistance through axial different products or reinforcement concepts, under
clamping: shear loading.
• Borehole and element geometry • The opportunity to provide a more scientific (hence
efficient) basis for optimising element reinforcement
• Element orientation relative to discontinuity designs for mines (in both primary and secondary
• Element and grout geomechanical properties support).
• Geomechanical properties of test block • Clear safety implications for the industry through
• Block geometry achieving the above outcomes.
• Element pre-tensioning • Further training of industry geotechnical engineering
specialists through this project, and the opportunity
• Characteristics of discontinuity for others through similar future work.
• Discontinuity aperture

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Understanding the Performance of Rock Reinforcement Elements under Shear Loading through Laboratory Testing:
A 30-year History

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Mining Engineers, Transactions Vol. 260, pp. 32-41.
Bjurstrom, S. 1974. Shear strength of Hard Rock Joints
Reinforced by Grouted Un-tensioned Bolts. Proceedings of the Haas, C.J. 1981. Analysis of rock bolting to prevent shear
3rd International Congress on Rock Mechanics, Denver movement in fractured ground, Mining Engineering, Vol. 33,
Colorado. National Academy of science, Washington, Vol. 2, No. 6, pp 698-704.
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Hibino, s and Motojima, M. 1981. Effects of Rock Bolting in
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in open mines. PhD Thesis, Monash University, Australia. Weak Rock, 1057~1062, Tokyo, 1981.

Egger, P. and Pellet, F. 1990. Behaviour of reinforced jointed Ludvig, B. 1983. Shear Tests on Rock Bolts. Proceedings of the
models under multiaxial loadings. Rock Joints - Barton & International Symposium on Rock Bolting, Abisko 28 August –
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Egger, P. and Zabuski, L. 1991. Behaviour of rough bolted McHugh, E., and Signer, S. 1999. Roof bolt response to shear
joints in direct shear tests. Proceedings of the 7th ISRM Cong., stress: laboratory analysis. In Proceedings, 18th International
Aachen, Germany, pp. 1285-1288. Conference on Ground Control in Mining (Morgantown, WV,
Aug 3-5, 1998). Dept. of Min. Eng. WV, Univ., pp232-238
Fernando, S. 2001. An engineering insight to the fundamental
behaviour of tensile bolted joints. Steel Construction, Vol. 35, Pellet, F. and Egger, P. 1996. Analytic model for the
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support units and systems currently used with a view to Roberts, D.P. 1995. Testing of mining tunnel support elements
improving the effectiveness of support, stability and safety of and systems for hard rock mines. Master of Science in
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pp 91-94. University of Natal, South Africa.

Gerrard, C. 1983. Rock Bolting in Theory – A keynote lecture. Schubert, P. (1984). Das Tragvermogen des mortelversetzten
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Abisko, 28 August – 2 September 1983. Balkema. Thesis, Montan-Universitat Leoben, Austria, 1984.

Goris, J.M., Martin, L.A. and Curtin, R.P. 1996. Shear Spang, K. and Egger, P. 1990. Action of fully grouted bolts in
behaviour of Cable Bolt Supports in Horizontal, Bedded jointed rock and factors of influence. Rock Mechanics and Rock
Deposits. Proceedings of the 15th International Conference on Engineering, Vol. 23, pp 201-229.
Ground Control in Mining, Golden, Colorado.
Windsor, C.R. 1992. Block Stability in Jointed Rock Masses.
Grasselli, G., Kharchafi, M and Egger, P. 1999. Experimental Submitted for Publication in the Proceedings of the Conference
and numerical comparison between fully grouted and frictional on Fractured and Jointed Rock Masses. Berheley: University of
bolts. Swiss Federal Institute of Technology, Lausanne, California, Vol. 1. Pp 65-72.
Switzerland. Proc. Int. Congress on Rock Mechanics, Paris
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