Clarifying Cable Bolting
Practices and Principles
Jim Galvin
Mine Managers Association of Australia
2012 Annual Seminar
7 – 8 June 2012
Support Mechanics and Design
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Characterised currently by:
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Controversy
Contradictions
Conflicts
Competition
= CONFUSION
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Amongst practitioners
Amongst those statutorily accountable
= RISK
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Objective
Take one of these issues – cable bolting - and
provide a basic understanding of cable bolting
principles and practices to assist mine
managers in evaluating advice relating to cable
bolting:
 Pattern
 Anchorage
 Length
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Some Basic Principles
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Tendon Support Systems
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Tendon
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All forms of bar, tube, cable (wire stand) support
systems anchored to the rock mass in boreholes
Fully Encapsulated
End Anchored
Friction Anchored
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Stiffness
Ultimate
Stiff
Yield
Soft
Unencapsulated
Cable Length
Extension at Yield
(1%)
Extension at Ultimate
(6%)
1
10 mm
60 mm
4
40 mm
240 mm
8
80 mm
480 mm
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Fully Encapsulated Tendons
Load Transfer
• For parting to open, rock mass on free
side of parting has to slide past the
tendon
• This generates shear stress in the
encapsulation medium, or grout,
which is then transmitted to the
tendon where it generates axial load
• The converse then occurs on the
opposite side of the parting
• Tension created in tendon acts to
clamp parting and resist further
opening
• Clamping force increases with
displacement across parting, until
peak load capacity of a support
system component is exceeded
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Fully Encapsulated Tendons
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Considerations
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Load Transfer

Exponential axial load transfer
Cable
Bar
Signer, 1990
Moosavi et al, 2002
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Fully Encapsulated Tendons
Q. What does face plate display?
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Fully Encapsulated Tendons
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Location of Parting
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Multiple parting planes
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End Anchored Tendons
Load Transfer
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Pre-tensioning
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Learnings
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A given amount of displacement, or opening of a parting,
generates a much higher load, or resistance against
further displacement, in a fully encapsulated tendon than
in an end anchored tendon. That is, fully encapsulated
support systems are stiffer.
A broken fully encapsulated cable can still provide
considerable confinement across partings.
An end anchored cable is susceptible to unloading if the
rock around the collar crushes, or the face assembly is
damaged.
Pre-tensioning significantly increases the stiffness of the
loading system, thereby generating higher reaction load
for a given amount of displacement
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Tendons
What do tendons do?
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Support deadweight load - Suspension
Increase self supporting capacity of rock
mass – Reinforcement
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Suspension
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Suspension
Suspension
Pressure Arch Conceptual Model
Pressure Arch:
• Transfers weight of overlying strata above arch to the abutments
• Immediate roof relies on its own capacity to span the excavation
Immediate roof strata bends, or sags, due to:
• Its own weight (transverse load)
• Axial (horizontal) stress
• Expansion of failing strata in the roof (transverse load)
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Beam Theory
Beam with Clamped Edges
Sag = B1/t2
Tensile stress = B2/t
Where B1, B2, are
proportional factors
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Reinforcement – Laminated Strata
• In order to sag, layers
have to slide past each
other
• No sliding at centre of
each beam
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Learnings

Principle of beam building is to prevent
sliding between layers so as to increase the
effective thickness of beam.
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If four beams of thickness t/4 are formed into
one composite beam of thickness t:
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Maximum deflection (sag) is reduced 16 fold
Maximum horizontal stress is reduced 4 fold
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Tendon Reinforcement Pattern
Stretched a lot, to
generate very high
clamping force
No Change in
Length and so
no change in
clamping force
CompressedTendon
shortened and
so unloaded
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Tendon Reinforcement Pattern
Impact of Bolt Location and Patterns
Number of Bolts
(Spann et al, 1983)
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Tendon Angle
Effect of Tendon Inclination on Shear Resistance
Generated by the Tendon
(After Windsor et al, 1993)
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Tendon Angle
Example of Effectiveness of Angled Bolts in Controlling
Failure of a Laminated Coal Roof
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Tendon Reinforcement Pattern
Eastern Distributer Motorway, Sydney
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Angled Tendons
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Angled Tendons
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Learnings
In a laminated roof environment:
 Shear stresses increase towards flanks of excavations.
 Therefore, tendons provide most resistance to shear
when concentrated towards flanks of excavation.
 Tendons angled outwards (towards flanks) load up more
for a given amount of shear displacement
 Tendons angled inwards (towards centre of excavation)
initially unloaded by shear movement.
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Reinforcement Mechanics

What naturally determines resistance to sliding?
𝜏 = 𝐶𝑖 + 𝜎𝑛𝑖 𝑇𝑎𝑛 ∅ + 𝜎𝑛𝑖 𝑇𝑎𝑛 𝑋
= 𝐶𝑖 + 𝜎𝑛𝑖 𝑇𝑎𝑛 ∅ + 𝑋
where:
Ci = initial cohesion
σni = initial normal stress
∅ = friction angle of joint surface fabric
X = equivalent friction angle for joint waviness
(Galvin, 1990)
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Bolting Mechanics
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How do tendons modify resistance to sliding?
1. Increase n by prestressing at time of installation
Applies to both fully encapsulated tendons and
end anchored tendons
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Bolting Mechanics

How do tendons modify resistance to sliding?
2. Increase n due to axial force developed by
sliding along parting
• Develops earlier in fully encapsulated tendons – less sag
• Develops faster in tendon angled towards rib – less sag
• Benefits lost in bolts angled towards roadway centre
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Bolting Mechanics

How do tendons modify resistance to sliding?
3. Increase n due to axial force developed by opening of
parting
Applies to both fully encapsulated tendons and
end anchored tendons
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Bolting Mechanics

How do tendons modify resistance to sliding?
4. Increase Co due to lateral component of axial
force
• Develops earlier in fully encapsulated tendons
• Develops faster in bolts angled towards rib
• Lost in bolts angled towards roadway centre
Bolting Mechanics
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How do tendons modify resistance to sliding?
5. Increase Co due to lateral resistance to shear
provided by dowel
Develops earlier in fully encapsulated tendons
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Reinforcement Mechanics
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How do tendons modify resistance to sliding?
𝜏 = 𝐶𝑖 + 𝐶4 + 𝐶5 + (𝜎𝑛𝑖 + 𝜎𝑛1 + 𝜎𝑛2 + 𝜎𝑛3 ) 𝑇𝑎𝑛(∅ + 𝑋)
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What gives the most benefit?
Say30 tonne (300 kN) Ultimate Tensile Strength tendon
• Shear strength = 50% UTS = 150 kN
• 5 tonne (50 kN) pretension = n1
• Bolts 1m apart, 2m row spacing = 2 m2 per bolt
Therefore
•
C5 = 150/2 = 75 kPa
•
n1 = 50/2 = 25 kPa
•
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Reinforcement Mechanics
n1
(kPa)

()
 = Tan()
f = n Tan ()
(kPa)
c = C5
Slickensided
25
17
0.3
7.5
75
Shale
25
26.6
0.5
12.5
75
Sandstone
25
45
1.0
25
75
30 t UTS bolt
5 t pretension
In a laminated (sliding) setting:
• Dowelling benefit far out-weighs pretension benefit
Provided tendon is fully encapsulated
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Reinforcement Mechanics
What about C4, n2 and n3 ?
Can get a variety of outcomes depending on properties
of rock mass, angle of bolt, bolt diameter, whether bolt is
fully encapsulated in hole.
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Why Install Centre Tendons?
• Most obviously, to reduce effective span – sag is
proportional to “span x span x span x span”
• Limit span between bolts of immediate roof plies so they
do not fall under their own weight
• Suspend delaminated immediate roof plies to higher
stable strata – using cables
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Why Fully Encapsulate Centre
Cables if They are not Subjected
to Shear?
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They may be subjected to shear, as loading regime is
not always symmetrical. Varies in mining cycle.
They have superior load/displacement characteristics to
end anchored tendons (any claims to the contrary should
be tested to confirm that tendons, rock properties,
anchorage characteristics and installation mediums are
comparable).
They are not susceptible to unloading as a result of
damage and rock crushing at the collar.
They can still perform a support function in a broken
state.
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When might a Centre Cable not be
Fully Encapsulated?
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To avoid breaking cable bolt in a high
displacement environment.
Requires strata to reach a quasi state of
equilibrium with sag
An option is to allow a certain amount of
“stretch” (sag) before then encapsulating cable
to make it stiffer and give it a higher resistence
to further displacement
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When might a Centre Cables not
be Fully Encapsulated?
8m long cable
Yield load = 60 t
Elongation at yield = 1%
Elongation at ultimate load = 6%
2m encapsulated, 6m free
Elongation at yield = 0.01 x 6 m = 60 mm
Elongation at failure = 360 mm
Fully encapsulated
Load transfer rate either side of parting = 60 t/m
Therefore, at yield, load is distributed over a length of 2 m, so elongation at yield = 20
mm.
The same displacement in the 6 m free end of an end anchored cable would give a strain
of 20/6000 = 0.33%, at which point the load in the cable would be only 20 t.
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When might a Centre Cables not
be Fully Encapsulated?
The end anchored cable can sustain 60 mm of displacement before
reaching yield as compared to 20 mm for the fully encapsulated cable.
Some mines do not immediately fully encapsulate in order to allow rock
to relax and to reach a quasi state of equilibrium without cable going
into yield, before then fully encapsulating cable.
In this example, if the cable were not to be fully encapsulated until after
20mm of convergence, it could still sustain another 14mm of
convergence before going into yield.
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Conclusions
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When it comes to support design, there is no “one size fits all”.
There are many and varied factors that impact on the design and
performance of cable bolting systems.
These should be assessed through the application of applied
mechanics supported, preferably, by numerical modelling.
Some designs need to accepted with caution. Whilst they may have
application in some circumstances, both theoretical considerations
and operational experience suggest that, in general, the benefits
associated with them are not commensurate with the risk presented
by pushing the limits to such an extent.
Subject all ‘novel’ advice to robust independent risk assessment
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Third party review
Premised on data
Supported, if necessary, by numerical modelling
Subject to cost/benefit analysis in respect of material and personal consequences
if design fails – Is it worth the risk?
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