Applicability of D-Bolt in High Stress Conditions
“A Step Toward Better Performance and Safety”
Francois Charette Dynamic Rock Support NA
September 2012
Presentation Overview
Introduction: What is the D-Bolt What kind of conditions are we trying to control What are the design criteria used in the bolt concept What are the properties that make the D-Bolt different Conclusion: Implications for safer operations
What is a D-Bolt: a rockbolt that is strong AND ductile
High Strength Energy-absorbing rock bolt
Strong rock bolt
Deformation
Load
Energy Absorbing Ductile rock bolt
Strength (e.g. Rebars)
Ductility (e.g. Split set)
Energy- absorbing
Combines strength and ductility to control energy release conditions
D-Bolt description
• Smooth steel bar with a number of anchors along its length. • Only fixed with the grout at the anchors’ positions. • The smooth sections between the anchors can freely deform
when subjected to rock dilation. • Typical 3 or 4 sets of anchors. • Special steel with enhanced mechanical properties for high
strength and ductility
The O - Anchor
Bolts sizes and threads
Diameter available: 22.2 mm and 20.5 mm diameter Lengths: from 1.8 m to 3.2 m Threads: M24 for the 22.2 mm 7/8 inch UNC for the 20.5 mm Anchor type: Oval shape double paddle anchor
Page 6 March 2, 2010
Concept comparison of D-Bolt and standard rebar bolt
1. D-Bolt and rebar bolt installed (resin not shown)
2. As the D-Bolt can deform together with the rock movement, it is still intact and maintains its properties while the rebar fails
Where is the D-Bolt Needed?
To Control Difficult Ground Conditions
1. Broken Ground and Squeezing Ground
Rock bulking without ejection
Other ground conditions to control
1. Squeezing Grounds 2. Seismic Conditions
a) Strain bursts i. small ii. large
b) Rock bulking with ejection
c) Rock ejection due to energy transfer
d) Rock falls by seismic shaking
Design Approach
• Typically in many systems: combination of strong rigid reinforcing elements and damping elements linked by surface support • Surface support needs to be linked to the damping
elements to reduce shearing between rigid elements • Damping elements provide little static load to the
system: the rigid elements are failed when the damping elements start to pick up load
Stiff rockbolts and Deformable rockbolts
In heavily squeezing or highly stressed grounds, often the stiff rockbolts cannot accomodate the large displacements and failure leaves the ground support system with only the lower static capacity of the deformable rockbolt
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00
Load
(ton
nes)
Displacement (mm)
Rebars Deformable rockbolts
What happens with a dual stiffness system?
Plate Failure? Mesh Failure? Shotcrete Failure? Broken Ground
Discontinuity of Deformation
Stiff bolt Soft deformable bolt
Under Heavy Loading Conditions
Higher initial stiffness and strength lead to less deformation at the excavation surface
Load
Deformation
Characteristic curve of excavation
Different Design Approach with D-Bolt • Another Suggested Approach is a single stiffness system
• Rigid elements are stiff but more ductile at high load than typical systems (rebars)
• Surface support can deform in a more uniform fashion: less shear in rock and shotcrete
0.75 inch Rebar
D-Bolt22 D-Bolt20
What happens in a balanced stiffness system?
Less rock failure Less Plate Failure Less Mesh Failure Less Shotcrete Failure
Broken Ground
More uniform Deformation
Total deformation is function of
stiffness/density of bolting pattern
Surface support can deform in a more uniform fashion: less shear in rock and shotcrete
Field Examples
D-Bolt In a Dynamic Condition (Long Hole Blast)
Applicability of D-Bolts
• D-Bolts can be used as damping elements • As strong as stiff elements like rebars • As deformable as most yielding rock bolts
• D-Bolts can be used as single reinforcing elements that combine strength and ductility of a 2 bolts system • Advantages of simplicity, single pass, and better static control • Provides similar reinforcement as rebars with much larger
convergence capability • Can dampen dynamic events of small and large magnitude
Safety Implications
Meeting Required Capacity for Dynamic Conditions • Simple evaluation of required capacity (Kaiser et al, Hudyma) based on
observation of damages • Displacements based on maximum capacity of rebars and cone bolts combined
• D-Bolt meets or exceed requirements Damage
mechanism Severity Load
Capacity (kN/m2)
Displacement (mm)
Ve (m/s)
Energy (kJ/m2)
Rock bulking without ejection
Minor 50 30 < 1.5 NC Moderate 50 75 < 1.5 NC
Major 100 150 < 1.5 10 - 15 Rock bulking with ejection
Minor 50 100 1.5 – 3 10 - 15 Moderate 100 200 1.5 – 3 10 – 20
Major 150 >300 1.5 – 3 20 - 50 Rock bulking from seismic
energy transfer
Minor 100 150 > 3 10 – 20 Moderate 150 300 > 3 20 – 50
Major 150 >300 > 3 > 50 Rockfall due to seismic shaking
Minor 100 NA NA NA Moderate 150 NA NA NA
Major 200 NA NA NA
Table 1. Damage severity and system support properties (modified after Kaiser et al, 1995, Hudyma, 2007) NA-not applicable; NC-not critical; Ve=ejection velocity.
0
20
40
60
80
100
120
140
160
180
200
0 10 20 30 40
Disp
lace
men
ts N
eede
d (m
m)
Energy (kJ)
Displacements Needed to Dissipate Energy
NewMCB33
Rebar+NewMCB33
DBolt20
DBolt22
Rebar 22 mm
Yield Lok
Hybrid AE
Roofex Rx20
From published data
Bolts with extended stiffness
Michael, if you look at this figure, you will see that the guidelines given by Kaiser (previous slide) are based on the capacity of rebars and cone bolts combined, not on anything else
Other Important Requirement • Capacity to resist to multiple impacts
• Bolt should stay ductile after loading/impacts
Multiple Drop Tests on D-Bolt
Design Requirements For Performance and Safety Support System Design Requirements Dynamic Loading Conditions Squeezing Ground Conditions
Static load capacity (shank and threads) 200 kN/m2 200 kN/m2 Energy damping capacity 20 kJ/m2 to 40 kJ/m2 30 kJ to 40 kJ in extreme
conditions Controlled convergence without failure At least 5% to 7% convergence
capability At least 10% convergence
capability Adequate coverage: span to remain small enough to
minimize flexural failure of surface support 0.9 m x 0.9 m pattern typically, or
less 0.9 m x 0.9 m pattern
typically, or less Face plates compatible with load 150 to 240 kN, depending of yield
load of bolt 150 to 240 kN, depending of
yield load of bolt Can be installed manually, or with semi-mechanized or
fully-mechanized rigs Easy insertion and spinning with
equipment available Easy insertion and spinning with equipment available
Possibility to install heavy weld mesh with additional surface support (mesh plate, mesh straps)
At least with manual and semi-mechanized operations
Corrosion resistance Grouted or coated bolts Grouted or coated bolts Easy repair or addition of surface support
Use of grouting material if needed Standard resin or cement Standard resin or cement Method of installation Preferably standard equipment
and methods Preferably standard
equipment and methods Quality control methods Same or better as for existing
systems Same or better as for existing
systems Competitive Total installation costs
Others operational advantages: set up time, scheduling, rehabilitation costs
Installation Methods
• Similar to rebar • Manual with stoper/jackleg • Semi-mechanized (MacLean bolters) • Fully mechanized
Market utilisation • Accepted as rock burst rock bolt at LKAB (Sweden) • Used commercially at Boliden for squeezing ground conditions • Used in 3 Canadian mines for squeezing and/or seismic
conditions • In test in 6 other key Canadian mines, and in one major mine in
Chile • On-going activity with BHP throughout Australia
Thank you for your attention!
EXTRA MATERIAL FOR DISCUSSION
Field Tests Pull tests on threads in the field: results of D-Bolt 22 mm
Yield Load 200 kN
Location of failure on the bar – not at ovals
Matching Capacity Domed Plates for D-Bolt 20 mm
Dynamic Results of 20 mm D-Bolts With 1.5 m free bar split tube configuration
• Bolt section: 1.5 m. • Drop weight: 2675 kg • Drop height: 1.5 m • Velocity: 5.4 m/s • Single-blow energy: 39.4 kJ
Sample M-5 Drop 1
CANMET dynamic test of the 20 mm D-Bolt (1.5 m segment) 39 kJ without failure
0.8 m
Analytical simulation of circular excavation in squeezing conditions, Sh = 2 Sv
0
50
100
150
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250
300
350
2.5 3.5 4.5 5.5 6.5 7.5
Conv
erge
nce
Ur (
mm
)
Original Radius R
Series1
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1.0%
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10.0%
0 2 4 6
Elon
gatio
n (%
)
Distance from Wall (m)
Series1
Analytical simulation of circular excavation in squeezing conditions, Sh = 2 Sv
0.000000
0.001000
0.002000
0.003000
0.004000
0.005000
0.006000
2.5 3.5 4.5 5.5 6.5 7.5
Delta
Elo
ngat
ion
per 6
.3 c
m (m
)
Original Radius R
Series1
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