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

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erge

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Original Radius R

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Elon

gatio

n (%

)

Distance from Wall (m)

Series1

Analytical simulation of circular excavation in squeezing conditions, Sh = 2 Sv

0.000000

0.001000

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Delta

Elo

ngat

ion

per 6

.3 c

m (m

)

Original Radius R

Series1