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Development of Yield-Lok* Yielding Rock Bolts

Rocky Y. Wu, Vice-President of Engineering, Jennmar of Canada, Sudbury, ONT

John Oldsen, Vice-President, Research & Development, KMS, Jennmar Corp., Pittsburgh, PARob Gagnon, Technical Sale Representative, Jennmar of Canada, Sudbury, ONT

ABSTRACT

A rock burst is one of the greatest challenges to ground control in themining industry. There are

more and more industry requirements on yielding rock support. Since 2008, JennmarCorporation has been conducting large scale research and development work to develop a

technically reliable and cost effective yielding rock bolt. This paper introduces a new yielding

rock bolt --- Yield-Lok*

. The bolt is characterized by the designed yielding ability to produce150 ~ 200mm of deflection at 8 ~ 10 tons loads for every 16.4kJ energy input. Its performancecharacteristics are consistent through multiple and varying amplitude of impacts. In this paper,

the design criteria of the bolt and the principle of performance are described; the dynamic testing

results are discussed; and the features and application of the bolt are presented.

INTRODUCTION

Rock burst is a dynamic failure of rock mass accompanied with seismic and collapse of stopes

and drifts, which can result in injury to workers and damage to equipment and infrastructure.Rock burst is one of the greatest challenges to ground control in Canada and around the world.

This is because:

The occurrence of a rock burst is unpredictable in its time, location, magnitude, and thescale of damage.

All of the mining countries in the world have experienced rock bursts in mining and civilengineering projects.

With the increasing mining depths and mining scale, there will be more and more minesfacing rock burst hazards

Since 1980, extensive research and development on yielding rock bolts has been conducted. Afew kinds of yielding bolts have been successively developed and some have been applied in

mines. The typical products are the cone bolt developed by COMRO, South Africa (Jager 1992),the modified cone bolt developed by Noranda, Canada (Simser, Joughin and Ortlepp 2001), the

Dynamic Solid Bolt produced by Garford in 2008 (http://www.garfordcablebolts.com.au/), and

the Roofex yielding rock bolt produced by Atlas Copco (Harven and Ozbay 2009).
http://www.garfordcablebolts.com.au/http://www.garfordcablebolts.com.au/


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*Yield-Lok, Patent Pending

Basically, the yielding mechanism of these bolts can be classified into two categories. Theyielding of the cone bolt (or modified cone bolt) results from pulling the cone though grouting

agents, such as resin or cement, when a rockburst event occurs. Its performance, therefore,

significantly depends on the properties of the grout, the diameter of drill hole, mixing efficiency

and encapsulation condition. Since in most cases these factors are not completely under control,the performance of a cone bolt may be less consistent and repeatable. The yielding of Dynamic

Solid Bolt and Roofex is produced by pulling the bolt through a dynamic device. The dynamic

device is manufactured in a workshop, enabling the bolt to perform consistently, but is inherentlycostly.

There are more and more industry requirements on developing a performance-reliable and cost-effective yielding rock support.

Since 2008, Jennmar has been conducting research and testing to develop a new yielding rock

bolt. Its trade name is Yield-Lok*. In this paper, the design and components of the Yield-Lok

*

bolt are introduced, the dynamic testing results are presented, the mechanism of yielding isdiscussed, and the feature and application of this bolt is summarized.

DESIGN CRITERIA OF YIELD-LOK*BOLT

The general principle of ground control in rock burst prone conditions is to transfer the dynamic

energy of a rockburst event to the yielding support system to facilitate absorption and controlled

deformation of rock mass while providing containment of materials, or simply helping the rockmass to support itself (Hoek, 1980).

When subjected to dynamic loading from a rockburst event, the rock mass will experience shear

and dilatory displacement and fracture as illustrated in Figure 1 (Ortlepp 1992). Therefore, in

addition to the specified axial yielding performance, the yielding bolt should be able to provide

high and stiff shearing reinforcement to prevent the rockmass from shearing displacement andfracture.


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Figure 1 Inferred fracture of rockmass resulting from a rockburst, after Ortlepp (1992)According to Jager (1992) and rockbursting ground support practices in Canadian hard rock

mines, the design criteria of Yield-Lok*bolts are specified as below:

1.

The total energy absorption capacity > 25kJ2. Static yield load >10 ton3. Average dynamic yield of 8 ~ 10 ton at 150 ~ 250mm displacement per input of 16.4kJ

energy at 5.4 m/s loading speed.4. Be able to withstand multiple and varying amplitude of impacts and performance

characteristics must be reliable and repeatable

5. High shear stiffness and strength

DESIGN AND COMPONENTS OF YIELD-LOK

*

BOLT

The design and components of Yield-Lok*bolts are illustrated in Figure 2. The bolt is made of

, grade 75 round bar with the minimum yield and ultimate tensile load of 12.5 ton and 16.7

ton, respectively. The bar is upset to specified dimensions at one end and partially or fully

encapsulated in an engineered polymer coating to achieve designed yielding performance under

dynamic loading. The end profile of the polymer coating is configured to aid insertion of the

bolt and, along with mixing/centering paddles longitudinally spaced over the length of the

coating, provides shredding of resin cartridge packaging. The other end of the bar is threaded for

tensioning with a nut. A dome plate and spherical washer are used for angle compensation and

to load the bolt axially.

Upset

Polymer Coating


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Figure 2 Design and components of Yield-Lok*bolt

PRINCIPLE AND FUNCTION OF YIELD-LOK*BOLT

The Yield-Lok

*

bolt is fully or partially resin grouted. The principle of yielding performance isbased on the inter-action between the upset, polymer coating, and resin. The function of each

element is illustrated in Figure 3.

Resin

Drill hole in rock

Polymer encapsulation

Upset and bolt

Plowing marks left in

Polymer after pulling

the upset through it

Figure 3 Interactions between the upset, polymer and resin

The angled segments of the polymer coating aids to shred the resin cartridge packing during

insertion of the bolt into resin and enhances anchorage. Resin mixing is facilitated by

deformations on the polymer coating similar to rebar. The bolt is tensioned and providesimmediate primary support on installation. In static loading conditions, the Yield-Lok

* bolt

performs completely similar to a rebar bolt, providing stiff reinforcement and detainment of the

rock mass. In dynamic loading conditions, the upset transfers the impacts on the surroundingpolymer coating, resulting in confined compression, thermal softening and flow of the polymer

around the upset, and creates a plowing effect. The dynamic energy is therefore absorbed by

pulling the upset through the polymer. A part of dynamic energy is consumed in the frictionbetween the smooth bar and the polymer coating.


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Since the yielding elements (upset and polymer coating) are controlled in engineering design and

manufacturing, the product quality is reproducible. Therefore, the performance of the Yield-Lok

*bolt is constant throughout the full length of polymer encapsulation.

The function of the resin with Yield-Lok*bolt is only to provide confinement to the polymer as

opposed to a yielding element with the cone bolt. Hence, if it is fully grouted and solidlyconfined, the performance of the Yield-Lok*bolt is basically independent of the type of grouting

media, mixing status, and drill hole diameter. Since the displacement mechanism is contained

within the polymer, debonding agents, such as grease, are not required to achieve the specifiedplow effect and consistent performance.

DROP TESTS WITH YIELD-LOK*BOLT

Large scale drop tests were conducted at the CANMET testing facility in Ottawa to optimize the

polymer coating material and the geometry and size of the upset to achieve the specified

performance. As many as 50 bolt samples were tested and in total more than 100 drops wereconducted.

Boreholes were simulated by 12 mm-thick steel tubes with the internal diameter of 34.5mm. The

steel tube preparation included a slight roughening of the inside surface over approximately the

last meter. This roughened section is referred to as the top of the tube where the bolt is groutedwith resin.

Figure 4 Drop test equipment and configuration at CANMET, Ottawa


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Similar to rebar installation on site, the tube was first loaded with resin cartridges. A bolt was

then slowly spun into the tube at a steady advancement rate. Once the bolt reached the bottom,the advancement was stopped and the bolt was rotated at a full speed of 220 ~ 350 rpm for an

extra 5 seconds to fully mix the resin.

Drop tests were conducted in a test rig with a capacity of 3 tons from a height of 2m (Figure 4).For comparison with the existing yielding bolts, a drop weight of 1115kg and drop height of

1.5m were used for all tests. This corresponds to a loading speed of 5.4m/s and an input energy

of 16.4kJ.

The bolt sample was placed in the test rig by inserting it through the center of a magnet and

weight. The weight was lifted by the magnet to 1.5m above the plate and then the magneticforce was de-energized to drop the weight on the plate.

Instrumentation consisted of plate and bolt end displacement monitors, and frame and plate load

cells. The instrumentation was connected to the data acquisition system which collected the data,

sampling at the rate of 10000 per second.

Table 4 Average values of displacements and loads obtained for each drop

Drop

No.

Plate

displ.(m)

End

displ.(m)

Steel

stretchplastic (m)

Steel

elong%

strain

End

displ.%

total

Peak

load(kN)

Avg

load(kN)

Peak

plateload

(kN)

Avg

plateload

(kN)

1 0.207 0.198 0.008 0.48 96.0 129 99 114 93

2 0.218 0.211 0.007 0.42 96.7 187 87 159 81

3 0.239 0.230 0.009 0.53 96.3 167 81 142 73


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Jen-43

Jen-45

Jen-46

Jen-48

Jen-49

Ave

rage

Drop 1

Drop 2

Drop 30

40

80

120

160

200

240

280

Enddispl.(mm)

Sample Series

No. of Impact

Figure 5 Summery of displacement obtained from tests with prototype bolts

The test results from 6 prototype Yield-Lok*bolts are summarised in Table 4 and Figure 5. As

indicated, all samples can withstand 3 impacts with minimal elongation of the steel. All samples

performed consistently as designed at a displacement of 170-230mm and average load of 8 10

tons (73 - 93 kN). The average displacement from plowing the upset through the polymercoating accounts for 96% of the total displacements, while the steel elongation accounts for 4%

of the total displacement, or only 0.48% steel strain.

A typical testing result is presented in Figure 6. The upper is the autopsy of polymer

encapsulation after testing, while the lower is the load versus displacement curve of the bolt from

each drop test. As can be seen, the vibration frequency of the curve is very consistent with the

plow marking left in the polymer during pulling the upset though it. This verifies the yieldingmechanism of the Yield-Lok

*bolt as described in the sections above.


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Jen-50 - Drop 1

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150

Displacement (mm)

Load

(x1000kg)

Figure 6a Typical test results from sample Jen-50 (Drop 1)


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Jen-50 - Drop 2

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150 200

Displacement (mm)

Load(

x1000kg)

Figure 6b Typical test results from sample Jen-50 (Drop 2)


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Jen-50 - Drop 3

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150 200 250

Displacement (mm)

Lo

ad(x1000kg)

Figure 6c Typical test results from sample Jen-50 (Drop 3)


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YIELD-LOK*CONVERGENCE ROCK BOLTS

Varying ground conditions and mineral deposits require ground control mechanisms that allow

convergence of the supported ground over time. Through modifications to the upset diameter of

the Yield-Lok

*

bolt, a bolt that responds to convergence over time can be designed. Figure 7 and8 graphically depict such a bolt. As can be seen, a designed bolt yield can be engineered for

specific mine conditions. The performance mechanism is similar to the Yield-Lok*bolt in that

the upset is pulled through the polymer coating under confined compression. Softening andflowing of the polymer coating is also accomplished. The desired length of convergence allowed

is directly correlated to the polymer coating encapsulated length. In comparison with the

traditional yielding bolts such as Friction-Lok Stabilizer and Expanbolt (Swellex), the Yield-Lok

* bolt can provide high pre-tension and stiff shear resistance to the ground in addition to

consistent axial yielding, which is greatly beneficial for consolidating and stabilizing the rock

mass.

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160

L

O

A

D

T

O

N

S

Displacementmm

CONVERGENCEBOLTSTATICPULLTEST

UPSETDIAMETER"A"

Figure 7 YIELD-LOK*Convergence Bolt


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0

2

4

6

8

10

12

14

16

18

20

0 20 40 60 80 100 120 140 160

L

O

A

D

T

O

N

S

Displacementmm

CONVERGENCEBOLTSTATICPULLTEST

UPSETDIAMETER"B"

Figure 8 YIELD-LOK*Convergence Bolt

STUDY OF POLYMER FOR LOW TEMPERATURE

Transportation and handling of ground support products can encounter harsh and demanding

environmental conditions. Besides rough handling, extremely low temperatures are routinely

encountered during delivery of ground control products in Canada. To insure product

performance would not be comprised, several engineered polymer coatings were evaluated forlow temperature sensitivity. The selected polymer coating, as tested, was cycled to minus 70

degree centigrade from typical ambient mine conditions. Impact from a hammer as well as

striking polymer coated elements together at this temperature, did not result in damage or

deterioration of the polymer coating or its performance.


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CONCLUSIONS

The Yield-Lok* bolt is characterized by consistent performance, wide suitability and cost

effective installation and application. The features and benefits of Yield-Lok* bolt can be

summarized as following:

1. Yield-Lok* bolt is suitable in static and dynamic load conditions and applicable asprimary support.

2. The performance characteristics of Yield-Lok*bolt are consistent through multiple andvarying amplitude of impacts, and the performance is independent of bore hole diameter

and resin properties.

3. Yield-Lok*bolt is suitable for typical mechanized and/or hand-held installation practices.4. Installation and application are cost effective.5. Yield-Lok*bolt can be configured to provide constant yield in convergence conditions,

providing high pretension and shear resistance.

6. The engineered Polymer coating performs down to -70 degrees centigrade.

REFERENCES

Jager, A.J., 1992, Two New Support Units for Control of Rockburst Damage,Proceedings ofthe International Symposium on Rock Support, Rock Support in Mining and Underground

Construction, Sudbury, ONT.

Simser, B., Joughin, W. and Ortlepp, W.D., 2001. The Performance of Brunswick Mines

Rockburst Support System During a Severe Seismic Episode,The 5th

International Symposium

on Rockburst and Seismicity in Mines, Johannesburg, South Africa.

Haven, S. and Ozbay, U. 2009, In-Situ Testing of Roofex Yielding Rock Bolts in Coal Ribs,

28th

International Conference on Ground Control in Mining, Morgantown, WV, USA.

Hoek, E. 1980, Underground Excavations inRrock,London, UK, Inst. of Min. & Metal.

Ortlepp, W.D. 1992, Invited Lecture: The Design of Support for the Containment of RockburstDamage in Tunnels An Engineering Approach,Proceedings of the International Symposium

on Rock Support, Rock Support in Mining and Underground Construction, Sudbury, ONT.

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