The Influence Of Rock Size Sample On

The Load Carrying Capacity Of Cable

Bolts Under Confined Conditions

Ibad Ur-Rahman

Student– UNSW Mining

Engineering

Introduction•The objective is to develop a standardised methodology for an

experiment to assess the load carrying capacity of cable bolts.

•Aimed at designing a system that compares the different size of

sample effects on the performance of cable bolts under confined

conditions

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Background• 1990- ‘Experimental and numerical

investigations of cable bolt support system’ by

Hossein Rajaie

• RDP-Howden electrohydraulic servo-controlled

testing system was used with following

characteristics

– 1000KN capacity

– Working stroke of 100 mm

– Pull out rate of 0.3 mm/s

• Pull out test performed under constant

parameters;

– Embedded length

– Borehole diameters

• Sample diameters were varied; 100mm,

150mm,200mm,250mm and 300mm

Preparation of sample (Rajaie,1990)

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Rajaie’s Results

• Diameter of sample effected the

load carrying capacity, as diameter

increased the load carrying capacity

also increased .

• Significant increase in load carrying

capacity from 100mm to 200 mm

sample

• stress distribution in cylinders

affects the confining pressure on

the grout and the cable grout

interface

• Test standardised to 250mm in

diameter

Effect of diameter of sample on peak load ( Rajaie,1990)

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Short comings of Rajaie’s Research

• Research is out dated

• Research performed on standard plain 7 strand wire cable bolt common at

the time but newer technology been developed

• Bulbed and birdcage cable bolts are commonly used in the industry today,

does Rajaie research remain true?

• Newer types of cable bolts are likely to induce higher lateral stresses during

failure requiring larger rock samples to deal with the dilation generated by

the bolt

• Research performed on samples in an unconfined state

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Past UNSW Research

• 2013- ‘The Size effect of Rock

samples used in anchorage

performance testing of cable bolts’

by Matthew Holden

• Similar to Rajaie’s research

• Test involved using Bulbed cable

bolts

• 16 samples were tested with 300mm

length and diameters of 150mm,

215mm, 300mm and 400mm

• Bore hole diameter of 38mmTest Arrangement (Holden,2013)

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Holden’s Results• Two Types of Failures

occurred:• Type 1 where failure

occurred at the grout/rock interface

• Type 2 where failure occurred at the bolt/grout interface.

• Shortcomings of research;• Borehole contained

un-realistic smooth wall

• Testing performed on samples under unconfined conditions

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Methodology

• Sample length= 320 mm

• Diameters of samples; 150mm, 250mm,300mm,350mm and

500mm

• Sample borehole diameter= 42 mm

• Cable bolt used- Bulbed Sumo stand with indented wire

• Each sample was formed in 3 step process

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Step 1-preparing samples• Rock samples casted in molds

• Riffling effect created using 42 mm

diameter PVC tube and 3mm

diameter wire wrapped around it

with 35mm in lay length

• UCS of cement =32 MPa

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Step 2-Sample Curing• Molds removed the following day after

cement pouring

• Research In 2002 by A. Kılıc, E. Yasar

and A.G. Celik showed that curing time

had significant effect on pull out load.

• Samples left to cure for 28 days

Curing time of sample vs. pull out load (Kılıc, Yasar and Celik , 2002)

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Step 3-Grouting cable bolt into sample

• Cable bolts were grouted into each rock cylinder using slow set

polyester resin.

• Setting time of resin 20-25 minutes

• Resin and oil catalyst mixed using electric mixer to combine the

two components thoroughly and provide even distribution of

catalyst.

• First resin was poured into borehole to a height 50mm below the

top of the borehole, allowing for displacement of the resin after the

cable bolt was installed.

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Test Arrangements

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Setting-up Test with Confinement• Confinement of sample was

achieved by adding metal

plates to the sample.

• Metal plates designed

specifically for each diameter

leaving one centimeter gap

between the sample and the

metal plate

• Foams were added in between

the two clamps to provide

additional displacement for

different level of confinement

• Micrometer torque wrench was

used to tighten the screws to

the specific torque required

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Results and Analysis

The following tests were performed:

• size effect of rock sample on pull out load of cable bolt under unconfined

conditions

• Size effect of rock sample on pull out load of cable bolts under confined

condition with zero torque

• Size effect of rock sample on pull out load of cable bolts under confined

condition with 40 N.m (30 ft.lb)

• Size effect of rock sample on pull out load of cable bolts under confined

condition with 80 N.m (60 ft.lb)

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Cable Bolt-Unconfined

Rajaie’s Results (1990)

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Cable Bolt- Zero Torque

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Cable Bolt- 40 Nm Torque

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Cable Bolt- 80 Nm Torque

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Cable Bolt Results

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Conclusion

• pull out tests found an increase trend in cable bolt pull out load capacity

with size of the sample diameter.

• Also found that different level of confinement levels also influence the pull

out load capacity, as the confinement increases so does the pull out load

capacity of the cable bolt.

• Pull out test for cable bolts standardised to 350 mm diameter with a

confinement pressure 40 Nm