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
3
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)
4
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)
5
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
6
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)
7
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
8
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
9
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
10
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)
11
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.
12
Test Arrangements
13
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
14
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)
15
Cable Bolt-Unconfined
Rajaie’s Results (1990)
16
Cable Bolt- Zero Torque
17
Cable Bolt- 40 Nm Torque
18
Cable Bolt- 80 Nm Torque
19
Cable Bolt Results
20
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
Top Related