1. Recent research into hollow, smooth and indented cable bolt mechanical shear strength

2. Contents Introduction Identification of the knowledge gap
Mechanical direct shear test method
Shear test results
Conclusions

3. Introduction
This presentation discusses mechanical direct shear tests for cable.
The 90° shear test provides the minimum shear property of the cable.
This is important as it gives the failure load under worst-case 90° shear conditions (it is not the same relationship as bar products).
The laboratory-derived minimum shear failure value can then be used as a point of reference for comparison with cable shear properties derived from embedded shear testing.
Some key learnings come from the test method:
Evidence of the combined stress relationship
Smooth and indented strand cable perform the same in a mechanical shear test
Importance of the cable to grout bond to embedded shear test results

4. Contents Introduction Identification of the knowledge gap
Mechanical direct shear test method
Shear test results
Conclusions

5. Contents Introduction Identification of the knowledge gap
Mechanical direct shear test method
Shear test results
Conclusions

6. Identification of the knowledge gap
Cables are subject to both tensile and shear loads caused by roof displacement.
Embedded tensile (pull-tests) and shear tests aim to simulate underground conditions.

7. Identification of the knowledge gap
During embedded cable pull-tests the mechanical tensile properties of the cable are known:
For example, DSI 21.8mm diameter plain strand cable:
Tensile Strength – 595kN
Yield Strength - 525kN
There is a point of reference
From Thomas 2012

8. Identification of the knowledge gap
During embedded cable shear tests the minimum shear load of the cable are not published/known.
minimum (direct) shear load vs embedded shear load ???
No point of reference
From Aziz et al 2015

9. Contents Introduction Identification of the knowledge gap
Mechanical direct shear test method
Shear test results
Conclusions

10. Contents Introduction Identification of the knowledge gap
Mechanical direct shear test method
Shear test results
Conclusions

11. Mechanical Direct Shear Test Method
Aims of the mechanical direct shear test method:
Shear the cable at 90° minimising bending or tensile forces
Measure shear load minimising friction from the shear faces
Remove the influence of resin or grout embedment on the shear load results
Remove the influence of the cement block strength on the shear failure mode (and results)
Remove the influence of differing bond strength (due to smooth or indented strands) on the shear load results
Evaluate the influence of various axial tensile loads (cable pre-tension) on the peak shear load

12. Mechanical Direct Shear Test Method
The mechanical direct shear test procedure requires:
An inner and outer test cylinder made of hardened steel
A Universal Test Machine (UTM)
A length of cable (sole consumable)
+/- pre-tensioning equipment

13. Mechanical Direct Shear Test Method
Cable passed through holes drilled in the steel cylinders
Two methods used, single shear plane and double shear planes.

14. Mechanical Direct Shear Test Method
Tensioning equipment can be used. Axial tensile is isolated from the test cylinder. When tensioned the cylinders are not connected to the frame used to pre-tension the cable. This eliminates additional friction on the shear faces.
We have shear tested cables with pre-tension of 0, 10 and 20 tonnes using a commonly available hydraulic tensioning device.
The set-up below is then placed on the UTM for shearing.

15. Mechanical Direct Shear Test Method
The UTM displaced the inner cylindrical jig downwards at a constant rate.
Cylinders minimise rotation and tilt during testing.
Oil is used to reduce shear surface friction.
before
after

16. Mechanical Direct Shear Test Method
The tests were continued until complete hole overlap (23mm displacement for 21.8mm cable), or until complete loss of load was recorded. Data collected was:
Observations and photographs of the test samples, and
Force versus crosshead displacement (from the UTM).

17. Contents Introduction Identification of the knowledge gap
Mechanical direct shear test method
Shear test results
Conclusions

18. Contents Introduction Identification of the knowledge gap
Mechanical direct shear test method
Shear test results
Conclusions

19. Mechanical Direct Shear Test Results
DSI supplies four general cable specifications. The diameter, strand geometry, UTS, shear force (without pre-tension), and shear as a percentage of UTS are shown below:
Cable Diameter
Number of Wires
UTS
Mechanical Shear
(no pre-tension)
Shear as Percent of UTS
15.2mm
7 x 5mm Ø
250kN
142kN
56%
17.8mm
7 x 6mm Ø
350kN
181kN
52%
21.8mm
19 (various)
595kN
304kN
50%
TITAN
Smooth
9 x 7mm Ø (hollow core)
605kN
367kN*
61%*
TITAN Indented
600kN
359kN*
60%*

20. Mechanical Direct Shear Test Results
Double direct shear tests of 28mm diameter internally-grouted hollow cable found:
Smooth strand TITAN cable peak shear = 367kN (no pre-tension)
Indented strand TITAN cable peak shear = 359kN (no pre-tension)

21. Mechanical Direct Shear Test Results
Double direct shear tests of 21.8mm diameter 19 wire cable found:
Peak shear = 304kN (with no-pretension);
253kN (with 10t pre-tension); and
213kN (with 20t pre-tension)

22. Mechanical Direct Shear Test Results
Average shear force vs pre-tension – combined stress relationship:
Single shear tests had a higher peak shear of approximately 30kN
43 to 47 kN reduction in peak shear force for every 100kN increase in pre-tension

23. Mechanical Direct Shear Test Results
Average shear force vs pre-tension – combined stress relationship:
Extending the previous double shear plot shows the theoretical effect of higher axial cable load on shear failure load

24. Contents Introduction Identification of the knowledge gap
Mechanical direct shear test method
Shear test results
Conclusions

25. Contents Introduction Identification of the knowledge gap
Mechanical direct shear test method
Shear test results
Conclusions

26. Conclusions
Mechanical direct shear tests provide the minimum shear value of the cable
The test measures the worst-case cable failure mode
It can be used in combination with UTS which measures the best-case cable failure mode
Some key learnings from the test method include:
The combined stress relationship – as axial tension on the cable is increased the load required to fail the cable in shear is reduced (and vice-versa)
Smooth and indented strand cable return the same results in a mechanical shear test
The test results are very reliable and make sound engineering sense
However:
How do these results relate to surface or underground embedded cable tests?

27. Conclusions
ACARP C24012 – Shear Testing of the Major Australian Cable Bolts Under Different Pre-Tension Loads (Aziz, et al 2017). Used the Megabolt single shear test apparatus.
Conclusions:
Smooth strand cable returned higher peak shear load and greater displacement at peak shear load compared with indented/spiral strand cable
All smooth strand cables debonded the 1.8m embedment length during shearing
Debonded cable failure mode was dominantly tensile (shear not reported)
Indented/spiral strand cable failure mode was dominantly tensile but low angle shear was also observed
Increased pre-tension generally reduced peak shear failure load

28. Conclusions Common finding:
Cable in tension returns lower peak shear load – combined stress relationship
Divergent finding:
Mechanical direct – indented and smooth have the same peak shear load
Embedded – smooth strand returns higher shear load and displacement
Embedded finding different because:
All smooth strand cables de-bonded, indented did not (stiffer)
Stiffer cable performance means more rapid development of axial tension
Axial tension can be applied (pre-tension), or developed with strong bonds
Stiffer cables will fail in a more shear-like mode
Mechanical direct tests always fail in 90° shear

29. Conclusions What did industry learn from this research:
Mechanical tests allow us a means of determining the mechanical behaviour of cable and bar elements
Cable and bar conforms to combined stress relationship
Embedded tests recognised the role of:
The confining-medium strength (concrete, etc)
The embedment-material strength (resin, grout, etc)
Bond strength of different cable surface profiles
Pre-tension on shear-plane friction and cable stiffness
Overall, the research gives site engineers multiple cable support design directions:
A stiff cable system using indented strands, bulbs and high pretension
A higher displacement system using smooth strand or de-bonded sections
A myriad of combinations of the above elements

30. Recent research into hollow, smooth and indented cable bolt mechanical shear strength