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

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

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

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

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

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.

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

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

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

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

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

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

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

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.

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

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).

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

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

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%*

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)

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)

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

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

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

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

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?

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

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

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

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