ACARP PROJECT C42012: Shear testing of the major Australian cable bolts under different pretension loads

ACARP PROJECT C42012: Shear testing of the major Australian cable bolts under different pretension loads

This report deals with shear testing of the major Australian cable bolt types using the Megabolt Integrated Single Shear Testing Rig (MISSTR). The MISSTR apparatus was selected to determine the cable shear strength values and to evaluate the debonding characteristics of various profiled cable bolt strand wires during shearing with respect to wire surface roughness. The optimum debonding length of the cable during cable shearing failure was determined by appropriate instrumentation.

The study examined both the shear behaviour and the failure mechanism of various cable bolts of different designs and constructions under zero and 15 t pretension loads, although some cables were also tested at other pretension loads in 40 MPa concrete cylinders using Stratabinder grout. The use of 40 MPa concrete and stratabinder grout was to complement the pull testing ACARP Project (C22010) undertaken at the School of Mines, UNSW. While attention to the strength of cable bolt was generally focused on the tensile strength, very little attention has been given to the cable bolt strength in shear. Ironically, failure in shear represents one of the most important aspects of the cable bolt integrity. Rock bed sagging and horizontal stresses cause shearing along fracture planes as well as along bedding planes, thereby placing shear forces on the cable. In some cases, the combined tensile and shear forces are sufficient to cause failure of cable bolts as experienced and reported from numerous mines.

The project was a laboratory based testing programme together with analysis of the field data supposedly from various mines with active shear testing studies. This was proven to be not the case and thus no field study results were provided and hence no analysis was undertaken. The duration of the study was over a period of 24 months. The study included plain and rough surface wire strands and indented and spiral strands in bulbed and un-bulbed cables. Other issues examined include cable bolt rotation during shearing and the contribution of the sheared concrete joint surfaces to the total cable bolt shearing stress and suitability of cable types installed in varying roof strata formations.

Various parameters considered included, cable type, grout type, hole size, and hole rifling, concrete confinement and cable tensioning, rate of shear loading and location of cable bulbs with respect to test block length. This project was part of a joint programme of study funded by ACARP and looked at both the cable load transfer capacity evaluation by pull testing (ACARP project C21010), undertaken at the School of Mines, UNSW and the shear strength characterisation of various Australian market cables (ACARP project C24012). The dual study was considered to lead to better outcomes in understanding load transfer mechanism by pull testing and shearing characteristics of the tested cable bolt under different test conditions.

The project had the support from the coal mining industry as well as manufacturers and suppliers of cable bolts and resin/grouts. A total sum of $389 600 000 was sought from ACARP to fund this labour intensive project. This project is likely to lead to the establishment of an Australian standard method of testing cable bolts in shear. The main findings from this study were:

MISSTR was a correct method of evaluating the shear properties of various cable bolts. The methodology enabled researchers to shed light on the effectiveness of cable installation bonding. Plain cables were found to debond much more readily in comparison with rough /indented cables for a given cable encapsulation length.
No debonding was observed in spiral and indented cable bolts because of the influence of increased interlocking in the cable/grout interface.
The failure modes in cable bolt strand wires were mostly a combination of pure tensile and tensile/shear. No strand wires failed in pure shear. Shear testing of the cable bolt will be unlikely achieved in full shear because of the relatively low strength properties of the host medium, resulting in excessive wires bending and elongating around the sheared zone, leading to near tensile shear failure.
In general, the failed cable strand peak shear load was lower with increased pretension load. Higher pretension load causes the cable to stiffen and fail with lower vertical shear displacement.
Modelling simulation demonstrated that the finite difference based FLAC software is capable of simulating underground roadway stability, supported by both rock bolts and cable bolts. The large scale horizontal deformation on cable bolt elements as experienced in the field was captured by the model. The simulation was limited to two dimensional analyses. It is recommended to carry out a comprehensive numerical study in three dimensions to classify the most optimum support system for various strata conditions.

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