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Optimising the Selection of Fully Grouted Cable Bolts in Varying Geotechnical Environments

Underground » Strata Control and Windblasts

Published: February 15Project Number: C22010

Get ReportAuthor: Paul Hagan | University of New South Wales

The primary objective of this research project was to develop a laboratory-scale test facility in Australia that can be used to determine the behaviour and assess the load transfer performance of fully grouted cable bolts under axial loading conditions. The facility had to be capable of testing the range of cable bolts available for use in the Australian coal mining industry under varying conditions.

 

Recent work found a number of shortcomings with the current test method available in the U.K. These shortcomings made it difficult to draw firm conclusions concerning the performance of 14 cable bolts tested in that study. The lack of a locally available reliable testing method capable of assessing the extensive range of cable bolts has hamstrung geotechnical engineers in their ability to optimise the design of ground support systems from a cost and performance perspective at a time when mining conditions are increasingly more arduous.

 

As part of this ACARP project, a review was undertaken of the test methods developed for cable bolts. It confirmed several deficiencies in the range that impact on the integrity of some test results. For example, the tendency of some helical cable to unwind during a test induces an additional load on the cable bolt, grout and rock that can compromise performance and, the use of a steel pipe to confine a cable bolt that does not reflect conditions in the field.

 

The Laboratory Short Encapsulation Pull Test (LSEPT) forms the basis of the current British Standard and is the latest of testing methods. It was developed to suit a type of cable bolt that was prevalent in the British coal mining industry at that time but which is not necessarily suited to some of the modified, high capacity cable bolts developed since. A project recently funded by the Australian coal mining industry highlighted deficiencies in the LSEPT test and recommended the development of a modified or new test design that would address its shortcomings.

 

As part of this project, a new cable bolt testing facility based on the LSEPT test has been designed and commissioned together with a procedure capable of testing the full range of cable bolts including the high performance modified cable bolts available in Australia.

 

A series of tests were undertaken to gauge the sensitivity of several testing parameters. Many tests including the double-embedment test and the LSEPT comprise two sections; the embedment section where one length of a cable bolt is grouted into the borehole of a rock or test sample intended to model behaviour of the cable bolt in situ and, the gripping section that comprises the remainder of the cable bolt that is grouted in this case into a steel pipe or anchor tube.

 

With regards to the embedment section, previous work found the size of test sample can directly affect the load transfer behaviour of a cable bolt. Tests in this project found that the size effect is a function of the type of cable bolt. While a cable bolt in the field is confined within an otherwise infinite rock mass, in the laboratory there is a practical limit to test sample size. Hence in order to ensure cable bolt performance is not compromised by the size of test sample, it was important to determine an appropriate size of test sample. In the worst case scenario of testing a modified high capacity cable bolt when the highest stresses are induced in the test sample the limiting sample diameter was found to be in the order of 300 mm above which there was little variation in performance. It was further found that variation in performance can be minimised by placing the 300 mm test sample within a large diameter steel cylinder, the latter providing passive confinement to the sample. This sample size is nearly double that currently used in the LSEPT test and uses a pressurised bi-axial cell to confine the test sample. Further, tightening of the bolts that join the two halves of the steel cylinder to a torque of 40 NŸm provides a low but nevertheless consistent level of confinement to the sample.

 

A further point of difference to the standard LSEPT test is the embedment length of the cable bolt which has been increased from 320 mm to 360 mm. This longer length is particularly important when testing some types of modified cable bolts with a long bulb length.

 

Another factor considered was borehole roughness. As expected this affected anchorage performance depending on the type of cable bolt tested and strength of test sample and whether failure occurred at the cable/grout or grout/rock interface. A technique was developed that can produce a manufactured rifled borehole in test samples that will provide consistent anchorage conditions.

 

Due to the much larger size of test sample, the samples are cast from a cement-mortar material rather than prepared from a cored sandstone sample. Variability in material properties can be reduced by mass casting of the samples. This approach has reduced the cost of sample preparation, ensures uniformity of material properties and importantly provides an ability to simulate rocks of differing strength such as coal and sandstone.

 

With respect to the gripping section, the effect of termination of the cable bolt was examined. During a test, load is gradually applied to the cable bolt via a constant displacement pump and hydraulic cylinder at the interface between the test sample and anchor tube. To minimise slippage of the cable within the anchor tube, various cable termination methods were examined such as the bail and wedge. It was found that a modified anchor tube with an internal machined thread and length of 600 mm prevented any slippage.

 

The final stage of the project involved a preliminary assessment of two types of cable bolt. This was to confirm the newly constructed axial-loading laboratory-scale testing facility could distinguish differences in the anchorage performance between two cable bolts at either end of the performance spectrum (a modified bulb cable and a plain strand cable), in materials of two different strengths (10 MPa and 60 MPa) and, in two different borehole diameters (the "as recommended" standard diameter borehole and a +10 mm diameter borehole).

 

As expected, a marked difference was found in the behaviour of the two cable bolts. The modified bulb cable was much stiffer and attained a high pull-out load. Once with the peak load was reached, the load bearing capacity of the cable bolt reduced quickly to an insignificant level within the measured displacement range of 100 mm. Whereas the plain strand cable attained a much lower pull-out load however post-failure the cable bolt was able to provide a quite substantial level of load equivalent to 75% of the maximum load over the same range of displacement.

 

The strength of the test sample had a marked effect on cable bolt performance. For example, the maximum load of the modified bulb cable in the strong test sample material was almost twice that achieved in the weak or soft material. Conversely performance of the cable bolt in soft material was nearly half that in strong material. Hence performance of the bulb cable is sensitive to the properties of the host rock mass.

 

Finally regarding borehole size, surprisingly a larger borehole in weak rock improved performance of both cable bolts. It is thought the additional grout acts as a large diameter plug and in effect increases the surface area in contact with the test sample thereby increasing the effective resistance to load.

 

With respect to the revised proposal, all three project outcomes were achieved in this project, these being respectively:

· To develop an axial-loading test procedure for cable bolts used in Australian underground coal mines;

· To develop a laboratory-scale, axial-load test facility suitable to test cable bolt anchorage devices that can be used in Australian underground coal mines; and

· Complete a preliminary investigation of two cable bolts to enable some understanding of what range of testing would be appropriate in any subsequent research.

 

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