Mechanical behaviour of reinforced elements subjected to shear

Executive Summary

The results and conclusions of this research project are based on experiments undertaken using a laboratory-scale, single shear rock re-enforcement test facility that was designed, constructed and commissioned in the School of Mining Engineering at the University of New South Wales.

The test facility was developed to improve the level of understanding related to the behaviour of rock reinforcement elements when subjected to shear. The project examined some of the parameters that can influence the performance of reinforcement elements in order to better manage shear loading conditions and thereby contribute to better design and application of these elements in underground mine environments.

The test results demonstrated that interaction between the different rock reinforcement elements in the underground environment can be markedly different to the properties and behaviour of the individual elements when observed in isolation; that is the rock environment behaves as a system with synergy between the individual elements.

Background

As outlined in the original project scope of work, strata control is one of the core risk areas in underground coal mining. The use of appropriate technology for ground support whether it be roof or ribs, primary or secondary and the effective management of this technology can be pivotal to achieving a safe and economically viable mine.

While much research in the past has focused on the effects of axial loading of reinforcement elements in rock, less attention has been directed at understanding how these elements behave when subjected to shear loading. It is widely acknowledged that shear loads can have an important bearing on the stability of underground excavations; however, progress in this area has been hampered by the greater complexity of creating a physical model that can reliably simulate shear loading conditions.

The intention of this project was to develop a facility that could be used to gain this understanding of the interaction between rock and reinforcement elements under shear loading conditions. This would help to close the loop in terms of assessing the various conditions that are critical to the effective reinforcement of bedded/laminated roof conditions which is the domain of underground coal mines.

The objectives of the project were to:

· review the current state of knowledge related to shear behaviour;

· develop a suitable laboratory test facility to simulate shear loading; and

· undertake the first round of experiments to begin to understand the behaviour of reinforcement elements in a rockmass.

After completion of the review phase on the current state of knowledge, a report was prepared entitled “Understanding the performance of rock reinforcement elements under shear loading through laboratory testing: a 30-year history.” This was presented at the 1^st Australasian Ground Control in Mining Conference hosted at UNSW in November 2003.

The findings in the review report confirmed that there is limited understanding in terms of the effect of some parameters. The report also highlighted there is less understanding as to the effect of the method of installation of reinforcement elements and the influence of different loading conditions such as loading rate, pre-tension, torque, axial tension and/or normal loading.

Shear Testing Facility

Considerable effort and resources were devoted to the construction of a laboratory facility that could be used in testing rockbolts and cablebolts. The review report commented that in the past, poorly designed or constructed test facilities had had a detrimental impact on the quality subsequent research in terms of integrity of results and/or validity of the findings. It became apparent that the design of such a facility would require greater consideration than had been originally envisaged.

The design objective of the test facility was to be able to simulate conditions commonly encountered in underground mining environments. The initial design had focussed on a double shearing action, however, it was found that with careful analysis and design, a facility based on a single shear failure plane could be constructed with sufficient rigidity that would minimise block rotation about the reinforcement element. This would have important advantages in that it halved the load required to shear the reinforcement element and reduced the effects of interaction of double shear on the re-enforcement element and the rockmass. Various loading scenarios were analysed using a numerical modelling package to optimise the design of the facility.

The as-built facility is based on a hydraulically actuated Avery-Denison compression test machine located in the School of Mining Engineering. It has a shear loading capacity of 600 kN and is capable of isolating many of the operational variables necessary for experimentation.

Several improvements were made to the facility after commissioning and during the initial experimentation phase of the project. The data acquisition system incorporates pressure transducers, LVDT displacement transducers and a load cell to monitor various parameters.

Each test “rock” sample comprised two concrete blocks cast in separate steel casings. After a suitable curing period, the two blocks were bolted together with a 50mm offset from the centre-line. A hole was drilled through the two blocks and a rockbolt installed as per current mining practices. Separate casting of the two blocks enabled a relatively smooth surface to be produced in the two blocks for the shear test.

Test Program

As more time was required in the design, construction and commissioning phase of the test facility than had been originally anticipated in this first stage fixed duration project, the remaining period left for initial experimentation had to be shortened.

After consultation, it was decided that most benefit would be gained from investigating a few parameters in detail than a low-level investigation of the original extended set of parameters. The following parameters were investigated:

· the effect of rockmass material strength;

· the effect of loading rate; and

· the effect of pre-tension.

Three series (or stages) of tests were undertaken in the test program. Stage 1 tests primarily confirmed the functionality of the test facility.

Stage 2 incorporated additional monitoring instrumentation as well as several modifications to the facility and involved six test samples. Stage 3 again included six test samples and three strain-gauged rockbolts.

For practical purposes and due to the nature of the test samples, rockbolts were installed using an ARO roofbolter at the Hydramatic Engineering plant in Newcastle. This is an industry-scale rig in common use in underground coalmines. Use of this roofbolter entailed some minor issues especially with regard to the control of variables and repeatability of installation practices.

Results of Investigations

In summary, the main findings of the experimental test program are as follows.

1. A standard BX rockbolt exhibited a greater resistance to shear loading than had been anticipated; greater than both the ultimate tensile strength (UTS) and shear strength of the individual rockbolt element. The amount of shear displacement and deformation of the rockbolt was also much greater than expected; nearly double that which had been allowed for in the original design.

Failure loads of nearly 400 kN were observed compared to typical UTS values of 250-300 kN.

This result emphasises that behaviour of the complete system cannot be reliably predicted based solely on the individual elements that makeup that system such as the rockbolt tendons. Rather behaviour is significantly influenced by the interaction the different elements such as between the rock reinforcement elements and the rockmass.

One potential ramification of this finding is that the extent of resistance to shear and the degree of deformation which is allowed for in the design of underground support systems may be underestimated and hence there is a possibility of some over-engineering.

2. Strength of the rockmass affects the performance of the system. In tests using higher strength concrete, the amount of shear displacement was less than that observed in comparable tests with weaker strength concrete samples. Stiffness of the system increased strength of the rockmass. Conversely, maximum load resistance decreased with rockmass strength.

The stronger concrete is thought to have limited the extent of the “activation zone” along the rockbolt. Less crushing of the concrete about the rockbolt was observed in the stronger concrete samples indicating the material was less compliant.

Hence in the design of an underground support system, cognisance should be given to the strength parameters not only of the rockbolt but also of the rockmass. The result indicates that in endeavouring to design for a level of performance some account is also required of the rockmass:

· In stronger rock, the support system is likely to be less compliant and stiffer. The system is better able to maintain the integrity of the laminated beds and hence contribute to overall stability.

· In weaker rock, the system is likely to be more compliant and allow for more differential movement between bedding plains. Conversely the strength of the system should be enhanced with the rockbolt capable of sustaining a higher level of resistance to shear load than can be achieved in a stronger rockmass.

3. The performance of the rockbolt as characterised in a plot of applied shear load versus shear displacement demonstrated two distinct behaviours. Initially, the system was relatively stiff and exhibited little shear displacement with load until a load was reached beyond which the stiffness decreased and remained constant until eventual failure of the rockbolt.

4. Stiffness of a system is unaffected by cyclic loading. In early tests, with the limited offset between the two test blocks, load on the sample had to be withdrawn to allow packers to be installed and load subsequently re-applied. The load-displacement curve was found to follow a similar path as in continuously loaded tests.

Hence loading and unloading cycles are likely to have minimal impact on the performance of the rockbolt support system.

5. Over the range of loading rates examined, stiffness of the system varies with the rate of load application; higher loading rates result in greater stiffness.

6. Examination of the fracture surfaces of the failed rockbolts showed they failed in a typical ductile manner and not in a manner usually associated with shear failure. Failure was initiated in the centre of the necked region of the element with cracks radiating outwards towards the surface. The fracture was completed via a shear lip on the outer extremities of the element.

7. Hence although a rockbolt may have failed axially at the rockbolt/resins/rock interface, it may still be capable of offering some appreciable resistance to shear loading and hence support to a rockmass.

In tests where no face plate and nut were used at the collar of the borehole, the rockbolt generally could not be made to fail. At some point during a test, the limited length of encapsulation in the shorter block was insufficient to resist the axial load generated along the rockbolt resulting in failure at the rock/resin and/or resin/rockbolt interface. With continual shear displacement between the two test blocks, the level of resistance remained constant as the rockbolt was extruded through the borehole.

8. Pre-tensioning of a rockbolt increases its resistance to shear displacement. When pre-tensioned, a rockbolt would initially exhibit a higher level of stiffness and minimal shear displacement. With continual loading, eventually a point was reached when shear displacement increased with load at a rate similar to that observed in untensioned elements.

The magnitude of load necessary to initiate shear displacement increased with the level of pre-tension.

Hence pre-tensioning is beneficial to increasing the stiffness of a rock support system dependent on the level of pre-tensioning.

9. The use of strain-gauge rockbolts confirmed that shear loading generated an axial load along a rockbolt.

When the orientation of the strain gauges was aligned with the shear plane, failure of the rockbolt was initiated in the corner of the one of the longitudinal slots of the strain-gauged rockbolt where the bending stress is at a maximum. Here plastic hinges are created that fractured the rockbolt.

Results of the testwork have been peer-reviewed in a paper entitled “Development of a laboratory facility for testing shear performance of installed rock reinforcement tendons” which was presented at the 2005 Conference on Ground Control in Mining, Morgantown, USA.

Future Actions

In summary, with completion of this first-stage project on shear loading, the following have been achieved:

· an understanding of the current state of knowledge related to the performance of reinforcing elements subjected to shear loading conditions;

· the creation of a single-shear test facility capable of simulating behaviour in a controlled laboratory environment;

· an initial series of controlled laboratory experiments using the test facility; and

· an understanding of some of the factors that influence the performance of a rockmass when subjected to shear.

Further work using this new test facility is recommended specifically with regard to:

· borehole and element geometry;

· orientation;

· element, encapsulation and rockmass properties;

· block geometry;

· element pre-tensioning;

· differences in discontinuity surfaces and aperture.

Further theoretical, mechanistic and computational studies are also recommended to complement the work with the laboratory test facility. In combination, this research will provide a better understanding of the factors that affect the performance of rock re-enforcement systems and contribute to more effective design of support systems used in underground coal mines.

Draft Guidelines

Consideration Of Shear Behaviour On
Best Practice In The Design Of Rock Support Systems
In Underground Coal Mines

Based on the findings of initial test work undertaken as part of ACARP Project C12010, the following guidelines have been developed to improve the design of underground excavations with respect to re-enforcement elements subjected to shear loading.

1. In the design of the support system for underground excavations, the loading bearing behaviour of re-enforcement elements is a function not only of the material property characteristics of the elements but also of the rockmass in which the element is embedded.

a. In stronger rock types such as sandstones, the support system is likely to be less compliant and stiffer. The system is better able to maintain the integrity of the laminated beds and hence contribute to overall stability.

b. In weaker rock types such as coal and shale, the system is likely to be more compliant and allow for more differential movement between bedding plains. However the strength of the system should be enhanced with the rockbolt capable of sustaining a higher level of resistance to shear loads.

2. The loading capacity of a re-enforcement element in shear when embedded in a rockmass may be greater than the rated tensile and shear specifications for that element.

3. The stiffness imparted to a rockmass by a re-enforcement element can be high and independent of the level of shear displacement or shear load applied. With continual increase in load eventually the re-enforcement element with begin to yield resulting in a reduction in stiffness.

4. Even after a re-enforcement element has yielded, it can still provide support to a rockmass in terms of resistance to further shear loading.

5. Stiffness of the rockmass with re-enforcement elements varies with the rate of load application. The rockmass has a greater propensity to resist sudden loading conditions. Alternatively, resistance is less at lower loading rates as in the case of creep.

6. Even where it may be evident that the anchorage mechanism of a re-enforcement element has failed and can provide little axial resistance, the element may still be capable of providing some resistance to any shear load.

7. Pre-tensioning of a re-enforcement element can considerably increase the stiffness of the system and hence minimise any differential movement between bedding planes. The level of resistance varies with pre-tension.

	Paul Hagan		Bruce Hebblewhite
	Luke Mahony		Sean Egan
	Wouter Hartman