ACARP ACARP ACARP ACARP
Underground

Anchorage and Failure Mechanisms of Fully Encapsulated Rockbolts: Stage 2

Underground » Strata Control and Windblasts

Published: September 04Project Number: C10022

Get ReportAuthor: Paul Hagan, S Weckert | University of NSW

The results of this second stage project have contributed to the state of knowledge and further understanding of the mechanics of fully encapsulated or full column rockbolts (FERB). The project included several investigations intended to improve current rockbolting practices in the field and into the mechanism of rockbolt failure.

Background
The project followed the earlier Australian Coal Association Research Program (ACARP) project C7018. Recommendations from the peer review of that project concerning improvements to the test facility and follow-up investigations formulated the scope for the second stage project.

The defined objectives of the Stage 2 project, with funding of $135,000 provided by ACARP were to:
  • enhance and upgrade the control and monitoring capabilities of the testing facility established in the earlier project;
  • modify and refine the testing procedures to ensure more reliable research outcomes; and
  • expand on the earlier research activities covering a greater level of detail and a wider range of principles that affect the performance of rockbolts.

The project was again undertaken as a collaborative venture combining the collective expertise at the University of New South Wales (UNSW) and Strata Control Technologies (SCT) to ensure the maximum benefit would be gained from the project.

Management &Communication
The project was principally undertaken in two phases over a two and one half year period that began in April 2001. A progress report detailing the interim findings from the first phase was completed in April 2002. The project involved several students in the School of Mining Engineering; seven students at Honours level and one student at Masters level.

Results of Investigations
Considerable insight has been gained as a result of this project into understanding the effect of changes in a range of parameters on anchorage performance as well as the load transfer behaviour and failure mechanisms of fully encapsulated rockbolts.

Several investigations were conducted using the upgraded test facility; these included an investigation into the effect of changes in the design as well as the conditions concerned with the installation of fully encapsulated rockbolts. In particular the investigations focussed on the effect of changes in:
  • resin annulus;
  • deformation geometry of the rockbolt; and
  • resin spin time.
A further investigation was undertaken to better understand the nature of load transfer of a fully encapsulated rockbolt. The various findings are as follows.
  • In terms of load transfer, load was found to decrease with distance along an encapsulated rockbolt. The load being greatest nearest the rock free surface (that is nearest the point of load application), gradually reducing with distance along the rockbolt into the rock mass. This finding supports the theory that an externally applied load on an encapsulated rockbolt is gradually transferred into the rock mass, creating a localised stress field that aids in clamping together the rock mass. This finding was predicted in the numerical modelling work by Whitaker and reported earlier in the Stage 1final report. There has been some conjecture as to the nature of the load transfer function. To better understand this, two arrangements of load application were investigated. One arrangement modelled the standard rockbolt pull-test. This tends to confine the surface of the rock surrounding the rockbolt. The second arrangement modelled loading resulting from the separation of the partings in a rock mass. In both cases load transfer occurred over the entire albeit short length of encapsulation. Minor differences were observed in the load transfer function between the two loading arrangements. An earlier test indicated a non-linear rate of load reduction with distance using the first arrangement while in the second arrangement the trend appeared to be linear. In a subsequent repeat test using a rockbolt with twice the number of strain-gauges, differences in the load transfer function between the two loading arrangement were less apparent. Significantly, load transfer appeared to be nonlinear in both cases with 50% of the load transfer taking place within approximately 54 mm or 2.5 rockbolt diameters from the free surface. Interestingly cratering of the test sample immediately surrounding the rockbolt was observed in the laboratory tests with the second loading arrangement, a result that was predicted following the numerical modelling analysis undertaken in Stage 1.
  • Following on from these results in terms of the mechanism of rockbolt failure, when a high load is applied to a rockbolt the resulting stress induced in the surrounding rock mass may exceed its strength causing localised rock failure. It is reasonable to consider that the load will transfer along the rockbolt until either confinement of the rock mass is sufficient to withstand the load transfer; the rockbolt intersects a higher strength rock mass; or, the end of the rockbolt is reached. In either case this transfer of load extends the length of localised failure with permanent de-coupling of the rockbolt from the rock mass.
  • The strength or load bearing capacity of FERB was found to be independent of changes in resin annulus where resin annulus remained less than 4mm. With greater resin thickness there was a reduction in the magnitude of anchorage strength and an increase in the variability. In addition, the stiffness of the anchorage system reduced as the influence of the properties of the resin in the anchorage system became more prominent. A near halving in anchorage strength and system stiffness was observed with an annulus size of 9mm compared to 2mm. This indicates that the extent of acceptable tolerance when drilling for rockbolt installation is around 4mm.
  • Underspinning or overspinning of a fast-set resin cartridge had the same result in terms of diminishing the anchorage performance of a rockbolt and are equally undesirable in practise. Both had a pronounced negative effect in terms of increasing the variability in rockbolt performance. More consistent performance was achieved in those tests that were closer to the recommended spin time.
  • A near doubling in anchorage strength was observed with a changeover to a mix-and-pour resin from resin cartridges when anchoring a rockbolt. There was also greater variation with resin cartridges even under supposedly the same conditions.
Future Actions
While the insights gained from Stage 2 represent an advance in the state of knowledge of the mechanics of rockbolts as well as some of the factors involved with the practise of installing rockbolts, it has lead to even more fundamental questions being raised. For example:
  • how much difference is there in the anchorage performance of the different types of rockbolts currently available on the market?
  • what is the critical range in spin times for other resin cartridges on the market?
  • what is the effect of glove fingering on anchorage performance with different cartridge designs - length of encapsulation, type of plastic etc?
  • do highloads lead to de-coupling of a rockbolt from the rock mass through load transfer?
  • what influence does resin annulus have on the effectiveness of load transfer between a rockbolt and rock mass?

Underground

Health and safety, productivity and environment initiatives.

Recently Completed Projects

C29009Control Of Transient Touch Voltages During Switching

There have been an increasing number of electric shock incidents rep...

C29025Effectiveness Of Shotcrete In Underground Coal Mines

The primary objective of this project is to quantify the effectivene...

C34019Longwall Bretby Cable Handling Monitoring With Fibre Optics

This project examined the potential of using fibre optic sensing tec...

Underground

Open Cut

Safety, productivity and the right to operate are priorities for open cut mine research.

Open Cut

Coal Preparation

Maximising throughput and yield while minimising costs and emissions.

Recently Completed Projects

C33053Improving Centrifugal Dewatering Via Modelling And Analysis

The aim of the project was to develop a model for screen bowl centri...

C33042Coal Quality Borecore Methods Amalgamation Guide

Sampling and testing of borecores for coal quality outcomes has a lo...

C34050Hand Held Sensor For Real Time Measurement Of Fluorine Mineral Contamination In Coal

The project's objective was to create a new type of hand-held se...

Coal Preparation

Technical Market Support

Market acceptance and emphasising the advantages of Australian coals.

Recently Completed Projects

C34059Coke Reactivity With CO2 And H2O And Impacts On Coke Microstructure And Gas Diffusion

With the global shift to low-carbon ironmaking, partial substitution...

C33066Washability And Distribution Of Sulfur And Trace Elements For Different Size And Density Fractions Of Raw Coals

Based on the hypothesis that the levels of sulfur and other toxic tr...

C34060In-Situ Investigation Of Coke Structure Formation Under Stamp Charged Coking Conditions

Stamp charged cokemaking has emerged as an effective technique to im...

Technical Market Support

Mine Site Greenhouse Gas Mitigation

Mitigating greenhouse gas emissions from the production of coal.

Recently Completed Projects

C34066Safe Operation Of Catalytic Reactors For The Oxidation Of VAM Operating Under Abnormal Reaction Conditions

The catalyst Pd/TS-1 has shown excellent activity in oxidising venti...

C28076Selective Absorption Of Methane By Ionic Liquids (SAMIL)

This third and final stage of this project was the culmination of a ...

C29069Low-Cost Catalyst Materials For Effective VAM Catalytic Oxidation

Application of ventilation air methane (VAM) thermal oxidiser requir...

Mine Site Greenhouse Gas Mitigation

Low Emission Coal Use

Step-change technologies aimed at reducing greenhouse gas emissions.

Recently Completed Projects

C17060BGasification Of Australian Coals

Four Australian coals were trialled in the Siemens 5 MWth pilot scale ga...

C17060AOxyfuel Technology For Carbon Capture And Storage Critical Clean Coal Technology - Interim Support

The status of oxy-fuel technology for first-generation plant is indicate...

C18007Review Of Underground Coal Gasification

This report consists of a broad review of underground coal gasification,...

Low Emission Coal Use

Mining And The Community

The relationship between mines and the local community.

Recently Completed Projects

C16027Assessing Housing And Labour Market Impacts Of Mining Developments In Bowen Basin Communities

The focus of this ACARP-funded project has been to identify a number...

C22029Understanding And Managing Cumulative Impacts Of Coal Mining And Other Land Uses In Regions With Diversified Economies

The coal industry operates in the context of competing land-uses that sh...

C23016Approval And Planning Assessment Of Black Coal Mines In NSW And Qld: A Review Of Economic Assessment Techniques

This reports on issues surrounding economic assessment and analysis ...

Mining And The Community

NERDDC

National Energy Research,Development & Demonstration Council (NERDDC) reports - pre 1992.

Recently Completed Projects

1609-C1609Self Heating of Spoil Piles from Open Cut Coal Mines

Self Heating of Spoil Piles from Open Cut Coal Mines

1301-C1301Stress Control Methods for Optimised Development...

Stress Control Methods for Optimised Development and Extraction Operations

0033-C1356Commissioned Report: Australian Thermal Coals...

Commissioned Report: Australian Thermal Coals - An Industry Handbook

NERDDC