ACARP ACARP ACARP ACARP
Underground

Geophysical Assessment of Fracture Grouting

Underground » Strata Control and Windblasts

Published: May 04Project Number: C11056

Get ReportAuthor: Gary Fallon, Binzhong Zhou, Andrew King, Peter Hatherly | MIM Exploration, CSIRO Exploration & Mining

There are very high safety and production risks associated with mining through any weak zone associated with a fault or fractures. The mining industry seeks to reduce this by using fine, high pressure grouting techniques to stabilise such geomechnically weak zones. However considerable uncertainty exists on the effectiveness of grouting fault and fracture zones.

The objectives of this project were to assess the impact of a grouting program on the properties of the rock mass.  Geophysical monitoring techniques and a laboratory study of the petrophysical properties of the strata and grout were undertaken. The geophysical techniques deployed are crosshole seismic tomography, microseismic monitoring and down-hole stress monitoring.

MAIN FINDINGS AND CONCLUSIONS
The mass properties from 9 samples at Oaky Creek exhibit a relatively low permeability and porosity. Vacuum imhibition trials failed to force any microfine grout into the adjacent pore space within the rock volume.  Velocity anisotropy is significant with five samples averaging a 25% increase in the compressional velocity measured parallel to bedding compared to the velocity orthogonal to bedding.

Three different grout products were used in the laboratory and all products required a 28 day curing time to reach specification. 

The addition of each sawn synthetic fracture to the core reduced the compressional velocity by 4%. The addition of the Alofix grout product to these synthetic sawn fracture planes restored the core compressional velocity to approximately the original value. This would suggest that under confinement there is very little energy lost from the interfaces between the mineral components and the cured grout particles. After the specified curing time the core on either side of these fractures was easily pulled apart. A possible mechanism whereby the grout is improving the “strength” of jointed and faulted interfaces is through increasing the frictional coefficient between joint planes and thus reducing the amount of movement that occurs.

Sigra developed and installed in-situ stress change monitoring cell(s), with the data recorded via unmanned data loggers.  As the data indicates, all but two of the vibrating wire gauges, appeared to perform well. The work showed that there were no sudden major changes in stress levels until those caused by the passage of the longwall, though little jumps may have occurred. While the direction of principal horizontal stress change was not known its orientation was shown to be relatively constant.  The longwall induced stress is estimated to increase the vertical field by approximately 32 MPa.

The microseismic system was successfully installed and operated over the period of production.  The data indicate a majority of events occurred within a two week period prior to the shearer arrival and within the first 15m of immediate roof strata.  A rigorous error analysis and probabilistic uncertainty modeling concludes the fault zone was not activated by longwall mining.  The laboratory study also suggested that there was little uptake of grout into the formation.

RECOMMENDATIONS
This project has demonstrated some of the methods that can be used to assess the effectiveness of injecting grout into fault zones prior to longwalling. In this case, the fault did not appear to affect face control or caving behaviour. A need for improved up-front assessment methods to develop better management strategies for mining through faulted areas is therefore indicated. It is recommended more information be used in deciding on whether to grout and how to design the drill pattern.

Apart from consideration of previous mine experience, these methods should involve detailed geological assessment and the analysis of any seismic reflection data that might be available. If it appears that grouting is necessary, the drilling of an orientated, angled, core hole from the surface allowing the recovery of geotechnical geophysical logging data would be the next logical step. This would allow the in-situ properties of the fault zone to be assessed. To complete the assessment, the potential grout uptake should be evaluated by testing for the porosity and permeability of representative core.  By undertaking this step, it is estimated the total cost of a grout program would be increased by just 5%.

If a grout injection programme is to be undertaken then it should be planned on the basis of the previous investigations. Meticulous record keeping should be enforced on the grout injection team to ensure that the specified design volume is injected. The three microfine products tested in this project performed similarly under equal and ideal conditions.

The water to grout ratio is a significant factor in determining the overall grout strength. The production based data indicates that contractor procedures are likely to dilute the desired product ratio before it is injected into the hole. Most joints and faults within an underground coal operation become conduits for mining induced dewatering. This coupled with the optimal design criteria of a grouting pattern to maximise the exposure to the fault and joint planes suggest that further dilution of the grout mix is a certainty. This must be a consideration in the implementation of grouting programs.

The geophysical monitoring techniques employed here should also be considered for further studies when there are concerns about the fault behaviour. Microseismic monitoring will reveal fault movement and stress monitoring may indicate if non-brittle failure is occurring.  While there are issues in conducting the surveys and producing tomograms of sufficient resolution tomography still provide a method of assessing the changing properties of the rock mass as it responds to the mining process.

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