Underground » Strata Control and Windblasts
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.