Deep diligence before the dig: How thorough geotechnical investigation lifts raiseboring safety and efficiency concerns

A surface vent was installed after the raisebore was constructed. This is the result of a well-planned investigation and design process.

Phillip Quinn, Senior Mining Engineer, Cartledge Mining and Geotechnics

Raiseboring – creating a vertical shaft into an underground mine without explosives – is conceptually straightforward. In practice, however, it can be a complex story, with many factors including location, ground condition, projected service lifespan and other engineering considerations at play. In this article, Phillip Quinn, Cartledge Mining and Geotechnics Senior Geotechnical Engineer, explores the process and how thorough geotechnical investigation, in collaboration with all stakeholders, can greatly improve a raiseboring project’s safe completion.

“What could possibly wrong? It‘s only a vertical shaft constructed through the ground, to an underground mine.”

We’ve all heard a horror story or two about the construction of a raise-bored shaft that has not quite gone to plan, resulting in a collapse or, painfully, abandonment of a very expensive reaming head.

As geotechnical engineers, with safety constantly front of mind, it’s our duty to carry out proper due diligence before the dig to ensure safe and efficient short- and long-term project outcomes.

What is a raisebore and what is its purpose?

Raisebore drilling is an underground drilling application used to create a vertical or sub-vertical excavation between two or more levels of a mine without the use of explosives (DEMIRS, 2023).

The most common method is the piloting method, whereby pre-drilling is undertaken from the surface to the underground. Drilling rods are lowered down the hole, and the reaming head with cutter tools is attached with raise-bore drilling from the brow to the surface then commencing.

The shaft excavation or ‘structure’ (see Western Australia’s Work Health and Safety Act 2020) is usually used for ventilation to provide clean air, reducing hazardous dust and gas emission exposure to workers underground. This puts emphasis on the importance of long service life of the ventilation shaft, which could be up to 50 years, and involves cooperation between various stakeholders such as a construction team and a ventilation management team.

Raiseboring is not without its dangers, with one fatality occurring in Western Australia during 2022 during construction of a raisebored shaft at a gold mine. This was deemed likely the result of a worker being ‘in line of fire’ and working under unsupported ground, being the raisebored shaft itself.

Why thorough geotechnical investigation is critical

If rock mass was homogenous, strong and unjointed (structureless), constructing a shaft would be far less complex, without the need for geotechnical engineers to undertake detailed analysis.

However, geology and hydrogeology – as we know – can be variable and the requirement for a thorough assessment of the ground conditions at the shaft location is important to reduce uncertainty and risk.

Ideally the geotechnical investigation drillhole should be located within the footprint of the shaft. However, there is usually a reluctance for this practice, due to the small likelihood of drill rods becoming stuck during the geotechnical drilling investigation, which could compromise raisebore drilling at the planned shaft location.

However, if the drillhole is planned outside of the footprint of the shaft, appropriate judgment to determine the alternative drillhole location should be made, which would be dependent on the known variability of the ground conditions at the site.

If the underground working for the raisebore shaft has already been constructed under the assumption of good ground conditions down the shaft prior to the geotechnical investigation, this ultimately has used time, at a financial expense, with a possible move of the shaft location also being required. Detailed planning and consultation with a geotechnical engineer pays dividends!

Often, collaboration between civil and structural engineers and mining geotechnical engineers is needed during a raisebore geotechnical investigation where variability in ground condition at the collar of the shaft is encountered, i.e. transition from soil to rock, in weathered zones that can be extremely weak and saturated.

Engineering solutions – such as piling for the shaft collar – are often required to depths greater than 10m to provide support near the surface.

The use of Cone Penetration Testing (CPT) to help determine appropriate parameters for pile design is often neglected during raise-bore geotechnical investigations, leading to a reliance on empirical relationships to derive these geotechnical parameters. By collecting more data outside of traditional intrusive means, smart engineering and design can be implemented, likely reducing concrete consumption for the pile construction, thereby also reducing CO2 emissions.

The structural and civil engineer who designs the piles is often reliant on the geotechnical logging data, which is usually undertaken by a mining geotechnical engineer who normally has more experience logging rock core rather than soil material. The geotechnical logger also has the responsibility of collecting suitable samples for subsequent laboratory testing. The geotechnical logger/engineer must ensure that the samples are representative of each geotechnical domain encountered from surface and down through the shaft.

Once a geotechnical investigation has been completed, an empirical relationship using the McCracken and Stacey method (1989) is widely used to determine the stability of a shaft. They recognised that rock mass classification systems developed for tunnelling stability had similar factors to raisebore stability and the Q system was then modified for the purpose of raisebores. This method should not solely be relied upon and only accounts for the initial stability and not stability of the shaft over time. However, other methods from Peck and Lee (2007) do account for standup time. Numerical modelling has now also become widely used to predict overbreak/breakout within the raisebored shaft and can be supplementary to the McCracken and Stacey analysis method.

Use of technology such as downhole geophysical survey i.e. calliper, acoustic/optical televiewer, P&S wave etc. has become more widely used during a raisebore geotechnical investigation and can support the geotechnical logging data. After the raisebored shaft has been constructed, LIDAR scanning of the sidewalls to verify as-built to the design is becoming frequent practice. The point cloud data from LIDAR scanning can show areas that have been subject to stress damage and be used to determine where ground support is required.

Conclusion

The reporting of geotechnical raise-bore investigation findings should clearly communicate to the raisebore contractor the ground conditions encountered at the shaft location, via detailed core photos and easy to understand graphs.

Often the report readers are not technical specialists and the report should take this into account, communicating where high-risk areas are located and where cutterhead changes should not be undertaken during reaming to the surface.

If this is not achieved, then the expensive geotechnical investigation could end up being conducted somewhat in vain.

To further support thorough investigation, regular reporting of the drilling progress during raise-boring to geotechnical engineers should be adopted, including where abnormal groundwater flow or loss is observed, giving an early indication of where ground support may be required and boosting safety and success overall.

References

Government of Western Australia - Department of Mines, Industry Regulation and Safety (2023). Information sheet: Focus on compliance - ‘Known hazards when conducting raise-boring activities in underground mines.’ https://www.commerce.wa.gov.au/sites/default/files/atoms/files/231241_br_raisedbores.pdf

McCracken A, Stacey TR. (1989). Geotechnical risk assessment of large diameter raise-bored shafts. Shaft Engineering, Inst Min Met, pp. 309–316.

Peck, WA & Lee, MF. (2007). ‘Application of the Q-system to Australian underground metal mines’, in C Mark, R Pakalnis & RJ Tuchman (eds), Proceedings of the International Workshop on Rock Mass Classification in Underground Mining, National Institute for Occupational Safety and Health, Cincinnati, OH, pp. 129-140.