Rapid development techniques maximise activity at the tunnel face and advance completion rates without compromising safety. Good project planning should prepare for both expected ground conditions and unexpected variations in the rock mass to be excavated. This will reduce delays and mitigate the effect of poor ground conditions, which can slow advance rates by 50%.
It is no surprise that poor ground slows advance rates. For example, the industry-leading development practices at Newcrest’s Cadia East operation in New South Wales produced better than average advance rates in the access decline. Rapid development rates of over 8 m/day were achieved using longer drilling rounds and emulsion explosives, but the project still suffered low advance rates of less than 3 m/day as a consequence of poor ground.
Investment in a “rapid development” program is usually justified to realise higher Net Present Value for a project. The risk of not being able to excavate the rock mass as planned can result in inadequate prediction and preparation to deal with the actual ground types encountered.
A Ground Control Management Plan (GCMP) created from site investigation data can mitigate this risk. An active GCMP uses data collected prior to and during excavation. The aim is to predict ground conditions and select design control methods through a structured risk management approach.
Confidence in predictive models increases with geological, structural and geotechnical mapping, and ground behaviour monitoring. Feedback is essential to keep the GCMP “live”.
The Modified Rock Mass Rating (MRMR) system is one of the tools than can help to identify zones, based on similar ground behaviour.
”Rapid development” ground control can include techniques such as phased installation of ground support and drilling short rounds. Common practice, such as pattern rock bolting, meshing and shotcreting, could be complemented with practices such as setting arches and stabilisation grouting.
In conclusion, it was proven that active Ground Control Management Planning has an essential role in all rapid development projects.
