THERMAL COAL
Digging Things Up
March 2019
As resources go, coal presents relatively few technical or chemical challenges to extract, store, and prepare for sale. While many minerals and metals require extensive crushing, heating, and chemical treatment, coal processing largely relies on water-based washing and physical separation techniques. This isn’t to say that coal mining doesn’t present significant technical challenges, though.

A key issue in the industry for new projects is the time taken to gain access to underground deposits—and the ability to do it safely. Rapid development of a site can be greatly beneficial, and technology that decreases the time to mine without compromising safety or tunnel quality is greatly valued.

February 10, 2015 saw Anglo American’s completion of the Grosvenor project’s underground people-and-equipment drift tunnel by a specialised piece of equipment: The Tunnel Boring Machine. In this case, the TBM was named Lucia. The tunnel was completed 29 days ahead of schedule—an impressive achievement, given that TBM drilling already gave significant time and quality improvements over more traditional Roadheader excavation equipment.

 

A Head-to-Head Comparison

Weighing up the advantages and disadvantages, Anglo chose the TBM for two core reasons: speed and quality. Case studies have shown that TBM’s can be up to ten times faster at digging than roadheaders and come with numerous advantages that make tunnel construction easier, safer, and more durable—although it is often a more expensive method.

Where roadheaders excavate with a smaller drill head attached to a moveable arm, a pressurised TBM has a fixed drill head the same size of the tunnel it creates. The size of the TBM means that the tunnel is supported as it is drilled, reducing risks of a structural failure during digging. Once the drill reaches the end of the tunnel, the inner machinery can be extracted while leaving the exterior support shield in place, ready to be grouted and become a permanent part of the tunnel supports. As the tunnel is excavated, concrete rings are laid, providing a secure foundation that, in Grosvenor’s case, is expected to last for more than the 40 years required for the project.

The specific kind of TBM used, a Dual Mode Crossover model, allowed the mining crew to drill through both hard rock and softer ground found at the Grosvenor project without swapping approaches or large amounts of machinery.

The area in front of the drill head is sealed and pressurised, meaning that gas is easier to deal with and contain during operations and preventing gas from escaping into the rest of the tunnel. The machine was also equipped with a methane detector and snuffing box evacuation system, able to draw the methane out of the spark-resistant conveyor and directly into the tunnel’s ventilation system.

 

Construction with the TBM

The TBM allowed for a far faster drift construction. Drilling moved faster and enabled the installation of supports and flooring at a faster rate than previous techniques. Full sealing of the tunnel—to prevent gas leakage from the numerous stringer seams—also took considerably less time, due to the aforementioned pressurised zone that exists in the space in front of the machine as it operates.

Dewatering was also handled in a straightforward manner, with initially encountered groundwater incorporated into cuttings and removed along with other excavated material through the drift’s screw conveyer system. To prevent inflow into already-tunnelled areas, grouting was included between installed concrete rings.

The 813m-long conveyor drift was completed over five months from Dec 20th to 15th May 2014, and Construction of the 994m-long transport drift begun after May and was completed in February of 2015, meaning that both drifts were constructed in just over a year’s time.

While TBM’s had seen usage before, the Grosvenor project contains the first precast concrete-lined TBM-excavated set of drifts in Australia—and may have been the first usage of a TBM on Australian soil.