To meet schedule requirements, the mine opted for the use of a Tunnel Boring Machine (TBM), marking the first use of TBM technology in a mine on the continent. Due to the geological and mine conditions, the TBM was highly customized. The Robbins Company, the world’s first and foremost TBM manufacturer, supplied the Dual Mode Crossover TBM that could excavate in both hard rock and mixed ground, and in the presence of potentially hazardous methane gas.

Project Background

The Grosvenor Decline Tunnel is an ASD $1.95 billion coal project, and part of an initiative aimed at tripling Anglo American’s metallurgical output to 20 million tonnes by the year 2020. The new mine targets the Gonyella Middle coal seam, and is expected to produce five million tonnes of coal per annum from its underground longwall operation over the next 26 years.

Two decline tunnels were required for mine access to the coal seam at the shallowest depth of 130 meters. Longwall panels are planned to be 300 meters in width with lengths up to 6200 meters. The first 1:8 grade decline tunnel is a conveyor drift for coal transportation; the second 1:6 grade decline tunnel is a transport drift designed for people and equipment to provide underground access once the mine is operational.

Commissioning of the longwall is targeted for late 2016 and the coal will be processed through an existing Moranbah North coal handling and preparation plant and train loading facilities owned by Anglo American.

Innovative versus Traditional Methods

Anglo American made the choice of a TBM over the more commonlyused roadheader method for two main reasons: speed and maintenance. In terms of speed, TBMs have proven to be about ten times faster than roadheaders (50 m per week for a TBM vs. 5 m per week for a roadheader). As for maintenance, Anglo American wanted lasting tunnels that didn’t require any upkeep.

– The final tunnels need to remain intact for the life of the Grosvenor Mine [about 40 years], and be maintenancefree with cement linings, said Adam Foulstone, Underground Construction Manager at Anglo American.

When asked to compare the two methods, Foulstone replied of TBMs:

– This is the better methodology. [Use of TBMs] opens up a new chapter not just with Anglo American, but with the whole coal industry in Australia. Now we can draw up a new coal mine in less than a year, compared with two to three years if we use roadheaders.

Foulstone also noted that there are few limitations for TBMs in mines.

– Anywhere we need to get men and materials into an underground environment is an opportunity to use a TBM, he said.

With the construction methodology choice made, Anglo American opted for an 8 m diameter Robbins Dual Mode Crossover TBM and continuous conveyor system, to be assembled on location using Onsite First Time Assembly (OFTA) with onsite support from Robbins’ experienced Field Service team.

Geological Conditions

Ground conditions varied throughout each tunnel. Both drifts contained geology consisting of mixed soil and rock conditions, with the first 300 m or so of each tunnel containing the majority of the mixed ground such as soft clays and soils.

Tunnel length was dependent on the location of the coal seam. Called blind headings, the TBM ceased excavation in each tunnel once the coal seam was reached. Once the coal seam took up approximately 50 percent of the tunnel diameter, tunnelling for that access decline was considered complete. For both tunnels, this was around the 1,000 m mark.

The Grosvenor TBM is a Dual Mode Crossover TBM capable of “crossing over” from hard rock to mixed ground excavation, depending on tunnel geology.

About the Machine

The unique Dual Mode Crossover machine could convert between two modes: Earth Pressure Balance (EPB) mode for soft and mixed ground, and Single Shield mode for hard rock. This machine type is a recent innovation in the tunnelling industry, intended to provide the contractor with crossover capabilities; meaning, a machine that can function in vastly varying ground.

As the drives at the Grosvenor Project contained a mixture of hard rock and soft ground, a Crossover solution was selected. The machine was equipped with a combination of features found on Earth Pressure Balance (EPB) machines (for soft ground) and Single Shield machines (for hard rock). While the machine types have some similarities (they are both shielded to protect workers and they erect segments for a tunnel lining) they are also quite different. The main contrast of EPB-type and Single Shield-type machines is that EPBs are for water-bearing ground and are built to handle pressure below the water table, while Single Shields are for hard, fractured rock already at atmospheric pressure.

By combining features of both machine types into one, Anglo American was prepared to handle the combination ground that lay ahead. The machine was optimized towards Single Shield excavation, as only the early portions of each tunnel were in the softer ground. The cutterhead could be outfitted with different types of cutting tools depending on the ground, and a two-stage screw conveyor worked in both geologies.

The machine was also designed to meet the continuous ground support requirements of Australian mines. In order to do this, the components were bolted together with the protective shields able to be easily disassembled from the inner core of the machine. At the end of each tunnel, these outer shields were left in the ground and the cutterhead and other machine parts were disassembled and retracted from the tunnel for reassembly above ground. The outer shields were then grouted into place to provide life of mine roof support for the final length of each tunnel, which was lined up to that point with concrete segments.

The Anglo American crew gathers to celebrate the breakthrough of February 9, 2015. Image courtesy of Anglo American.

Planning for Gassy Conditions

Project geology consisted of soft soils to hard rock with sections of mixed face and potential gassy conditions. A machine with EPB capabilities was chosen not only due to the presence of softer ground, but also to contain the methane gas where it could then be diluted or safely removed from the tunnel.

At the beginning of the design process, an extensive risk assessment was conducted to determine the potential zones of accumulation and flow of methane gas. In accordance with Queensland Coal Mine Standards, these zones were classified as:

• NERZ (Negligible Explosion Risk Zone) with methane concentration below 0.5 percent
• ERZ1 (Explosion Risk Zone) with methane concentration between 0.5 and 2.0 percent
• ERZ 0 with methane concentration over 2.0 percent

The coalmine-compliant machine was therefore designed as NERZ-ERZ1, Class 1 Division 2 Explosion proof. To ensure worker safety and avoid explosions, gas levels were kept under 2 percent at all times. If any methane leakage was detected, a snuffing box evacuation system drew the methane out of the spark-resistant screw conveyor and directly into the ventilation system. An NERZ/ERZ1 boundary was set up on the front of the machine to ensure methane gas did not reach higher concentrations in the rest of the tunnel.

Excavation and Breakthrough

Construction on the Grosvenor project began in July 2012. The first of the decline tunnels, for conveyors, was excavated between the end of December 2013 and the beginning of May 2014, after achieving advance rates of up to 90 m per week. The Quick Removal System was a success, allowing the TBM inner core to be retracted back to the surface from a 160 m depth using specially designed transport dollies. In order to transport the machine to the next tunnel 2 km away, the TBM had to be split into two sections and required a large 600 metric ton lift.

The machine was re-commissioned for the men & materials tunnel with a new set of shields, and commenced boring in November 2014. Excavation was completed in 88 days at an average of 10.9 m per day, with a best day of 25.2 m. The TBM averaged 70 m a week, about 14 times faster than a roadheader. The bore itself was similar to the first, with few challenges encountered other than elevated methane gas levels that required several temporary stoppages in order to safely remove the gas from the tunnel. Final breakthrough was reached on February 9, 2015. Upon completion of the second tunnel, the machine shields were again left in place to provide continuous support.

Currently, the machine is being prepared for its final retraction and roll out, and will be on the surface by mid-spring. After that, the TBM will be stored on the Grosvenor site for future Anglo American mining projects.

Liz Stone,

Marketing Manager,

The Robbins Company,

stonel@robbinstbm.com

Desiree Willis,

Public Relations Specialist,

The Robbins Company,

willisd@robbinstbm.com