Boring the Blue Mountains: The Inside Story on the World’s Fastest TBM
Over 20 years ago, a Robbins open-type machine set three world records while tunneling in the picturesque Blue Mountains in Australia. You may be asking yourself, why is this significant? Why drudge up a project that is surely outdated at this point in our industry’s history? The fact of the matter is, two decades have passed and the Robbins open-type TBM chosen for this project is still considered to be the world’s fastest TBM.
In 1993, the 3.4 m (11 ft) diameter TBM was chosen to bore two sewage tunnels in the Blue Mountains near Sydney, Australia. At the time, there had been a rapid expansion of urban developments within the Blue Mountain National Park, causing an influx of pollution to enter streams as the result of septic tank runoff and outdated sewage treatment plants.
The Blue Mountains Sewage Transfer Project comprised of approximately 40 km (25 mi) of tunnels, two of which were TBM-driven using the Robbins machine. The first, the Katoomba Carrier tunnel, was 13.4 km (8.3 mi) long and the second, the Lawson Carrier, was 3.5 km (2.1 mi) long. While the Lawson Carrier tunnel was finished five weeks prior to the expected completion date, it was during the Katoomba Carrier tunnel that all three records were set. During excavation the machine set the following world records: best day of 172.4 m (565.6 ft), best week of 702.8 m (2,305.7 ft), and a best monthly average of 1,189 m (39,000 ft) within its size range of 3 to 4 m (9.8 to 13.1 ft) diameter machine. Two of those records—the best day and best week—are the fastest ever recorded and have yet to be surpassed by any TBM of any size.
Custom Machine Design
There were many factors that played into this machine’s success—the system utilized on this project required a well-planned design, careful operation, and regular maintenance. As many people involved in tunneling know, choosing the right equipment for the geology can make or break a project. Detailed empirical data allowed the contractor to accurately predict what kind of ground they would encounter and prepare accordingly. The National Park is located within the Triassic Sydney Sedimentary Basin and is comprised primarily of sandstones and claystones. Anticipating this geology, the 3.4 m (11 ft) diameter machine’s cutterhead was dressed with 25 Robbins 17-inch diameter disc cutters designed for soft yet abrasive rock formations.
The 13.4 km (8.3 mi) long Katoomba Carrier tunnel was originally planned to comprise of multiple short tunnels, several kilometers in length each, with intermittent portals to shorten drives. Ultimately it was decided to make the project one continuous tunnel, which led to higher advance rates and deemed it—at the time—the longest single-drive TBM tunnel. Not only did this change save time, but it also allowed boring to be less disruptive to the landscape.
In addition, the project was the first in Australia to utilize a continuous conveyor system. Due to the length of the tunnel, the use of traditional muck cars for muck removal would have taken too much time and were seen as an inadequate solution. The 106-m (347.8 ft) long system boosted production rates, with a best day of 1,565 m³ (55,267 ft³) of in-situ material removed from the Katoomba tunnel.
Ground support throughout the tunnel comprised of a combination of resin grouted bolts, mesh, steel straps and steel sets. In the sturdier sandstone, however, the tunnel was left largely unlined and instead shotcrete was applied to areas of poorer, softer rock.
As boring progressed, the sandstone proved to be softer than expected. Instead of the predicted average of 80 MPa ranging from 20 MPa to 150 MPa, it averaged 40 MPa to 50 MPa with a range of 10 MPa to 100 MPa. This ideal material could also have contributed to the Robbins machine’s record breaking results and its early breakthrough, which occurred nine months ahead of schedule.
The Robbins Mk 12C’s performance on the tunnel boring portion of the project substantially surpassed all expectations. Not only did it set the previously mentioned world tunneling records, but it also helped the project as a whole finish 17 months ahead of schedule, saving not only on time but on significant financial costs.
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