The 13.4 km Katoomba Carrier tunnel was to be one of the longest single-drive tunnel boring machine (TBM) tunnels at the time. The resulting excavation achieved landmark advance rates for any TBM of any size, which still stand today, nearly two decades later.
The Katoomba Carrier tunnel employed a 3.4 m diameter Robbins TBM between 1993 and 1996. The tunnelling was part of the country’s first BOOT project, a Build-Own-Operate-Transfer scheme for the New South Wales Government. The 35-year concession contract was awarded to a joint venture of McConnell Dowell and Japanese contractor Obayashi.
The TBM was launched in 1993 and began boring through competent rock. Much of the success, according to the contractors, came in choosing the right equipment for the geology. Russell Rooney, Operations Manager for the Tunnel & Underground division of McConnell Dowell, was onsite to witness the rapid excavation.
“The rock during the tunnel drive was as we predicted. Sydney sandstone has historically given contractors a lot of grief, but we had detailed empirical data so we knew what to expect.”
The 48 to 82 MPa UCS rock, with a high abrasivity and quartz content, did cause some wear to the cutterhead, though cutter consumption for the whole tunnel was less than predicted. Rock remained fairly consistent for the whole of the tunnel, so an open-type TBM was determined to be adequate.
During the excavation, the machine set three world records: best day of 172.4 m, best week of 702.8 m, and a best monthly average of 1,189 m within its size class of
3 to 4 m diameter machines.
The design decision to make the project one continuous tunnel likely led to improved advance rates, though it was not the first choice. An original proposal specified several tunnels with intermittent portals to shorten TBM drives to several miles each.
“We opted instead for a smaller impact, tunnelling from a single location. The Blue Mountains are a world heritage site and the new plan was less disruptive to the landscape. It also sped up the progress time with a less fragmented approach,” said Mr Rooney.
Long tunnel lengths also rendered more traditional muck cars inadequate for muck removal, as they would have taken nearly an hour to reach the tunnel portal. The project was the first in Australia to use a continuous conveyor system, which was designed by local company Prok.
“The continuously advancing conveyor played a very significant role in advance rates. It had never before been used at that length, about 13 km. It took a lot of planning to make it work with the TBM and back-up system, but it performed very well,” said Mr Rooney.
After TBM excavation, the tunnel was linked up with the existing Hazelbrook sewerage tunnel, also built using a Robbins TBM between 1991 and 1993.