FEATURE: Tunnelling challenges
By Helen Wright14 April 2014
There is no ‘one size fits all’ approach to tunnelling, as each contract presents unique challenges from tough ground conditions to extreme remoteness and even poor weather conditions.
The tools needed often have to be customised for the task at hand, and specialised new excavation, lining, drilling and boring equipment is being developed for the sector all the time.
The real beasts of the tunnelling sector are of course the tunnel boring machines (TBMs), which are tailor-made to the project they have been commissioned for, with new technology developed and integrated to fit demand.
Robbins, for instance, designed a hybrid TBM for work on an inclined tunnel at a mine in Australia. The 8 m diameter single shield earth pressure balance (EPB) machine was dubbed a hybrid because it combines features of two TBM types – EPB for softer ground, and a hard rock Single Shield.
The TBM can operate in pressurised EPB mode using a screw conveyor for soft to mixed ground under water pressure. The cutterhead allows for interchangeable cutting tools, and in hard rock mode, the rock is picked up by paddles that load the screw conveyor, as the machine is operating non-pressurised mode.
The TBM was built using Robbins’ Onsite First Time Assembly method in order to fit within a tight project schedule – a process that took place at the remote jobsite near Moranbah, Australia, and took about four and a half months to complete.
But things don’t always run quite so smoothly. Robbins also spent some of last year helping rescue a 2.6 m diameter Caterpillar EPB TBM which became stranded underneath New York harbour, US, in the wake of Super Storm Sandy, which struck the East Coast of the country during the autumn of 2012, causintg widespread damage.
The TBM had been excavating under high pressures in clay and sand for the harbour’s Siphon water line replacement project. But the storm flooded the entire tunnel, submerging the machine 460 m into the bore.
The joint venture contractors turned to Robbins for the extensive repair work needed, following Caterpillar’s announcement that it was exiting the TBM business. Robbins field service manager for the Americas, Jeremy Pinkham, said 99% of the electrical systems on the TBM needed replacing. “Some parts were corroded by the salt water—we had to inspect everything,” he added.
The machine had also been stopped with its thrust cylinders in, and as a result certain components could not be replaced before the machine started up. “Since we only had limited general assembly drawings from the Cat manual, we had to work backwards to create a new programme,” Mr Pinkham said.
The refurbishment project took four months, and was completed on schedule. Mr Pinkham said, “We took on a project that nobody else would. We were able to rebuild an unknown machine from an unknown supplier, working outside our system. There were no detailed drawings to refer to, just our observations.”
This demonstrates some of the dedication and innovation that is needed to rescue projects when things go wrong – innovation that is currently in demand on a project in Seattle, US, where a huge TBM has been stranded underground since December last year.
It is not uncommon for TBMs to run into unexpected difficulties during their bores. However, the level of downtime currently being experienced by the massive, US$ 80 million, 91.4 m long, 17.5 m diameter TBM known as Big Bertha is unusual.
Bertha started its Journey boring the 2.83 km long Highway SR 99 tunnel underneath Seattle, US, in July 2013. But the machine has been stranded 18.3 m underground near South Main Street since December, with repairs estimated to take six months or more.
The developer, the Washington State Department of Transportation, said contracting consortium Seattle Tunnel Partners (STP) and Bertha’s manufacturer, Hitachi Zosen, had identified a piece of steel pipe protruding through an opening in Bertha’s cutterhead.
STP – a joint venture between Dragados USA and Tutor Perini – is considering several options to remove the steel pipe and identify other potential obstructions. As the process to clear the blockage continues, workers are conducting critical maintenance, inspecting and replacing damaged cutter tools on the face of the machine. STP has estimated that digging might not resume until at least 1 September this year.
The US$ 3.1 billion tunnel is a replacement for the Alaskan Way Viaduct, and was initially scheduled to be completed by September this year, and operational by December 2015. The difficulties being faced by Bertha serve to highlight how critical the smooth progress of TBMs are to the completion of tunnelling projects.
Elsewhere in the sector, manufacturers continue to work closely with contractors to boost productivity and safety, developing customised equipment from new excavation units to flexible formwork designs.
GTA Maschinensysteme, for example, specialises in building machines that run on monorails for use in tunnels. It has introduced two new machines designed to support specific aspects of tunnel lining and excavation.
The first is a multifunctional uphill excavator, developed for the Crossrail metro project in London, UK – currently the largest infrastructure project in Europe (see box story).
GTA said the machine was designed and produced in six months, and was introduced in October last year. While the main tunnels for the Crossrail project are driven by TBMs, secondary incline shafts need to be dug out to connect the Crossrail stations to the new subway lines.The idea is that escalators will be installed here once complete.
These incline shafts are being driven from TBM-level to subway-level (bottom upwards), and GTA said the 30o tunnel incline was not possible to tackle with floor-bound machines. The uphill excavation machine and its working platform are also suspended on two overhead monorail tracks, and moved along by GTA push-pull units.
The machine features four breaking trolleys to keep it safe and secure at steep inclines, and digs forward using a specialised shovel. It also boasts a drill feed and be used to drill blast holes in case hard rock is met, as well as a hydraulic hammer for breaking up rock, and a nozzle and lines for shotcreting.
Another new machine from the manufacturer is the TAM 7500A lining machine for subway tunnels – a piece of equipment developed with the idea of mechanising the installation of cast iron or steel tunnel lining segments.
The TAM 7500A is a 28 tonne machine, also suspended on two overhead monorail tracks mounted on the inside of a tunnel. An additional rail track is suspended at the side and is used as travel rail for the transport unit. The machine has two booms which are separately mounted in two guideways, where they can be moved backwards and forwards to handle up to 1,000 tunnel lining segments.
Falsework & formwork
Meanwhile, several falsework and formwork manufacturers have added specialist tunnel formwork to their portfolio. Alsina, for instance, offers a self-supporting formwork system that is moved by a hydraulically-driven carriage along a tunnel, allowing the use of several formwork systems to line a tunnel.
Another method for tunnel building is Alsina’s Modular Curved formwork for the construction of false tunnels and tunnel entrances. This system supports a maximum pressure of 60 kN/m2, requires a minimum bend radius of 2.5 m and allows a modulation of 2.4 x 2.4 m.
This means it is adaptable to different radii, while Alsina said it also facilitated quick and secure mounting, boosting productivity.
For its part, Doka used different techniques to support construction of four tunnels during the building of the € 718 million (US$ 996 million), 22 km S 10 highway in the Mühlviertel district of Austria.
These included its modular heavy-duty supporting system SL-1 in combination with the large-area formwork Top 50 and high-capacity hydraulics, and its new cut-and-cover tunnel formwork system, Doka CC. This system supported construction of both tubes of the Pernau Subsurface Tunnel, each of which is 270 m long. The formwork can be adapted to the complex, slightly rotating geometry of the structure.
Doka CC also supported construction of the Ganglsiedlung Subsurface Tunnel, which consists of two tubes, each 275 m long. The manufacturer said adaptation to the structural geometry of this tunnel required little additional material cost.
The tunnelling sector is busy and ambitious, with a range of very large infrastructure projects currently underway around the world. Of course it is ideal when everything runs smoothly and projects are completed on schedule, but the sector’s ability to overcome unforseen challenges like TBM breakdowns also speaks volumes about the ability to solve problems.