Tunnelling advances

As in any other area of the construction industry, there is always pressure on tunnelling contractors to improve efficiency, and the last 12 months have seen some interesting advances in terms of the technology being applied to equipment like tunnel boring machines (TBMs), the specialist materials used in tunnels, as well as construction techniques themselves.

At 44.6 km, Malaysia's Pahang Selangor Raw Water Tunnel will be the longest tunnel in Southeast Asia when complete. The three 5.23 m (17.2 ft) diameter Robbins Main Beam TBMs excavating the tunnel are advancing well with a novel type of shotcrete used as primary ground support under high cover.

When complete, the tunnel will transfer water from the Semantan River in Pahang State to the Selangor / Kuala Lumpur region, travelling as far as 1200 m beneath the Titiwangsa mountain range in varying rock conditions with some ground water. TBM excavation is expected to be complete in 2013.

The shotcrete is applied manually and consists of a dry mortar mix with polypropylene fibres, and a fast-acting accelerator. It is thought to be the first time this material has been used outside Japan.

"The dry mix system is advantageous because it can be started and stopped whenever necessary, and can be mounted in the TBM back-up. Wet systems, by comparison, generate unused mortar if frequently started and stopped, requiring downtime for cleaning," said Nobuo Suematsu, marketing director of MCM, which developed the mix along with fellow Japanese company Denka.

MCM and Denka worked with Robbins to create a near-zero rebound shotcrete system to be used in the granitic hard rock of the tunnel, with rock strengths of up to 200 MPa. The shotcrete is applied to the top 180° of the tunnel following each TBM stroke in a 20 mm (0.8 in) layer directly behind the cutterhead support.

"The shotcrete is very quick setting with no rebound, and the mortar does not require high pressure application," said Yoshikuni Nakano, deputy project manager for contractor SNUI JV (a consortium of Shimizu, Nishimatsu, UEM Builders, and IJM).

The shotcrete system has so far resulted in less downtime compared to conventional ground support, as the shotcrete can be applied during excavation. In the fractured ground conditions currently being encountered, the machines would have required an estimated two hours of ground support work per excavated meter with conventional methods. This would have added 210 days to excavate and support the ground. With the fibre mortar system, this time has been reduced to 158 days - a saving of 52 days.

The TBMs themselves feature custom cutterheads, where the disc cutters can be changed from within the TBM. The operator's cabs on each of the three machines include industrial computer systems and wireless routers, allowing remote access and real-time data feedback to the surface.

"We have been keeping all real-time data on our servers. These are in our site offices, and there is one in Tokyo, which then routes the data on the Internet. Because of this our actual progress data is available anywhere in the world from our password-protected website," said Mr Nakano.

Beijing breakthrough

April 10 was milestone in the construction of the Metro Line 9 in Beijing, when a Caterpillar TBM made a major breakthrough in a section with very difficult geological conditions. Beijing Metro Line 9 will provide an additional 16.5 km of metro and is planned to be operational by the end of this year.

The construction of Lot 6 of the Beijing Metro Line 9 consists of two parallel tunnels with a total length of 1.2 km, each of which passes underneath a lake in the western part of the capital. It is believed to be the most difficult section that the alignment has to run through due to the water-rich conglomerate containing boulders up to 1.5 m.

Caterpillar Tunnelling Canada (previously known as Lovat) worked with contractor Beijing Urban Construction Group (BUCG) to design the 6. 28 m diameter, 1200 kW earth pressure balance (EPB) TBM for the difficult section. The excavation took 746 days, and BUCG staff nicknamed the TBM 'Diamond' due to the long life of its ripper teeth, which took care of the boulders.

Get a grip

Herrenknecht has supplied a 5.2 m diameter Gripper TBM to a pumped storage power project in Limmern, Switzerland. The challenging project involves the excavation of two 1.03 m diameter pressure tunnels at an angle of 40°, to achieve a 600 m change in altitude.

Breakthrough of the first of the two tunnels which will connect the pump and turbine hall at an altitude of 1700 m to the top cavern on Lake Muttsee was achieved in October, and work has now begun on the second tunnel.

The contracting consortium for the scheme is ARGE Kraftwerk Limmern, which is led by Marti Tunnelbau, and it was this company that placed the order for the 2205 kW Gripper TBM in September 2009.

A key concern due to the steep gradient is ensuring the TBM does not slip back down the tunnel it has dug. In order to ensure this, Herrenknecht and Marti devised a double anti-reversing lock, which means that at any given time, the 130 m long, 800 tonne TBM is braced against the tunnel walls with at least two out of three gripper systems. It does this regardless of whether it is advancing, at a standstill or re-gripping.

A mechanical failsafe means that even if there is a power and/or hydraulic failure, the TBM will stay locked in place, thanks to an automatic wedging system.

Following assembly and testing at the Herrenknecht plant in June 2010, the TBM was shipped to site and began excavation work in November 2010. It broke through the first tunnel after some 11 months of digging, and started on the second drive in March this year. The power plant is due to be commissioned in 2015.

New support

The new 150 m long, twin-track Torrebaso tunnel, which is currently being constructed on the Bilbao to San Sebastian railway line, will allow trains to pass through Amorebieta, in the Basque region, non-stop in both directions.

The poor ground conditions prompted contractor Geotunnel to adopt Atlas Copco's Symmetrix pipe roofing system - an easy choice as it was already using a Boomer L2 C drill rig for the drill and blast work and could use this same machine for the stabilisation work. The method involves installing steel pipes or casings in an umbrella pattern in the crown of the tunnel, providing support ahead of drilling and blasting.

The Symmetrix system has been used in similar ground conditions around the world, and in Spain it is one of Geotunnel's specialities. Borja Del Palacio of Geotunnel said, "We have installed hundreds of meters of pipe roofing around the country and I have to say Symmetrix is my favourite method. It is fast, reliable and economical and the quality of the bits and tubes we get from Atlas Copco is very good. Also, as we can use the same rig for pipe roofing as for drill and blast, it is not necessary to sub-contract this work to a specialised pipe roofing company."

Andoni Bonaechea, the company's technical office and purchasing manager, who decided to adopt the method at this site, added, "Pipe roofing using the Symmetrix system is by far the fastest and most economical way of stabilising unfavourable rock conditions in tunnelling and we have proved it at Torrebaso."

It is estimated that more than 5200 m3 of rock and soil will be excavated here and that the necessary stabilisation and reinforcement work will require the installation of 1824 m of steel pipes and 35 tonnes of cement.

For pre-support before each advance, the Boomer has installed five umbrellas with an average of 30 pipes per umbrella, each 12 m long. A starter pipe fitted with the standard Symmetrix ring bit set is used followed by extension pipes - in this case, 89 mm pipes with a 7.1 mm wall thickness using the Symmetrix
P89 system.

The pipes are installed at an angle of 4° and spaced 300 -
500 mm apart. Once installed, they are grouted in place to form a canopy around the upper part of the tunnel profile.
Net drilling time for a 12 m pipe averages 10 minutes and the total time for installation is 30 to 40 minutes per pipe, depending on the condition of the rock and other operational factors.

Excavation workhorse

Potain has supplied an MD 560 B top-slewing tower crane to construction of the 6.4 km, £ 635 million (US$ 995 million) Lee Tunnel in the UK. The scheme is designed to prevent the River Lee, a tributary of the River Thames, from being inundated with sewage in heavy rain. It is one of three major projects utility company Thames Water is undertaking to improve water quality in the river.

Construction began in September 2010, and tunnelling is expected to finish by the end of 2013, with final project completion in 2015. The contractor for the scheme is the MVB consortium, which comprises Morgan Sindall, Vinci Construction Grands Projects and Bachy Soletanche.

The tower crane is the first of its kind in the UK, and is being used to help excavate four 80 m deep shafts, from which tunnels will be driven. Two excavators at the bottom of the shaft load material into a 20 tonne capacity skip, which has a gross weight of 26 tonnes when full. It is this that the MD 560 B hoists to the surface.

Bernard Chatelet, plant manager for MVB, said there were two reasons for choosing the MD 560 B. "First, it has great lifting capability, so we knew the MD560 B would be able to handle the loads we need to excavate," he said. "Second, it has a powerful and fast winch. We need to complete the shafts before we can begin work on the tunnel itself, so it's important to have a productive winch that will help us complete this part of the work on schedule."

Once the six-month excavation phase is complete, the MD 560 B will be used for concreting and other duties.

These projects show not only how efficiency is being improved in the tunnelling sector, but how it often comes about through co-operation between contractors and equipment suppliers at an early stage.