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Amec tunnellers at full stretch on DLR

SITE REPORT

Forget Crossrail, the latest tunnelling project to go ahead in east London is for the Docklands Light Railway, and is already under way.

Joanna Booth went to see Amec's team in action

ASK A LONDONER to visualise the Docklands Light Railway and the chances are they will picture its carriages atop the now almost iconic bridge system, the chunky concrete arches which stretch away from the City across east London.

But the next section to be extended will be disappearing into the ground rather than rising above it. New tunnels will connect the existing DLR station at London City Airport to the overland rail station at Woolwich Arsenal, running underneath the Thames.

After a public enquiry, a transport works act order was granted in 2004 and the 30-year concession for design, construction, finance and maintenance of the Woolwich extension went out to competitive tender.

Amec was already working on the City Airport extension through a similar PFI contract and, with its tunnelling, civils and rail expertise, felt well suited to the next stage. The contract was awarded in May 2005 to Woolwich Arsenal Rail Enterprises, a joint venture between Amec and the Royal Bank of Scotland.

Completion of construction is scheduled for the end of February 2009. Amec will receive no payment until the line is in service, so it needed to borrow £210 million. The Royal Bank of Scotland is providing £110 million and the remainder comes from the European Investment Bank in recognition of the urban regeneration benefits the transport link will provide.

'We will be responsible for supporting the project throughout its life, ' says Harvey Pownall, general manager of WARE. 'Amec Developments is involved in negotiating the compensation elements for compulsory purchases. Amec Spie, the specialist rail arm, is reviewing design elements to ensure the end product will be maintainable, and it will be responsible for carrying out maintenance up until 2018, when WARE will need to retender.'

But first Amec must build the line itself, under a £177 million construction contract. A 1,000-tonne crane lifted the huge tunnel-boring machine pieces into place in March, ready to start on the first of two 5.3 m-diameter tunnels. The site is now running for 24 hours a day, five days a week, with two days free for maintenance.

Peter South, Amec's tunnelling director, explains why the team chose to use an earth pressure balance machine for the tunnelling process: 'The other technical option would be a slurry machine. It mixes the excavated material with water or another pumping medium within the machine's cutter head prior to it being pumped to the surface. On this project we're working predominantly in chalk and Thanet sand, which is notorious for breaking down into very fine particles, so it would be immensely difficult to separate out afterwards. There are also fissures in the chalk through which slurry could be lost. We have previous experience in these conditions and an earth pressure machine works well.'

The 450-tonne, 9 m-long tunnel-boring machine has a revolving head, the metal face of which cuts the rock in front of it. Long chain polymers and foam are added to the spoil in the head to help it move more smoothly and to reduce wear of the parent metal in the head and cutters.

Face doors are provided to give support to the face during man entries for cutter inspection.

'During excavation, it is essential that the head is kept full, ' explains Mr South. 'We do this by watching the pressures within the head and at the base of the screw conveyor which draws the excavated material out onto the tunnel belt conveyor. At no time do we allow the pressures at the base of the screw during the excavation process to drop below a set level.'

The tunnel lining is made of concrete segments, cast in a factory on site by Buchan Concrete Solutions, an Amec company. Eight 2-tonne segments form a ring 1.5 m in length. A vacuum pad erector lifts the segments into position, where they are bolted into place. It will take on ly half an hour to excavate and install one ring once the system is fully installed. The outside cut diameter of the tunnel is 300 mm larger than the outside diameter of the ring. To prevent the void collapsing, grout is injected and accelerated with sodium silicate as the machine advances, giving the ground no time to move. Brush seals within the tail of the TBM wrap the back of the extruding ring and, with the assistance of wax-like grease, stop water from entering from behind the machine.

Hydraulic jacks thrust the machine forward, bracing off the previous concrete ring installed. The control cabin sits on the gantry behind the machine, from which the driver can control all aspects of its operation, including the speed and torque of the screw and the cutting head. The front end of the machine articulates and is steered by rams. The tail skin articulates itself on hydraulic rams that move to suit the curve being negotiated. Seals protect the tunnel from water ingress at each articulation.

A 90 m double-deck gantry containing all the necessary transformers, electric panels, hydraulic power packs, grouting equipment and ventilation is pulled beh ind the mach ine. Access to the tunnel wall is also provided for back-grouting and service installation.

The launch chamber - a massive excavation within diaphragm walls 16 m deep - was excavated and propped prior to the insertion of the TBM, which broke ground just after Easter.

After 100 m of tunnel construction and the full installation of the machine's gantries, the tunnel passes immediately below the piles of a two-storey house. Bentonite grout was used to effectively create a mild concrete within the ground above the tunnel and beneath the piles. 'The team pumped thousands of litres of grout into the ground to make it solid , ' Mr South says. 'There was a 35 per cent volume acceptance in the ground at this location, which was considerably more than we expected.

'We move downwards at a gradient of 5.5 per cent, moving quickly into the chalk. When we are 35 m down, the outside pressure will be 3.5 bar. That's 50 lbs per square inch - about half what a pneumatic drill works at.'

The team navigates by surveying with lasers and theodolites. The path will level out under the Thames then climb up the other side out of the chalk to dig the last 300 m in Thanet sand, a particularly tricky material.

'It's murderous stuff, ' Mr South says. 'It has extremely low permeable capabilities, so water contained won't move through it, making it very difficult to de-water. Yet, if you stir it or loosen it, it's like quicksand.'

Once out the other side, the TBM will be extracted and taken back to North Woolwich to do the journey all over again boring the second tunnel.

The spoil, once removed from the excavation chamber by the screw, is dropped on to a conveyor belt runn ing back to the su rface. Th is belt is at tached to the TBM, spooling out behind it as the machine moves further underground. At the surface, the spoil is transferred from the muck bin to barges in the King George V Dock. These transport it to a land reclamation site in Tilbury, keeping road traffic to a minimum. By the end of the job, 104,000 cum will have been removed.

Traditionally, belt weighers have been used to measure the excavated tonnage, so it can be compared with predicted amounts, but Mr South is unconvinced of their consistent accuracy.

'It is absolutely essential that we know whether we are overexcavating on a regular and consistent basis.

We must also know what the average face density is at any point so we can understand what the results mean in mixed and varying ground, ' he says. 'Technology is moving on and we must explore more consistently accurate systems.'

So, in addition to using a belt-weigher, the team will also trial an isotope density measure. This takes its reading from the changes in received radioactivity from the source dependent on the density of material that passes it.

Supplies into the tunnel will be by train, driven by a single, diesel-powered 30-tonne locomotive. The train is geared to be self-braking if it overspeeds down a slope. At a defined speed it will automatically let itself down onto the rails where the weight will brake the rolling stock. Even though that theoretically cuts out any chance of something running into the TBM, the team has still mounted a buffer at its back to protect the face workforce.

Bogies built specifically for the project will be used to ferry segments and purpose-made flat wagons will transport rail, pipes and cable to the face, also converting to man riders for labour transfer at shift changes. All workers are directly employed and most have worked for Amec on previous jobs. Each shift sees 14 workers on the machine underground and six co-ordinating from the surface.

The TBM, named Carla, after the site receptionist, has completed ring 33. It is 50 m into its journey.

'We're pretty much on programme, ' Mr South says.

'Things always go slowly at the beginning because we're installing equipment as we go along. We've got gantries galore still to go in.'

Mr South has spent 37 years in the tunnelling sector, working on major projects like the Victoria and Jubilee lines for London Underground, the Channel Tunnel, the Newcastle Metro, Cairo waste water tunnel and many UK sewerage contracts. But he still would not describe himself as a tunnelling expert.

'You can't be an expert, ' he says. 'Knowledgeable is as far as you get. Every job offers something new.'

Access from above

THE GAP between the two tunnels becomes wider at one point to accommodate a 45 m-deep, 15 m-diameter access shaft that will provide emergency escape and ventilation.

The shaft will be constructed as a wet caisson sunk from the surface, using segmental concrete linings with an outside in situ concrete wrap. The shaft will be jacked down using 12 hydraulic jacks mounted on an outside concrete collar.

The cutting edge has point attack cutters to overcome the f lints expected at depth in the chalk. Once at depth the chalk is predicted to tighten and water flows should decrease.

At this point the water will be removed and excavation will continue as a dry caisson down to the base, where the excavat ion will be enlarged and an underreamed shaft plug will be formed.

Th is process is rendered even more complex by the fact that, at the top, the shaft cuts through existing anchors to the river wall, and the Environment Agency has stipulated that the wall must be re-anchored before any major civils works take place.

To get around this problem, the collar of the shaft was cast in two sections. The inner section was cast first and anchors were attached. Piling then took place around the shaft, then finally the outer section of the collar was cast. 'It was like knitting, ' Mr South says.

Cross and ventilation passages will be formed from the shaft base using shotcrete temporary and permanent lining after ground treatment has closed any open fissures in their path.

Woolwich starts at the top

THE NEW terminus for the DLR trains will join up with the Network Rail station at Woolwich and will be extensive enough to cater for three-car, 90 m-long trains, with platforms 6 m below ground level. Some shops and businesses in the town centre have needed to relocate and the site is adjacent to a BT exchange centre.

'It's a very tight area and we have to be careful not to vibrate their equipment or cover it in dust, ' Mr Pownall says. 'From the back of the piles for the station to the BT building there is only about half a metre.'

Demolition works have already started, and are being managed incredibly carefully so as not to affect either the BT building or the open market also next door.

The Amec team decided to build the new station by top-down construction.

'It requires less space and is less disrupt ive to local businesses than having a massive excavation, ' Mr Pownall says.

There will be extensive piling works within the Network Rail station and the it will be subject to a series of weekend possessions to allow this to happen. The whole works will be complete by early 2009.