Caution has tempered the almost runaway speed of tunnelling work on Channel Tunnel Rail Link Contract 220, discovers Bernadette Redfern.
The incredible boast 'I did it 19 times with Annie in one night' emblazoned on the Tshirts of crew working on one of the tunnels now racing towards St Pancras station in London, refers to prowess erecting the tunnel lining. Honest.
Annie is one of two earth pressure balance tunnel boring machines (TBMs) being deployed by joint venture contractor Nishimatsu/Cementation Skanska on Channel Tunnel Rail Link Contract 220, a £147M, 7.5km tunnelled link from Stratford station to CTRL's St Pancras terminus. Although Annie and her sister, Bertha, started two months apart, in August and October last year, there has been intense competition between the two teams to make fastest progress. The record, held by a crew operating Annie, stands at 19 tunnel lining rings erected in a 12 hour shift - equivalent to 28.5m headway.
The two 8.15m excavated diameter tunnels are being pushed under densely developed central London 30m-50m below ground level, along a layer of Thanet Sands. This consistent, non-water bearing stratum makes good tunnelling ground, but following trials, settlement was found to be far greater than expected. Fearing ground collapse of the kind seen on Contract 240 at Lavender Street, Stratford, in February (NCE 13 February) the team has advanced cautiously.
Earth pressure balance TBMs support unstable ground as the cutting head advances using compressed air to create force equal to that previously exerted by excavated material. This reduces settlement, so minimising the risk of walls and crown collapsing. Pressured is contained by closing an air lock behind the cutting face, referred to as 'closed mode'. However, in predictable ground it is normally considered safe to operate the TBM without need for compressed air.
'Initially we were only going to use the machine in closed mode for 20% of the time but to minimise risk of collapse we decided to keep it closed along the whole length, ' says project director Terry McDonald.
'The decision is achieving remarkably low settlement above the tunnels, but at the cost of tremendous punishment to the TBMs, ' says McDonald.
More torque is needed to turn the cutter head when the working face is pressurised.
Working under constant pressure also accelerates wear on the machine's seals.
In all Annie and Bertha will excavate 750,000m 3of spoil.
Behind the cutting head a 1.1m diameter inclined screw conveyor transports spoil away from the face. To keep up with the rate of advance it was decided to run spoil out of the tunnels using conveyor belts rather than trains, which are more conventional but offer lower capacity. To bulk up the dry Thanet Sand so it can be moved in large volumes, foam and polymer lubricants are pumped into the spoil as it joins the conveyor belt.
Tunnel lining is of precast concrete segments, making up rings 1.5m long and with a 7.15m internal diameter. Lining rings are 350mm thick, made up of 10 pre-cast concrete elements, each weighing in at 3.5t. Elements manufactured in a casting yard next to Stratford station are joined by shear pins and bolted connections.
'It takes around 30 minutes to dig far enough for one ring, ' says TBM driver Phil Birch. When the machine has advanced 1.5m it stops boring and bentonite is pumped to the face of the machine to fill the 20mm gap between the cutting head and shield, says senior tunnel agent Keith Nicholson.
This prevents settlement of the newly excavated walls.
At the back of the cutter head the huge jacks that push against completed lining rings to propel the TBM forward are then retracted one by one. A pneumatic erector arm lifts and places segments, starting at the invert, building up the walls, and installing a key element in the crown last of all. As each element is lifted into position, men rush in and bolt it to its neighbours. Work is carried out to a tolerance of 5mm.
The precise task of placing the 10,000 elements needed in each tunnel is carried out semiautomatically by the TBM, with some help from the TBM driver, who sits in a control cabin on the upper of the machine's two decks.
Grout is injected to fill the 130mm annulus between the completed ring and excavated diameter.
Amazingly, the entire process from starting excavation to the final bolt being tightened into the key element and grouting takes less than an hour.
Tunnelling is recognised internationally as a high-risk activity so site procedures are followed and risk assessments carried out obsessively to ensure that, as stated in project literature, 'all employees go home at the end of their shift, not to a hospital bed'. But is it really the safest tunnelling job ever?
Health and safety manager Dan Weathers thinks so. 'We have compared accident records from everywhere - Japan, Canada, Taiwan - and this is the safest tunnelling project in the world, ' he says.
Despite making amazing progress - Annie is 5km into her drive and Bertha is at the 4km mark - there have been significant challenges to overcome. Delays on construction of the Stratford station box on Contract 230 held up assembly of the first TBM: concrete on the box floor was still being poured when the 120m long machine was arriving in parts on site. It was built into larger elements off site which were then transported for final connection insitu, ready for the drive, as the concrete cured.
Meanwhile, a recent encounter with some unexpected limestone has left Annie out of action for a week. After pushing on and breaking through the rock, the machine now needs a significant number of steel and tungsten discs and pics replaced.
However, this seems unlikely to cause many long term problems as the project is ahead of programme. It was originally expected to finish in February 2004 but now it looks like the team will all be at St Pancras for Christmas.