When an engineer who’s been in the tunnelling game for nearly 40 years, having cut his teeth on London Underground’s Victoria Line, the London Ring Main, the Channel Tunnel and the Jubilee Line to name but a few, describes his current project as one of the toughest he’s ever worked on, you know it’s been hard graft. The engineer in question is Peter South, tunnelling contracts director for UK contractor AMEC. The project is London’s 2.5km long Docklands Light Railway extension that will further the DLR from its current terminus at the King George V Dock via twin 1.8km, 5.3m i.d tunnels under the River Thames to Woolwich Arsenal.
The twin tunnel drives start from a tight 30m long, 15m deep diaphragm wall launch box on the northern side of the Thames, and pass immediately on a 300m radius curve through a short stretch of alluvium into terraced gravels before descending into London’s upper chalk at a depth of 14 – 20m. They then head down through, and remain in, the chalk to a depth of 35m for the majority of the drives. On the southern side of the river, at roughly the 1.3km point, the tunnels climb back to surface passing through a thick band of Thanet Sands before breaking into the TBM reception chamber.
With this varied geology to contend with, and in particular the fissured upper chalk with its known troublesome flint inclusions and high 3.5 bar pressure water content, Amec opted to use a new 6m diameter Lovat EPBM to bore the tunnels and line them with a 1.5m long x 250mm thick, eight piece trapezoidal ring of fibre reinforced concrete segments and to cast those rings on site.
The EPBM was launched on the first drive in May last year and successfully broke through into the reception chamber some 30 weeks later in December. It was then disassembled and transported back across the river in just nine weeks and re-assembled in the launch shaft to attack the second bore. The EPBM had just gone to face as T&TI went to press.
Sounds straightforward? Well not really, as according to South, driving tunnel 1 produced a lessons learned list “as long as your arm!”
Launching tube 1
The tight confines of the launch box were to pose Amec its first serious headache. With just 30m to launch the 6m diameter EPBM from a box absolutely rammed full of equipment, the issue of mucking out the initial stages of the drive, bedding in the new equipment and adding backup equipment when space was available proved time consuming. It was simply impossible to install a conveyor system during the launch and the only alternative was to remove the muck a skip at a time until the machine’s back-up was buried and the conveyor could be added outside the tunnel within the box.
“It was slow at the beginning,” said South, “it’s 80 tonnes of muck per ring, and we had three 8 tonne skips to muck out with. Each skip allowed us just 150mm of advance before we had to take it out and put another one in. This went on for 100m and obviously got harder the further in we went. With machine teething problems adding to the situation it took four weeks to bore those first 100m adding gantries, as and when they fitted, followed by another two weeks to install the conveyor system. Given the space we had initially it’s difficult to see how it could have been launched any other way.”
With the tough start up completed and the backup and conveyor installed it was time to get up to speed, but not before the team had to pass within 3m of pile bases supporting some new residential buildings right above the alignment. In preparation Amec pre-consolidated the ground between the proposed bore and the piles before the machine passed under the houses. Amec spent some US$592,400 on permeation grouting, and left the steel tube-a-manchettes in the ground to allow for any necessary follow up compensation grouting. After the machine had cleared the area South confirmed that only a small amount of compensation grouting was carried out to lift the ground: “There was predicted damage and there were some internal repairs needed to minor cracks within the houses, but nothing major.”
Now it was time to head down into the upper chalk. With a grout block already installed for a planned head intervention at ring 600 and planned interventions scheduled at rings 300 and 900, where the geological predictions looked good for face entry, Amec felt ready for the drive.
But when the torque started to climb at ring 247, some 370m in, the problems seemed to have come sooner than anybody would have thought.
Tough intervention
The rising torque made Amec sure that the machine had suffered damage, so an intervention was planned as soon as possible, as South explained: “We knew we were damaging the head. It must have been a problem with the flints, most likely from the Bullhead Beds overlying the chalk, which caused it. The torques kept going up so we tried to get into the face, but we just couldn’t. There was too much water, far more than we ever believed possible from strata indicated by our borehole data. The fissures must have been a lot wider.”
The problem was exacerbated by the close proximity of the River Thames, the machine had gone down very quickly due to the alignment and was already under 2.5 – 3 bar of pressure. The Lovat was fitted with an airlock, but Amec felt it unwise to use it as the compressed air would have probably blown straight out through the fissures causing damage outwith the workings. So the team grouted hard behind them, and around the machine using a special gel, to try and control the severe water ingress, but to no avail.
Three times, roughly every 15m, the teams tried to access the head, whilst all the time the torque continued to rise. Eventually, and thankfully, at ring 288 a spot was found where access was just possible.
“We got in, after a great deal of hard work and a great deal of grouting. We had to really flush hard and tight against our brushes to get all the water out behind us. We found the damage – we had no cutters left worth having,” remembered South. “We pulled the head back 150mm and then broke out past the spokes and put up a 1.5m long timber heading across the top half of the face. There was water pouring in so we had to put up polythene over the top and use pumps. We’d lost about 75mm of metal off the extremity of the head. The outer edges were just gone. We had to re-build that up and then refit all of the outside sweepers, which was quite worrying. We employed welders around the clock who did a good job for us, and we pumped and pumped! The lads worked so very hard in very wet conditions.”
The stoppage cost three weeks to the programme whilst the team re-welded the cutterhead. The cutters in the outer edges were replaced with new ones featuring a revised design with more tungsten carbide protection, with normal cutters set in the centre.
Now equipped with a virtually new cutterface, the team set off to the next intervention point within the grout block at ring 600.
Ring 600 onwards
Progress to ring 600, under the Thames, proved far more efficient. The new cutters were obviously performing well and rates of advance of 9 and 10 rings per shift were achieved. The grout block at ring 600 was reached and access to the head was again attempted but problems due to high water inflows were to happen again. “It was very difficult to get in, in spite of the grout block, but we just managed it. Obviously there are large fissures in that chalk that fed water around the machine. We grouted the annulus tight but the cold wet stuff still found its way in over the length of the open machine,” said South. “The open fissures combined with the 3.5 bar head threw a lot of water at us. The chalk close to us must have been heavily fractured or there must have been some major fissures around the machine.”
The head inspection showed that very little damage had occurred between ring 287 and ring 600 (the tunnel low point), but the crew changed the cutters anyway, and headed back up the 1:20 hill towards the final intervention point planned at ring 900. Again, the journey began smoothly, but at ring 692 the tail seals were letting in so much water that ring building and grouting proved virtually impossible. “We had no option other than to effect a seal repair there and then,” explained South. “We had to grout up tight against the leading edge of the ring to stop water flow, push on to create an open skin and weld in a fourth seal to the face side of the last seal position. Having built that seal and greased it, we then built a ring on it and pushed forward to a point that would expose the seal behind, had the ring in the skin not been over it. Stopping at that point we removed the ring in the skin, segment by segment, replacing the front original seal piece by piece, and replacing the segments as we progressed. Once completed, we had two new working seals which took us through the remaining length of tunnel with no further incident.” This operation took two weeks to complete and when the machine reached ring 900 water was again a serious problem for face entry.
“We got there and it was as wet as hell, we just got in,” said South. “The water poured back and down towards the pumps installed at the tunnel low point. At the sump point with all pumps running there was 900mm of water in the tunnel invert. At this time however we were well up the hill, and with a man stationed at the low point watching the situation in constant touch with the face, we were never in danger. If all else failed all we had to do was shut the face door to recover the situation.”
At ring 900 everything was fine with the cutterface, but the crew changed the cutters again as the Lovat was about to enter the Thanet Sands for the last 300m and this was the last place it could be done.
“We were worried about wear in the Thanets, that’s why we put the air lock on the machine. Thanet sand is very hard wearing, and susceptible to liquefaction becoming very fine, like quicksand. So you keep the earth pressure balance high and the water away. It’s very impermeable so once there’s water in it, you can’t get rid of it. But we never had to use the air, it was immaculate, we went through the sand keeping good EPB using soap as a ground conditioner. The worst settlement we had within the Thanets was 7mm. We haven’t had to touch the buildings above, even though we had a compensation shaft complete with grouting arrays ready,” explained South.
The Lovat successfully broke through into the reception chamber in December 2006, completing a tough drive, even with the problems, that saw a best output of 14 x 1.5m rings in 12 hours and a best week of 87 x 1.5 rings (135m).
Grouting problems
Another problem on the first drive was with the grouting system through the tail. South believes this to be design related: “You have the grout tube that goes through the ring behind the brush. Within this grout tube is another one for the accelerator. This inner tube was retractable and had a rubber end to stop the accelerator coming out. What we found was there was an intrinsic pressure within the inner tube that remained longer than it should have. When you retract the tube, it still dripped accelerator into the main tube, so you were sending the end of it off. We went through all sorts of modifications. The thing you really need to do with grout tubes is pig them out with a sponge ball all the time, but you couldn’t here because of the inner tube. So to clean it out we thought we’d use a high-pressure water jet, but with it being under such high pressure from outside, as soon as you finished cleaning it all the gravels from outside were forced into the tube blocking it again. We just couldn’t get it to work in the environment we were in. So we went back to grouting through the ring. We’ve got a diamond system down there now. The rings are superb.”
Second tube
With the machine now heading into the second drive, South is confident that the lessons learned on the first drive will dramatically increase production on the second. The improvements made to the cutters, tail brushes and grouting system should stand the team in good stead. Also the fact that drive can now start with the extendable conveyor right to the face will speed up the initial 100m considerably. Interventions will still be carried out at rings 300, 600, and 900 but it is still anticipated that water could be an access problem.
“Hopefully we can make it through in about 16 weeks including interventions and full conveyor installation. That will bring us out on programme. This time we’re hoping for 16 x 1.5m rings in 12 hours. The potential is for 2 an hour, if the machine behaves itself. It would be nice to get,” said South. “But it’s the lads that are the thing. That’s what counts and how they are prepared to stick at it, and I respect them enormously. If it wasn’t for them it would have been a lot more difficult.”
With the lessons learned and the undoubted positive spirit among the contract team, it seems it would have to be something pretty un-expected to prevent the Lovat from breaking through on the second drive in June.
The tight launch from the box Water caused severe problems to the interventions Conveyor, grout lines and walkways were lifted to keep the rails clear Breakthrough of drive 1 Fig 1 – Map of the project
