It might be thought that driving a tunnel over 14m in diameter would give you a margin for error in proportion to its size. And that a tunnel boring machine the size of a small block of flats would handle like the civil engineering equivalent of a supertanker.
Not so on the new bore for the Elbe tunnel under Hamburg harbour in northern Germany. Machine control for the project is
finer than on most smaller tunnels. And tolerances are measured in fractions of millimetres, even though the 5.25m long by 2m wide tunnel segments are three times bigger than usual.
The 14.2m diameter Herrenknecht TBM now at work is claimed to be the world's largest soft ground machine, just outdoing the 14.17m diameter machine used in Japan to drive the Trans Tokyo Bay tunnel.
The Hamburg machine will drive a 13.75m tunnel with an internal diameter of 12.35m which will provide for a twin track road, an additional hard shoulder and two small emergency pedestrian ways. The existing tunnel has suffered from lack of emergency space and the extra cost was deemed worthwhile. The tunnel accounts for DM500M (£167M) of the total £250M which includes ramps and finishing works.
This price will increase to £585M with interest charges over a 15 year period. Funding is by a private bank loan to the German government.
The Elbe drive is not quite so long as Tokyo but long enough; the 2.6km tunnel increases to 4.4km with open cut and ramp sections. It will run alongside and supplement the six traffic lanes of the existing three celled immersed tube tunnel completed in the 1970s and long notorious for traffic jams. The two extra lanes allow better traffic control and more flexibility, before the road merges back into the six lane autobahn at either end of the tunnel.
Immersed tube tunnel construction for this project was ruled out by the Hamburg harbour authority, which foresaw disruption to harbour operations in one of Europe's biggest ports. An immersed tube would also have needed greater space to dredge a new trench, increasing the 70m maximum distance from the old tunnel. So a soft ground tunnel was called for, through the soft and mixed ground of the river estuary.
Precision is vital because, while the machine is huge, the depth at which it operates is not. When the main drive across the harbour begins this month, the TBM will be as little as 7m beneath the seabed in places, little enough cover in good ground but unheard of in bad. And Hamburg's ground is not good.
Greater cover is not possible because of the 3% gradient limit on the road path and also a length limit because it has to tie back into the existing route at either end.
The work is being carried out by a consortium of Germany's biggest firms - Hochtief, Philip Holzmann, Wayss & Freitag, Bilfinger & Berger, Dyckerhoff & Widmann, Zblin, and Heitmann. It covers construction of the tunnel and the approaches, plus complex work to extend the old box tunnel by 160m on the north side.
The new tunnel will be built through a succession of cohesive and non- cohesive soils including sand, silts, clays and marls. In the latter particularly, the team is expecting to find large boulders, though the first part of the drive, some 400m through sandy ground on the river bank has been free of obstacles. Later on there is an area of hard impermeable clay which should be easier.
Such conditions, adds Rolf Berger, project manager for the consortium building the project, would normally mean an automatic blow-out 'for example if you attempted to use compressed air for excavation, or for pressurising the machine head'.
The TBM therefore has been developed with very fine control. It uses a combined bentonite, compressed air head, the bubble of compressed air trapped behind a bulkhead in a secondary chamber linked to the face. No air actually sits in the face chamber. But both buffering and rapid adjustment of the face pressure is possible.
'We have 12 pressure detectors inside the bentonite space on the machine. The machine itself is 14.2m high so the pressure differential is quite high,' says Berger. At the base of the machine, pressures of around 50t/m2 are faced.
For the main drive under the harbour, data will be fed in from two piled detector points in the harbour measuring the level of the tide - a factor which has a more or less instantaneous effect on groundwater pressure. These inputs are connected to the computers on the machine to allow real time adjustment.
'Theoretically,' continues Berger, 'the machine face pressure needs to be constantly altered to keep inside a half bar differential with the ground as a maximum all the time. But we shall see.'
The sophistication of the machine is apparent once on board. The control cabin is a mass of computers and screens, which though now standard on modern machines provide a greater range of facilities. Explains shift controller Neumann: 'We have several monitors here, on each of which we can select various screens to display information.
'The trick is to learn what data is significant so that in an emergency we can act fast.' Compared to 'sitting up front over the pumps and hearing all the machinery and feeling it', the largely visual input of data is better and faster, he says. The machine had driven an initial 400m approach through mainly sandy soil when NCE was on site, and although progress was good, at some 10m per day, there had been a testing incident.
'We had a flexible hose blowout, and the operator was able to call up the screens and identify it, and in half a minute close all the necessary valves. It would have been a bigger problem before,' Neumann explains.
Herrenknecht's own laser targeting system is displayed on one screen while on another a radar-like coloured image runs through a sequence. This is a real time sonic detector display being used for the first time. Produced by Swiss firm Amberg Measuring Technique, it uses pulse generators and detectors in the machine head to scan forwards and pick up obstacles. In the section under the river, boulders up to several metres across, and possibly metal debris, could be met.
Boulders are taken in through the head and crushed by a pulverising unit at the centre, says Neumann. The head also contains 39 advance drilling points for detailed probing and grout injection to stabilise sand lenses.
During NCE's visit, the scan was showing even ground followed by a harder area at about 440m. This is a specially prepared 'railway station', a 10m thick section of overlapping piles where the machine was due to stop for head maintenance.
The giant cutting wheel has five hollow box spokes holding cutter wheels and picks which can be changed from the inside which is at atmospheric pressure. But subsidiary spoke tools can only be changed from in front.
'In the station the cutter head can be retracted by 800mm,' explains Martin Heimerdinger, spokesman for the consortium. The 6m or so thickness of treated ground ahead holds back water pressure, he says, allowing tools to be inspected.
On the harbour drive there will be another 'railway station' stop in the hard clay area. Here the completely impermeable material will allow inspection again.
Interestingly on this job, if there are problems, the cost will be shared by client, the German government and the contractors' consortium. The contract form is one of the first in Germany using a risk sharing partnership.
'We made a list of problems that could occur and there is an agreed price for them,' explains Neumann. Problems range from obstacles to tool replacement. If nothing happens both sides share the benefit, he says.
Unforeseeable risk is covered by an insurance policy contracted by the client and as usual the contractor is responsible for ground conditions as long as they are within the boundaries of soil investigation information provided.
So far the result is benefits not burdens. The ground has allowed a six week advance on schedule with up to five rings daily being placed, instead of the anticipated three. This translates as 10m a day since the rings are some of the biggest ever made.