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Boring Under the Channel: One Perspective on the Experience

Richard Robbins

T he Channel Tmmel project was probably the most visible tun nel job ever to be built. Other

tunnels have been as long and as diffi- cult (the Seikan Tunnel in J apan and the Storebeelt in Der~nark), but the Channel Tunnel has been a major Eu- ropean infrastructure plan for over a hundred years. I t was star ted at least four times and was beth a major source of embarrassment and a diplomatic t r iumph for the governments of Great Britain and France. ~ne whole world watched, with considerable skepticism, when the project was appreved for con- struction and the work began seven years ago.

However, this was to be the s tar t that saw an end to the project. That the job was finally carried to completion is more of an achievement in interna- tional financing than in engineering. Nevertheless, it was a major engineer- ing achievement thatwill be referenced and will be a source of learning for decades to come.

Lessons from the Channel Tunnel

I r a similar mega-preject were to be planned and built today, practically everyone who was involved in the Channel T-nnel would have opinions on what should be done differently. Still, one must conclude tha t in all the major categories, the r ight decisions were made, the right p]AnR and orga- nization were established, and the right people were breught in to do the job. I t was a difficult job by any mea- sure, and it was accomplished in a very professional way with a high stan- dard of engineering.

Present address: Richald J. Robb/ns, Vice Chairman, The Robbins Company, Puget Sound Plaza Bldg., 1325 Fourth Ave., Suite 1930, Seattle, WA 98101-2509, U.S,S,.

The management of the contracting groups on beth the French and British sides was willing to take chances with cutting-edge technology beyond the demonstrated state-of-the-art at the time, and had the patience to live with the time and expense needed to see through the "de-bugging" process. The payoff came in performance records that brought the construction program back to the original schedule. On the other hand, one may question whether the same people, i f they were to do the job again, would make the same rela- tively high-risk decisions.

Technical Decisions In 1974, The Robbins Company built

the service tunnel machine for the French-German joint venture, for a previous s tar t of the Channel Tunnel project. That machine, which was not put into operation because of an early cancellation of the job, was an open double-telescopic-shield machine that was designed to install cast-iren tun- nel segments.

The machine was dependent on grouting to seal the fractured chalk beds near the French shore. I t also was designed to incorporate very ef- fective probe drilling in a strict pat- tern of protection, well ahead of the tunnel face, in order to detect any zones of high water inflow. Grout drills could be positioned to completely solidify a zone ahead of the tunnel face if the probing indicated tha t this was required.

When the machines for the French side of the tunnel were under develop- ment for the recent tunnel s ta r t in 1986-87, it was feared tha t the middle and lower chalk beds could not be grouted effectively, and that therefore it might be necessary to advance the headin~ as a closed pressure bulkhead shield, similar to a slurry or earth- pressure-balanco CEPB) shield. This

necessitated a completely different ap- p roach- -and a much more sophisti- cated and complicated machine.

As it turned out on the job, the chalk formations could be grouted very effectively. Grouting was employed to solidify the face of the service tunnel several t imes in the early de-bugging stages ofthe tunnel. The two running tunnels on the French side (the main rail tunnels) were grouted in advance from the service tunnel to allow them to be bored in an unpressurized, open mode.

This leaves us with the question: Was it correct to specify a pressure bulkhead type of machine when the open type of shield machine, k la 1974, could have succeeded? I t would cer- tainly have reduced the cost, the startup time and the complexity of the ma- chines used under the French side of the Channel. On the other hand, work- ing from the information the contrac- tor had at the t ime of go-ahead, there was a danger that grouting might not be effective.

The French tunnel machines (see Fig. 1) were all built as pressure bulk- head shields. Theeretically, this meant that no probe drilling or grouting capa- bility would be required. The machines were designed to bore through any rock condition they encountered.

These machines were converted from a closed mode, with the cutterhead filled with sea water under pressure, to the open mode for low flow or dry con- ditions. This is accomplished by sim- ply openlng a gate and closing down the controlled muck discharge system.

In the case of the service tunnel, the muck discharge system was a pair of piston dischargers. The two runnln~ tunnel machines used double screw conveyors with a plug zone as the muck discharge control mechanisms. Re- vertln~ to a pressurized cutterhead was just as simple a procedure: close the gate and start the piston discharger.

Tunnelling and Underground Space Technology, Vol. 10, No. 1, pp. 23-25, 1995 Copyright ~ 1995 Eisevier Science Ltd Printed in Great Britain. All rights reserved 0886-7798/95 $9.50 + .00

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Figure 1. This 5.6-m Robbine TBM was used on the drive from the French side for the Channel Service Tunnel.

However, there is a catch to this system that must not be disregarded. If the machine is bering in wet faulted rock under pressure and a cutter fail- ure occurs, or if the cutterhead main bearing seal must be serviced, or if any other event requires human access in the cutterhead cavity, there are only two choices:

1. Put the cutterhead cavity under air pressure to displace the water; or ;

2. Bringit to atmospheric pressure. Thjis will require placing a grout plug around the machine.

There are many examples of severe difficulties in using air pressure at the cutterhead to keep the water at bay while workers are ahead of the pres- sure bulkhead. This may be the stan- dard emergency approach in urban slurry face tunneling or with an EPB, where the water pressure is not greater than approximately 3 atm.

At the ChAnnel T-nnel, the water pressure was as great as 10 atm for many of the difficult faulted zones. A t the Storebeelt Tunnel in Denmark, the pressure was up to 7 atm, which also put that tunnel out of the range of practical work under air pressure. Some workers can go under conditions

of"saturation diving preseures, ~ which would allow them to be under high pressure for long periods. However, the danger of a pressure blowout to the surface and sudden loss of pressure in the working chamber make this alter- native generally too risky.

This leaves the grouting solution. To be effective, the grout plug around the machine, or at least the front part of the machine, must be so effective that it will allow the pressure in the cutterhead cavity to be dropped to atmospheric while the water inflow remains low enough to be pumped. This requires a very capable grout drilling setup because the pos i t ion ing of the grout drills within the shield must be caable of producing complete coverage in order to create the grout bubble surrounding the machine.

Following the above requirements, we have a tunneling maehine with the grouting capability that it would need to bore the job as an open shield, that is, the type of machine that was pro- duced for the Channel T-nnel in 1974. The 1987 pressure bulkhead version did not move the machine beyond the requirements of the past, but rather added the pressure bulkhead features to them.

For rock bering or mixed ground bering under high water pressures, a grouting capability from within the machine is essential. This was a key deficiency at the Storetnelt T-nne], and caused severe problems for that job.

Choice of Segments The chalk conditions on the British

and French sides of the Cb~nnel were so different t h a t the contractor , Transmanche Link (TML), chose com- pletely different types of segments for the two sides of the tunnel. The ex- panded segments used on the British side proved to be appropriate for the job, especially in the dry sections, and they were very quick to install.

On the French side, sealed, bolted segments were definitely required, es- pecially for crossing the fractured zones and faults, which occurred out to more than 7 km offshore.

The problem with the expanded seg- ment system used on the British side was that a substantial distance, par- ticularly in the first few kilometers of the t-nnel , was not bored in the excel- lent ground conditions expected. In- stead, the drive encountered consider- able water inflow and some blocky

24 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 10, N u m b e r 1, 1995

rock sections. The water combined with the fines created mud tha t was plastered against thqv tunnel walls, necessitating hand cle~uaing before set- t ing of the segments. These problems could have been avoided or great ly reduced by the use of a segment sys- tem tha t was erected inside the skin of the tail shield.

The Bri t ish machines achieved maximum advance rates of 1,700 m per month. The expanded Hnln E used in good ground was ml important fac- tor in achieving this performance. On the other hand, the French machines achieved 1,200 m per month while in- stalling bolted and sealed segments. One lesson of this experience is that the extra security of a sealed llnlnE system in an undersea tunnel is worth careful consideration.

While the CbRnnel ~].~]nnel was near- ing completion, a tunneling machine at the Yindaruqin Proj,ect, in the Gansu province of China, was completing 1,300 m per month while installing hexago- nal segments. Hexagonal segments were ser iously considered for the French side of the CbAnne] l~mnel; however, on the advice of German con- tractors who had had serious difficul- ties with this system, the idea was abandoned.

The Chinese experience by CMC and SELI of Italy was in a mostly dry tlmne] where vurxsealed segments could be applied. These segments are in- stalled inside the protection of the shield tail and requi~ backfill grout. Hexagonal segments are an alterna- tive tha t should be considered for a similar tunnel projecL in the future. They offer the chance to use a simpli- fied shield machine without a tele- scopic joint, since bering can progress while segments are being erected.

Simplicity Versus Complexity All of the t -nnel ing machines used

on the Channel ~ m n e l project could have been build in much simpler con- figurations than the machines tha t were built. Their complexity arose from the requirement for high rates of advance.

I t is hard to argue with success. The machines did achieve monthly rates nearly double the original target rates. That was very fortunate, because they

had to catch up on the construction program which, after the first year of bering, was nearly six months behind schedule.

The late s tar t and long period of de- bugging were the result of the proto- type nature of the machines being used and their level of complexity. The financing and viability of the en- t ire job were jeopardized by the poor progress in the early stages of the job. At the beginning of any mega-project, s teady progress can be crucially im- portant. This factor may not have been seriously considered when the features of the custom-designed tun- neling machines were chosen.

Contracting and Procurement Policies

For many tunnel contractors and tunnel equipment suppliers, the Chan- nel ~ m n e l project has long been con- sidered the "ultimate job ~. The pres- t ige and in te rna t iona l references would be valuable for the companies tha t succeeded in building the tunnel. As a result, there was a very high level of competition, especially among prin- cipal suppliers.

At first glance, this may appear to be to the advantage of the contractor who is purchasing the equipment. On the other hand, it can introduce seri- ons risks, because of the need to be optimistic about the results of the con- tract in order to achieve the advantage of the prestige of the project. The purchaser of equipment is then pu t i n the difficult position ofevaluatlng pro- pesals that may be beth technically and commercially unrealistic.

I f the contractor were to select the supplier of custom equipment based on a competition demonstrating capabil- ity to perform the work, as would be the case in selecting a consulting engineer- ing firm, a different contracting form could be used for the design, manufac- ture, and supply of the machines after the supplier had been selected.

Such a selection method might re- sult in a more collaborative design, in which the contractor and the supplier establish a much closer technical part- nership in the development of the spe- cialized equipment. I t would certainly result in a much less risky position for the equipment supplier, who other-

wise is required to offer prototype equip- ment on a fixed price basis without knowing what the details of the final design will be.

A collaborative development pro- gram involving the contractor, the equipment supplier, and to some ex- tent the tunnel owner, probably would result in bet ter performance and a higher level of understanding of all of the technical and commercial risks in- volving all parties.

New Developments and Pretesting Prototypes

A case can be made tha t on any project of the size and scope of the CbRnnel T-nnel , specialized equip- merit should be tested off the job first, preferably on other projects on a full scale. I t is recognized tha t this is not always possible; nonetheless, for ma- jor projects, the tunnel owners should consider funding this type of develop- ment in a way that minimizes the risks when their tunnel job is approved to proceed. This problem could be ap- proached in several ways:

• First, by funding a study to iden- tify the major hazards to be faced that could have a serious effect on the cost and time to complete the job, especially hazards of geo- logic and safety nature.

• Second, by using a panel of ex- perts to identify what is state-of- the-ar t in dealing with these problems.

• Third, by using specialist engi- neers or suppliers to explore new advanced systems, beyond the state-of-the-art, which could be developed and tested before the s tar t of the project.

Such an integrated approach would have the advantage ofmM~ing it clear to all parties that they have the same understanding and agreement of the important hazards and how the risks should be covered or shared before the ~nA! plan for construction is developed.

The Channel rl~]nnel was indeed a classroom for large international con- struction projects. I t was a thrill to be associated with the designers and build- era of this epic job. We look forward to the operational phase and hope it can be as successful as the construction.

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Volume 10, Number 1, 1995 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 25