28. BRIDGE, TUBE OR TUNNEL?
Geological risk or engineering risk? This question pretty well' sums up the choice that is available when considering what form a permanent crossing of the English Channel might take. On the one hand, there is the bored tube; whether it can be made or not depends on the continuity of a chalk stratum from one side of the Channel to the other. On the other hand, there is the immersed tube and the bridge. There is also a fourth scheme that has been mooted - a dam, but this is both an engineering and a geological risk and does not enter into any immediate arguments as to what route shall be built within the next few years.
What do these several routes offer? The tube and the tunnel are alike in this respect. At today's standards, both of them can provide only rail facilities. Under no reasonable conditions is it practicable to provide the degree of ventilation that would allow road traffic through either of them. For those who do not accept the proposition that road traffic can be conveniently put on board train and ferried under the Channel, there is the bridge, or the dam. If the bridge can be built at all, there is no reason why it should not carry both rail and road traffic. If one did opt for the dam, then there could be both road and rail routes and a tidal power project thrown in for good measure.
Apart from the construction problems, the bridge presents a navigational problem for shipping. At this moment, navigational aids seem far too crude for it to be safe to move the volume of shipping through the restricted channels that would be available between the piers of the bridge. This does in fact appear to be the unanswerable objection to building the bridge.
Yet another prospect remains to be considered and that is a combination of bridge and tube or tunnel. The sponsors of this proposal suggest that a bridge would be used for the greater part of the route but that a tube or tunnel should be provided below the main shipping routes. Such a route might provide both rail and road facilities, for the length of tube or tunnel could conceivably be sufficiently short for it to be ventilated adequately.
Ignoring the geological risk for the moment, the bored tube is by far the simplest proposition. If it could be guaranteed that there were no faults in the chalk bed through which it is proposed to drive the tunnel, the engineering problems would be negligible compared to those facing the sponsors of the immersed tube or the bridge. The difficulty is that, not until the tunnel has been driven throughout its length, can it be certain that an insuperable difficulty will not be met, though the risk can be narrowed down by first driving a relatively small pilot tunnel along the route at a correspondingly smaller cost.
There is no similar geological risk with the immersed tube or the bridge. In both cases boreholes are sufficient to provide all the evidence that is necessary to estimate the engineering requirements of the structure.
In the case of both the bridge and the immersed tube it is the magnitude of the project that raises doubts about feasibility. In the case of the bored tube, it is only the length of the bore below water that is longer than has ever been attempted before. A number of immersed tubes for traffic have been built and the sponsors of the Channel tube argue that the step can be taken from what has been done to date to what will be required in the Channel.
(from Engineering, 18th May, 1962)