1. Fortifications
2. Closure or temporary obstructions
3. Explosion by mines or charges
4. Flooding
a. Temporary
b. Permanent
After reviewing the situation in great detail, and from every aspect, the committee suggested a long list of precautionary measures that, it said, it would be necessary to use, singly or in combination, to protect and seal off the tunnel against any enemy attempts to invade England directly through the tunnel or by seizing the English end from the outside and using it as a bridgehead for invasion. The list included these recommendations:
The mouth of the tunnel should be protected by "a portcullis or other defensible barrier."
A trap bridge should be set in connection with this portcullis.
Means should be provided for closing off the ventilation, and for "discharging irrespirable gases or vapors into the tunnel."
Arrangements should be made for rapidly discharging loads of shingle into the land portion of the tunnel, shutting it off.
The land portion of the tunnel should be thoroughly mined with explosives capable of being fired by remote control exercised not only from within the central fort at Dover but also from more distant points inland, so that even if the protective fortress fell to the enemy, the tunnel still could be permanently destroyed.
In addition, a truck loaded with explosives and equipped with a time fuse should be kept ready by the entrance, so that it could be sent coasting down into the tunnel for some distance, there to explode automatically.
Arrangements should be made for temporarily flooding the tunnel by means of culverts operated by sluice valves. ("If by chance the sluice valves should not act, Measure XVIII could be resorted to, or the tunnel could be blocked by one or more of the means ... mentioned in Measures VIII, IX, X, XI, and XII.")
The tunnel should emerge inland, out of firing range from the sea. And it was imperative that it emerge under the guns and "in the immediate vicinity of a first-class fortress, in the modern acceptation of the term, a fortress which could only be reduced after a protracted siege both by land and sea."