Engineers and Mechanics Encyclopedia 1839: Railways: Tunnels

The adoption of tunnels in lines of railway has been the subject of much discussion; for the most part apparently arising from individuals who are interested in the execution of certain lines of railway, in which tunnels are excluded.

Many timid and ignorant persons have thus been frightened into the apprehension of suffocation from the noxious state of the air, caused by the decomposition of the fuel in the locomotive engines. In order to show to what extent the air in a tunnel is thus contaminated, Mr. Gibbs, in his report already alluded to, observes

"Let us suppose a tunnel one mile in length, to be traversed by a locomotive engine and its train, of a gross weight of 100 tons. The experience of the Liverpool and Manchester Railway has shown that the average consumption of coke is considerably less than half a pound per ton for each mile it is carried on a railway; but taking the consumption at half a pound, the whole weight of 100 tons will require the consumption of 50 lbs. of coke. It may be calculated that every ten pounds of coke will evaporate a cubic foot of water; so that the whole 50 lbs. will convert into steam five cubic feet of water in the distance of one mile. Now, to convert into steam one cubic foot of water, requires 1,950, or say 2,000 cubic feet of air; then five feet of water will of course require 10,000 feet, and this will be the whole amount of contaminated air in one mile of tunnel.

To determine the proportion of such an amount of foul air, and the whole of the air contained in the tunnel, we may take, for example, a moderate sized tunnel, 30 feet high, having an area of 800 square feet. One mile in length of such a tunnel, will contain 4,224,000 cubic feet; hence the contaminated air will bear to the whole quantity in the tunnel, the ratio of 10,000 to 4,224,000, or it will be as 1 to 422.

It will scarcely after this, appear an valid objection to tunnels, to assert that an injurious effect must result from the contaminated air, when we find that the quantity of this description of air produced by the passing of the whole train will be no more than one part in 422 of the whole quantity in the tunnel."

On the preceding page is represented a longitudinal section of a tunnel, (supposed to be cut through marl, shale, or clay strata,) showing the proportion which an engine with its tender and train of carriages bears to the size of a tunnel. A transverse section of the same tunnel is given in the subjoined figure; in which the irregular diagonal lines are intended to represent the form of the strata in marl, shale, or plastic clay. In making tunnels through strata of this nature, it is an important consideration that no springs will encroach; on the other hand, inverted arches become necessary, as shown in the figure.

In the formation of tunnels through chalk rocks, the expense is less than through any other material; the excellent stone of which it is usually composed renders artificial side walls unnecessary, while the material will, in some case, exceed the cost of excavation. "As to the expediency of adopting tunnels at all," Mr. Gibbs observes, "it is certain that this ought not to be admitted until after much consideration and investigation, with a view if possible to avoid them; yet when, by the introduction of a tunnel a positive good might be effected, such as a shortening of the line, the means of penetrating a difficult summit, or of reaching a country which might otherwise be shut out from the advantages offered by railway, it might possibly be great injustice, alike to the shareholders in. the undertaking, and the surrounding district, to adhere too rigidly to the determination of excluding tunnels."

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