7. The chief engineer now finds himself engaged in a new struggle with man in addition to nature. In many instances the contractors let out a portion of the work they have engaged to perform to sub-contractors, who again “set” the earth-work to a body of “navvies,” who again among themselves sub-divide it among the three branches of which their State is composed, namely, “excavators,” “trenchers,” and “runners,” each party of whom appoint their own “ganger.”
The duty of effectually overlooking all these details, of preventing collusion as well as collision, of enforcing the due execution of the contract, and yet, where necessary, occasionally to alleviate the strict letter of its law, constitute perhaps the most harassing of the various difficulties which the chief engineer has to overcome: for it must be evident that if, by means of bribery, or from inattention, or from sheer roguery, any important portion of the work be “scamped,” or insufficiently performed, results may ere long occur of the most serious description.
Tunnels.
8. The brief history of the construction of the Kilsby Tunnel of the London and North-Western Railway very strikingly demonstrates the latent difficulties which occasionally evade the investigations, baffle the calculations, and which, by chastening as well as by humbling, eventually elevate the mind of every man of science who has practically to contend with the hidden secrets of the crust of the earth which we inhabit.
The proposed tunnel was to be driven about 160 feet below the surface. It was to be, as indeed it is, 2399 yards 2 feet 6 inches in length, with two shafts of the extraordinary size of 60 feet in diameter, not only to give air and ventilation, but to admit light enough to enable the engine-driver in passing through it with a train to see the rails from end to end.
In order correctly to ascertain, and honestly to make known to the contractors, the nature of the ground through which this great work was to pass, the engineer in chief sank the usual number of what are termed “trial shafts,” and, it clearly appearing therefrom that the principal portion of the stratum was the shale of the lower oolite, the usual advertisements for tenders were issued, and the shafts, &c., having been minutely examined by the competing contractors, the work was let to one of them for the sum of 99,000l.
In order to drive the tunnel, it was deemed necessary to construct 18 working shafts, by which, like the heavings of a mole, the contents of the subterranean gallery were to be brought to the surface.
This interesting work was in busy progress, when all of a sudden it was ascertained, that at about 200 yards from the south end of the tunnel, there existed, overlaid by a bed of clay 40 feet thick, a hidden quicksand, which extended 400 yards into the proposed tunnel, and which the trial shafts on each side of it had almost miraculously just passed without touching.
The traveller in India could scarcely be more alarmed at the sudden sight of a crouching tiger before him, than the contractor was at the unexpected appearance of this invincible enemy. Overwhelmed at the discovery, he instantly took to his bed, and though he was liberally, or, to speak more correctly, justly relieved by the Company from his engagement, the reprieve came too late, for he actually died!
The question then arose whether, in the face of this tremendous difficulty, the execution of the Kilsby Tunnel should be continued or abandoned. The general opinion of the several eminent engineers who were consulted was against proceeding, and certainly the amount of the difficulties which were subsequently incurred, justified the verdict. But in science, as well as in war, the word “IMpossible” can occasionally, by cool and extraordinary exertions, be divested of its first syllable; and accordingly, Mr. Robert Stephenson offering, after mature reflection, to undertake the responsibility of proceeding, he was duly authorised to do so.