The building of the Hudson River tunnels was probably one of the most daring engineering feats ever accomplished. As is well known, the Hudson River, for the length of Manhattan Island, is approximately a mile wide, reducing in width at the Palisades north of Hoboken. In consequence of the unusual geographical situation, all trunk lines and other transit facilities in New Jersey terminate on the westerly shore of the Hudson, and passengers were of necessity compelled to use ferries to reach New York. A conservative estimate, which was confirmed by various counts, indicates that, prior to the construction of the tubes, the annual passenger traffic between New Jersey and New York was 125,000,000, and to handle this great volume of traffic the transportation companies assembled in the Hudson River a fleet of rapid ferry boats and maintained them up to the highest and most modern standards. But this very expeditious ferry service was not enough, and for many years there was a demand for facilities for more rapid transportation of the tremendous population residing in the suburban district of New Jersey tributary to New York City. As far back as 1873, a company had been organized to construct a tunnel under the river, but had met with numerous and most discouraging difficulties and obstacles, so that it was finally compelled to abandon the work, although it succeeded in building a considerable length of structure. Efforts were made at various times after that date to revive the work, with little or no results. In 1902 it was resumed, however, and a few years later was pushed to a successful end.

During the undertaking, more than 40,000 men were engaged in air-pressure work and there were many thousand more who did not work under air pressure. This vast army of men consisted of all nationalities and all grades and conditions of labor. The skilled tunnel workmen are men of character and ability, usually young, of good intelligence and sound of body, without a streak of fear or cowardice in their makeup. All of those characteristics are essential to under-water air-pressure work.

As is quite generally known, air pressure and tunnel shields were used in all of the under-water work. It might be well to here correct the misconception which exists in the minds of many, that the use of air pressure for such purposes is something comparatively new. This is not the case. The use of air pressure was a very early invention, and it is a matter of record that in 1830, Admiral Cochrane, afterwards Lord Dundonald, was granted letters patent for the use of air pressure in tunnel construction. The modern engineer has merely developed the art to a high degree.

The method of construction used in the Hudson River tunnels has been designated the "shield method." In this type of construction, the primary part of the tunnel structure consists of an iron shell, formed of segmental rings, bolted together through inside flanges, and forming a large articulated pipe or tube, circular in section. This iron shell is put in place segmentary by means of a shield, an ingenious mechanism which both protects the work under construction and assists in the building of the iron shell.

A tunneling shield consists essentially of a tube or cylinder slightly larger in diameter than the tunnel it is intended to build, which slides over the exterior of the finished lining like the tubes of a telescope. The front end of this cylindrical shield is provided with a diaphragm or bulkhead in which are apertures which may be opened or closed at will. Behind this diaphragm are placed a number of hydraulic jacks, so arranged that by thrusting against the last erected iron ring the entire shield is pushed forward. The hind end of the shield is simply a continuation of the cylinder which forms the front end, and this hind end, or tail, always overlaps the last few feet of the built-up iron-shell tunnel.

*Illustrations by courtesy of Jacobs & Davies, Engineers.

The New Short Cut to New York Hudson River Tubes of the Hudson & Manhattan R. R. Co..

When the openings in the bulkhead are closed, the tunnel is protected from the inrush of water or soft ground, and the openings may be so regulated that control is maintained over the material passed through. After a ring of iron lining has been erected within the tail of the shield, excavation is carried out ahead. When sufficient excavation has been taken out, the jacks are again extended, thus pushing the shield ahead, and another ring of iron is erected as before.

One of the Sixty-seven-Ton Tunnel Shifelds.

For the erection of these heavy plates, a hydraulic swinging arm, called the "Erector," is mounted, either on the shield itself or on an independent erector platform, according to conditions. This erector approaches closely the faculties of the human arm. It is hydraulically operated and can be moved in any desired direction. This method of construction can be followed in almost every kind of ground that can be met with, and it is especially valuable in dealing with soft, wet grounds. In passing through materials saturated with water, the shield is assisted by using compressed air in the working chamber.

The employment of compressed air under such conditions is really a rather simple thing in itself, and means merely that the pressure of air in the chamber where men are working is maintained at a point sufficient to offset the pressure of the hydrostatic head of water and thereby prevent its inflow. A crude comparison may be inrush of water, would be to increase the air pressure, which might be a very dangerous thing to do. An experienced man, however, would very likely first lower his pressure in such an emergency, and then put up with the nuisance and difficulty of having a good deal of water in his working chamber. By doing this, he would permit the greater external pressure to squeeze the soil into the leaking pockets and thereby choke the leak.