ABSTRACT
One of the most important considerations in the formation of a tunnel is its size and shape. Its width on the narrow gauge should be about 30 feet ; and in depth it must extend 5 or 6 feet below the intended line of the rails, so that space may be allowed for the inverted arch, the ballasting, and the drainage. Where, however, the excavation is carried through rock sufficiently hard to form the bottom and side walls, 25 to 26 feet in width, and about 26 feet in height are sufficient. The brickwork, from the invert upwards, is oval, whereby greater resistance to side pressure is obtained than if the side walls were
level. This was formerly done with the aid of the horse-gin, now replaced by the steam-engine ; and, by the material raised,
perpendicular. The shape of the tunnel will also be determined by the nature of the ground through which it has to pass. In a wet quicksand approaching the nature of a fluid, the form will approximate to that of a circle.
