Tunneling: A Practical Treatise.

by Charles Prelini

EUROPEAN AND AMERICAN METHODS.

The more common method of tunneling through hard rock is to begin the work by a heading, instead of by a drift. This heading may be of small dimensions, and the remainder of the section may also be removed in successive small parts, or it may be the full width of the section, and the enlargement of the section be made in one other cut.

Fig. 61.—Diagram Showing Sequence of Excavation in Heading Method of Tunneling Rock.

General Discussion.

—When the tunnel is excavated by means of several cuts, which is the method usually employed in Europe, the sequence of work is as indicated by [Fig. 61]. Work is begun by driving the center top heading No. 1, whose floor is at the level of the bottom of the roof arch, and which is usually excavated by the circular cut method. This heading is widened by removing parts Nos. 2 and 3 until the top part of the section is removed, then the roof arch is built with its feet resting on the unexcavated rock below. The lower portion of the section or bench is removed by first sinking the trench No. 4, after which part No. 5 is taken out, and then parts Nos. 6 and 7, and the side walls built. Part No. 8 for the culvert is finally opened. The heading is, as a rule, driven far in advance, but the excavation of each of the other parts follows the preceding one at a distance behind of about 300 ft.

The strutting, when any is required, is usually the typical radial strutting of the Belgian method of tunneling. The masonry lining is constructed practically the same as in tunnels excavated by a drift. The hauling is done on a single track laid in the heading No. 1, which separates into double tracks where the full top section has been excavated by the removal of parts No. 2. These two tracks are again combined and form a single track along the top of part No. 5, which has been left wider than part No. 4 for this particular purpose. When part No. 3 is excavated a standard-gauge track is laid on its floor; and as the full section of the tunnel is completed by taking out parts Nos. 4 and 5, this single track is replaced by two standard-gauge tracks, into which it switches. Spoil is transferred from the narrow-gauge tracks on the upper level, to the standard-gauge tracks on the tunnel floor, by means of chutes, and building material is transferred in the opposite direction by means of hoisting apparatus.

When the excavation is made by a single wide heading, and a single other cut for removing the bench, which is the method preferred by American engineers, it is called the Heading and Bench method. The work begins by removing a top heading the full width of the section; this heading is usually made 7 ft. or 8 ft. high, and is excavated by the center cut method. The method of strutting usually employed is to erect successive three- or five-segment timber arches, whose feet rest on the top of the bench; when the bench is removed, posts are inserted under the feet of each arch. These arches are covered with a lagging of plank. In America it has often been the practice to let this strutting serve as a temporary lining, and to replace it only after some time, often after years, with a permanent lining of masonry. In a succeeding chapter, some of the methods adopted in relining timber-lined arches with masonry are described. The hauling is done by either narrow or broad gauge tracks laid on the floor of the completed section below. A device called a bench carriage is often employed to enable the cars running on the heading tracks to dump their loads into the cars below, without interfering with the work on the bench front. This device consists of a wide platform carried on trucks, running on rails at the sides of the tunnel floor, so that it is level with the floor of the heading. The front of this platform carries a hinged leaf which may be raised and lowered, and which forms a sort of gang-plank reaching to the floor of the heading. By running the heading cars out on to this traveling platform, they can be dumped into the cars below entirely clear of the work in progress on the bench front.

For the purpose of illustrating the two methods of driving tunnels by a heading, which have been briefly described, the St. Gothard and the Fort George tunnels have been selected. The St. Gothard tunnel is selected, as being one of the longest tunnels in the world, and because it was excavated by a number of small parts; and the Fort George tunnel, as being a double-track tunnel, driven by a heading, and bench, and having a concrete lining.