—The system of hauling adopted with this method of tunneling is very simple, since the excavation of the various parts is driven only from 6 ft. to 10 ft. ahead, and the work progresses slowly to allow for the construction of the heavy strutting required. To take away the material from the center bottom drift, narrow-gauge tracks carried by cross-beams between the side posts above the floor line are employed. This same narrow-gauge line is employed to take away a portion of parts No. 2, the remaining portion being left and used for the refilling after the bottom portion of the lining has been built, as previously described. The upper half of the section being excavated, as in the Belgian method, the system of hauling with inclined planes to the tunnel floor below, which is a characteristic of that method, may be employed. It is the more usual practice, however, since the excavation is carried so little a distance ahead and progresses so slowly, to handle the spoil from the upper part of the section by wheelbarrows which dump it into the cars running on the tunnel floor below. Hand labor is also used to raise the construction materials used in building the upper section. The tracks on the tunnel floor, besides extending to the front of the advanced bottom center drift, have right and left switches to be employed in removing the refilling in parts No. 2, the spoil from the upper part of the section, and the material of part No. 7. [Fig. 103] is a longitudinal section showing the plan of excavation and strutting adopted with the Italian method.
Fig. 103.—Sketch Showing Longitudinal Section of a Tunnel under Construction, Italian Method.
Modifications.
—It often happens that the filling placed between the side walls and the planking, which is practically the space comprised by parts No. 2, is not sufficient to resist the inward pressure of the walls, and they tip inward. In these cases a common expedient is to substitute for the earth filling a temporary masonry arch sprung between the side walls with its feet near the bottom of the walls, and its crown just below the level of their tops, as shown by [Fig. 107]. This construction was employed in the Stazza tunnel in Italy. In this tunnel the excavation was begun by driving the center drift, No. 1, [Fig. 104], and immediately strutting it as shown by [Fig. 105]. The other parts, Nos. 2 and 3, completing the lower portion of the section, were then taken out and strutted. While part No. 2 was being excavated at the bottom, and the center part of the invert built, the longitudinal crown bars carrying the roof of the excavation were carried temporarily by the inclined props shown by [Fig. 106]. After completing the invert and the side walls to a height of 2 or 3 ft., a thick masonry arch was sprung between the side walls, as shown in transverse section by [Fig. 107], and in longitudinal section by [Fig. 106]. This arch braced the side walls against tipping inward, and carried short struts to support the crown bars. The haunches of the arch were also filled in with rammed earth. The upper half of the section was excavated, strutted, and lined as in the standard Italian method previously described. When the lining was completed, the arch inserted between the side walls was broken down and removed.
Fig. 104.—Sketch Showing Sequence of Excavation, Stazza Tunnel.
Fig. 105.—Sketch Showing Method of Strutting First Drift, Stazza Tunnel.
