Fig. 76.—Diagrams Showing Sequence of Excavation in German Method of Tunneling.
Excavation.
—The excavation of tunnels by the German method is begun either by driving two bottom side drifts or by driving a center top heading. [Fig. 76] shows the mode of procedure when bottom side drifts are used to start the work. The two side drifts No. 1 are made from 7 ft. to 8 ft. wide, and about one-third the total height of the full section; the width of each heading has to be sufficient for the construction of the masonry and strutting, and for the passage of narrow spoil cars alongside them. These drifts are increased in height to the springing line of the arch by taking out the two drifts No. 2. Next the top center heading No. 3 is driven, and finally the two haunch headings No. 4 are excavated. The center core No. 5 is utilized to support the strutting until the side walls and roof arch are completed, when it is broken down and removed. In case of very loose material, where the first side drifts cannot be carried as high as one-third the height of the section, it is the common practice to make them about one-fourth the height, and to take out the side portions of the annular gallery in three parts, as shown by [Fig. 76].
Fig. 77.—Diagram Showing Sequence of Excavations in Water Bearing Material, German Method.
The top center heading plan of commencing the excavation is usually employed in firm materials or when a vein of water is encountered in the upper part of the section. In the latter contingency a small bottom drift A, [Fig. 77], is first driven to serve as a drain; but in any case the excavation proper of the tunnel consists in first driving the center top heading No. 1, and then by working both ways along the profile parts, Nos. 2, 3, 4, and 5 are removed. Part No. 6 is left to support the strutting until the side walls and roof arch are built, when it is also excavated.
Strutting.
—When the excavation is begun by bottom side drifts these drifts are strutted by erecting vertical posts close against the sides of the drift and placing a cap-piece transversely across the roof of the drift. The side posts are usually supported by sills placed across the bottom of the drift. These frameworks of posts, cap, and sill are erected at short intervals, and the roof, and, if necessary, the sides of the drift between them, are sustained by means of longitudinal poling-boards extending from one frame to the next. The cap-pieces of the strutting for the bottom drifts serve as sills for the exactly similar strutting of the heading next above. To support the additional weight, and to allow the construction of the side walls, the strutting of the bottom drifts is strengthened by inserting an intermediate post between the original side posts of each frame. These intermediate posts are not inserted at the center of the frames or bents, but close to the wall masonry line as shown by [Fig. 78]. This eccentric position of the post avoids any interference with the hauling, and also allows the removal of the adjacent side post when the masonry is constructed.
