Figs. 93 and 94.—Diagrams Showing Sequence of Excavation in Austrian Method of Tunneling.
Strutting.
—Each part of the section is strutted as fast as it is excavated. The center bottom drift first excavated is strutted by laying a transverse sill across the floor, raising two side posts from it, and capping them with a transverse timber having its ends projecting beyond the side posts and halved as shown by [Fig. 95]. The top center heading No. 2, which is next excavated, is strutted by means of two side posts resting on blocking and carrying a transverse cap as also shown by [Fig. 95]. Sometimes the side posts in the heading strutting-frames are also carried on a transverse sill as are those of the bottom drift. This construction is usually adopted in loose soils. When the sill is employed, the middle part, No. 3, is strutted by inserting side posts between the bottom of the top sill and the cap of the frame in the drift below. When, however, the posts of the top heading frame are carried on blocking, it is the practice to replace them with long posts rising from the cap of the bottom drift frame to the cap of the top heading frame. Further, when the intermediate sill is employed at the bottom level of the top heading it projects beyond the side posts and has its ends halved.
Figs. 95 to 97.—Sketches Showing Construction of Strutting, Austrian Method.
After the completion of the center trench strutting the next task is to strut parts Nos. 4 and 5. This is done by continuing the upper sill by means of a timber having one end halved to join with the projecting end of the sill in position. This extension timber is shown at a, [Fig. 96]. The next operation is to place the timber b, having one end resting on the cap-piece of the top heading frame and the other beveled and resting on the top of the sill a near the end. The timber b is laid tangent to the curve of the roof arch, and to support it against flexure the strut c is inserted as shown. To support the thrust of this strut the additional post d is inserted and the original bottom heading frame is reinforced as shown. The next step is to insert the strut e, and when this and the previous construction are duplicated on the opposite side of the tunnel section we have the strutting of the parts Nos. 1 to 5; inclusive, complete. Part No. 6 is then removed and strutted by extending the bottom drift cap-piece by a timber similar to timber a above, and then by inserting a side strut between the outer ends of these two timbers, as indicated by [Fig. 97]. As the final parts. Nos. 7 and 8, are removed, the inclined prop a, [Fig. 97], is inserted as shown. When the soil is loose some of the members of the framework are doubled and additional bracing is introduced as shown by [Fig. 97].
The frames just described are placed at intervals of about 4 ft. along the excavation, and are braced apart by horizontal struts. Some of the longitudinal bearing beams, as at b, [Fig. 97], also extend through two or three frames, and help to tie them together. Finally, the longitudinal poling-boards extending from one frame to the next along the walls of the excavation serve to connect them together. The short transverse beam c, Fig. 90, located just above the floor of the invert, serves to carry the planking upon which the train car tracks are laid. Besides the timber strutting peculiar to the Austrian method, the Rziha iron strutting described in a previous chapter is frequently used in tunneling by the Austrian process.
