AUSTRIAN METHOD.
The Austrian full-section method of tunneling through soft ground was first used in constructing the Oberau tunnel on the Leipsic and Dresden R.R., in Austria in 1837. It consists in excavating the full section and building up the lining masonry from the foundations as in the English, but with the important exception that the invert is built last instead of first in all cases except where the presence of very loose soil requires its construction first. A still more important difference in the two methods is that the excavation is carried out in smaller sections and is continuous in the Austrian method instead of alternating with the mason work as it does in the English method.
Excavation.
—The excavation in the Austrian method begins by driving the bottom center drift No. 1, [Fig. 93], rising from the floor of the tunnel section nearly to the height of the springing lines of the roof arch. When this drift has been driven ahead a distance varying from 12 ft. to 20 ft. or sometimes more, the excavation of the center top heading No. 2 is driven for the same distance. The next operation is to remove part No. 3, thus forming a central passage the full depth of the tunnel section at the center. This trench is enlarged by removing parts Nos. 4, 5, 6, 7, and 8 in the order named until the full section is opened. A modification of this plan of excavation is shown by [Fig. 94] which is used in firm soils.
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.
Fig. 98.—Sketch Showing Manner of Constructing the Lining Masonry, Austrian Method.
Centers.
—The two forms of centers used in the English method of tunneling are also used in the Austrian method. One of the methods of supporting these centers is shown by [Fig. 98]. The tie-beam of the center rests on longitudinal timbers carried by the strutting frames and intermediate props. In single-track tunnels it is the frequent practice also to carry the ends of the tie-beams in recesses left in the side wall masonry, with intermediate props inserted to prevent flexure at the center. When the Rziha iron strutting is employed, it also serves for the centering upon which the arch masonry is built.
Masonry.
—In the Austrian system of tunneling, the lining is built from the foundations of the side walls upward to the crown of the roof arch in lengths in consecutive rings equal to the lengths of the consecutive openings of the full section, or from 12 ft. to 20 ft. long. Except in infrequent cases in very loose materials the invert is the last part of the masonry to be built, since to build it first requires the removal of the strutting which cannot easily or safely be accomplished until the side walls and roof arch are completed. As the side wall foundations are built, however, their interior faces are left inclined, as shown by [Figs. 97] and [98], ready for the insertion of the invert, and are meanwhile kept from sliding inward by the insertion of blocking between them and the bottom of the strutting. [Fig. 98] shows the nature of this blocking, and also the manner in which the side wall and roof arch masonry is carried upward. Finally when the roof arch is keyed and the centers are struck, the strutting is taken down and the invert is built.
Advantages and Disadvantages.
—The principal advantages claimed for the Austrian method of tunneling are: (1) The excavation being conducted by driving a large number of consecutive small galleries, which are immediately strutted, there is little disturbance of the surrounding material; (2) the polygonal type of strutting adopted is easily erected and of great strength against symmetrical pressures; (3) the masonry, being built from the foundations up, is a single homogeneous structure, and is thus better able to withstand dangerous pressures; (4) the excavation is so conducted that the masons and excavators do not interfere, and both can work at the same time. The disadvantages which the method possesses are: (1) The strutting while very strong under symmetrical pressures, either vertical or lateral, is distorted easily by unsymmetrical vertical or lateral pressures, and by pressure in the direction of the axis of the tunnel; (2) the construction of the invert last exposes the side walls to the danger of being squeezed together, causing a rotation of the arch of the nature discussed in describing the Belgian method of tunneling.