ITALIAN METHOD.
The Italian method of tunneling was first employed in constructing the Cristina tunnel on the Foggia & Benevento R.R. in Italy. This tunnel penetrated a laminated clay of the most treacherous character, and after various other soft-ground methods of tunneling had been tried and had failed, Mr. Procke, the engineer, devised and used successfully the method which is now known as the Italian or Cristina method. The Italian method is essentially a treacherous soil method. It consists in excavating the bottom half of the section by means of several successive drifts, and building the invert and side walls; the space is then refilled and the upper half of the section is excavated, and the remainder of the side walls and the roof arch are built; finally, the earth filling in the lower half of the section is re-excavated and the tunnel completed. The method is an expensive one, but it has proved remarkably successful in treacherous soils such as those of the Apennine Mountains, in which some of the most notable Italian tunnels are located. It is, moreover, a single-track tunnel method, since any soil which is so treacherous as to warrant its use is too treacherous to permit an opening to be excavated of sufficient size for a double-track railway, except by the use of shields.
Excavation.
—The plan of excavation in the Italian method is shown by the diagram Fig. 99. Work is begun by driving the center bottom heading No. 1, and this is widened by taking out parts No. 2. Finally part No. 3 is removed, and the lower half of the section is open. As soon as the invert and side wall masonry has been built in this excavation, parts No. 2 are filled in again with earth. The excavation of the center top heading No. 4 is then begun, and is enlarged by removing the earth of part No. 5. The faces of this last part are inclined so as to reduce their tendency to slide, and to permit of a greater number of radial struts to be placed. Next, parts No. 6 are excavated, and when this is done the entire section, except for the thin strip No. 7, has been opened. At the ends of part No. 7 narrow trenches are sunk to reach the tops of the side walls already constructed in the lower half of the section. The masonry is then completed for the upper half of the section, and part No. 7 and the filling in parts No. 2 are removed. The various drifts and headings and the parts excavated to enlarge them are seldom excavated more than from 6 ft. to 10 ft. ahead of the lining.
Fig. 99.—Diagram Showing Sequence of Excavation in Italian Method of Tunneling.
Fig. 100.—Sketch Showing Strutting for Lower Part of Section.
Strutting.
—The bottom center drift, which is first driven, is strutted by means of frames consisting of side posts resting on floor blocks and carrying a cap-piece. Poling-boards are placed around the walls, stretching from one frame to the next. As soon as the invert is sufficiently completed to permit it, the side posts of the strutting frames are replaced by short struts resting on the invert masonry as shown by [Fig. 100]. To permit the old side posts to be removed and the new shorter ones to be inserted, the cap-piece of the frame is temporarily supported by inclined props arranged as shown by [Fig. 103]. When parts No. 2 are excavated the roof is strutted by inserting the transverse caps a, [Fig. 100], the outer ends of which are carried by the system of struts b, c, d, and e. The longitudinal poling-boards supporting the ceiling and walls are held in place by the cap a and the side timber e. To stiffen the frames longitudinally of the tunnel, horizontal longitudinal struts are inserted between them.
The excavation of the upper half of the tunnel section is strutted as in the Belgian method, with radial struts carrying longitudinal roof bars and transverse poling-boards. On account of the enormous pressures developed by the treacherous soils in which only is the Italian method employed, the radial strutting frames and crown bars must be of great strength, while the successive frames must be placed at frequent intervals, usually not more than 3 ft. After the masonry side walls have been built in the lower part of the excavation, longitudinal planks are laid against the side posts of the center bottom drift frames, to form an enclosure for the filling-in of parts No. 2. The object of this filling is principally to prevent the squeezing-in of the side walls.
Figs. 101 and 101A.—Sketches Showing Construction of Centers, Italian Method.
Centers.
—Owing to the great pressures to be resisted in the treacherous soils in which the Italian method is used, the construction of the centers has to be very strong and rigid. [Figs. 101] and [101A] show two common types of center construction used with this method. The construction shown in [Fig. 101] is a strong one where only pressures normal to the axis of the tunnel have to be withstood, but it is likely to twist under pressures parallel to the axis of the tunnel. In the construction shown by [Fig. 101A], special provision is made to resist pressures normal to the plane of the center or twisting pressures, by the strength of the transverse bracing extending horizontally across the center.
Fig. 102.—Sketch Showing Invert and Foundation Masonry, Italian Method.
Masonry.
—The construction of the masonry lining begins with the invert, as indicated by [Fig. 100], and is carried up to the roof of parts No. 2, as already indicated, and is then discontinued until the upper parts Nos. 4, 5, and 6 are excavated. The next step is to sink side trenches at the ends of part No. 7, which reach to the top of the completed side walls. This operation leaves the way clear to finish the side walls and to construct the roof arch in the ordinary manner of such work in tunneling. Since this method of tunneling is used only in very soft ground which yields under load, the usual practice is to construct the invert and side walls on a continuous foundation course of concrete as indicated by [Fig. 102]. The lining is usually built in successive rings, and the usual precautions are taken with respect to filling in the voids behind the lining. The thickness of the lining is based upon the figures for laminated clay of the third variety given in [Table II].
Hauling.
—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.
Figs. 106 and 107.—Sketches Showing Temporary Strutting Arch Construction, Stazza Tunnel.
Advantages and Disadvantages.
—The great advantage claimed for the Italian method of tunneling is that it is built in two separate parts, each of which is separately excavated, strutted, and lined, and thus can be employed successfully in very treacherous soils. Its chief disadvantage is its excessive cost, which limits its use to tunnels through treacherous soils where other methods of timbering cannot be used.