CHAPTER XV. SPECIAL TREACHEROUS GROUND METHOD; ITALIAN METHOD; QUICKSAND TUNNELING; PILOT METHOD. 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.

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.

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.

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.

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.

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.

QUICKSAND TUNNELING.

When an underground stream of water passes with force through a bed of sand it produces the phenomenon known as quicksand. This phenomenon is due to the fineness of the particles of sand and to the force of the water, and its activity is directly proportional to them. When sand is confined it furnishes a good foundation bed, since it is practically incompressible. To work successfully in quicksand, therefore, it is necessary to drain it and to confine the particles of sand so that they cannot flow away with the water. This observation suggests the mode of procedure adopted in excavating tunnels through quicksand, which is to drain the tunnel section by opening a gallery at its bottom to collect and carry away the water, and to prevent the movement or flowing of the sand by strutting the sides of the excavation with a tight planking.

The sand having to be drained and confined as described, the ordinary methods of soft-ground tunneling must be employed, with the following modifications:

(1) The first work to be performed is to open a bottom gallery to drain the tunnel. This gallery should be lined with boards laid close and braced sufficiently by interior frames to prevent distortion of the lining. The interstices or seams between the lining boards should be packed with straw so as to permit the percolation of water and yet prevent the movement of the sand.

(2) As fast as the excavation progresses its walls should be strutted by planks laid close, and held in position by interior framework; the seams between the plank should be packed with straw.

(3) The masonry lining should be built in successive rings, and the work so arranged that the water seeping in at the sides and roof is collected and removed from the tunnel immediately.

Excavation.

—The best and most commonly employed method of driving tunnels through quicksand is a modification of the Belgian method. At first sight it may appear a hazardous work to support the roof arch, as is the characteristic of this method, on the unexcavated soil below, when this soil is quicksand, but if the sand is well confined and drained the risk is really not very great. Next to the Belgian method the German method is perhaps the best for tunneling quicksand. In these comparisons the shield system of tunneling is for the time being left out of consideration. This method will be described in succeeding chapters. Whenever any of the systems of tunneling previously described are employed, the first task is always to open a drainage gallery at the bottom of the section.

Assuming the Belgian method is to be the one adopted, the first work is to drive a center bottom drift, the floor of which is at the level of the extrados of the invert. This drift is immediately strutted by successive transverse frames made up of a sill, side posts, and a cap which support a close plank strutting or lining, with its joints packed with straw. Between the side posts of each cross-frame, at about the height of the intrados of the invert, a cross-beam is placed; and on these cross-beams a plank flooring is laid, which divides the drift horizontally into two sections, as shown by Fig. 108; the lower section forming a covered drain for the seepage water, and the upper providing a passageway for workmen and cars. The bottom drift is driven as far ahead as practicable, in order to drain the sand for as great a distance in advance of the work as possible. After the construction of the bottom drainage drift the excavation proper is begun, as it ordinarily is in the Belgian method by driving a top center heading, as shown by Fig. 108. This heading is deepened and widened after the manner usual to the Belgian method, until the top of the section is open down to the springing lines of the roof arch. To collect the seepage water from the center top heading it is provided with a center bottom drain constructed like the drain in the bottom drift, as shown by Fig. 108. When the top heading is deepened to the level of the springing lines of the roof arch, its bottom drain is reconstructed at the new level, and serves to drain the full top section opened for the construction of the roof arch. This top drain is usually constructed to empty into the drain in the bottom drift.

Strutting.

—The method of strutting the bottom drift has already been described. For the remainder of the excavation the regular Belgian method of radial roof strutting-frames is employed, as shown by Fig. 109. Contrary to what might be expected, the number of radial struts required is not usually greater than would be used in many other soils besides quicksand. Single-track railway tunnels have been constructed through quicksand in several instances where the number of radial props required on each side of the center did not exceed four or five. It is necessary, however, to place the poling-boards very close together, and to pack the joints between them to prevent the inflow of the fine sand. In strutting the lower part of the section it is also necessary to support the sides with tight planking. This is usually held in place by longitudinal bars braced by short struts against the inclined props employed to carry the roof arch when the material on which they originally rested is removed. This side strutting is shown at the right hand of Fig. 110.

Masonry.

—As soon as the upper part of the section has been opened the roof arch is built with its feet resting on planks laid on the unexcavated material below. This arch is built exactly as in the regular Belgian method previously described, using the same forms of centers and the same methods throughout, except that the poling-boards of the strutting are usually left remaining above the arch masonry. To prevent the possibility of water percolating through the arch masonry, many engineers also advise the plastering of the extrados of the arch with a layer of cement mortar. This plastering is designed to lead the water along the haunches of the arch and down behind the side walls. In constructing the masonry below the roof arch the invert is built first, contrary to the regular Belgian method, and the side walls are carried up on each side from the invert masonry. Seepage holes are left in the invert masonry, and also in the side walls just above the intrados of the invert. At the center of the invert a culvert or drain is constructed, as shown by Fig. 110, inside the invert masonry. This culvert is commonly made with an elliptical section with its major axis horizontal, and having openings at frequent intervals at its top. The thickness of the lining masonry required in quicksand is shown by Table II.

Removing the Seepage Water.

—After the tunnel is completed the water which seeps in through the weep-holes left in the masonry passes out of the tunnel, following the direction of the descending grades. During construction, however, special means will have to be provided for removing the water from the excavation, their character depending upon the method of excavation and upon the grades of the tunnel bottom. When the excavation is carried on from the entrances only, unless the tunnel has a descending grade from the center toward each end, the tunnel floor in one heading will be below the level of the entrance, or, in other words, the descending grade will be toward the point where work is going on, while at the opposite entrance the grade will be descending from the work. In the latter case the removal of the seepage water is easily accomplished by means of a drainage channel along the bottom of the excavation. In the former case the water which drains toward the front is collected in a sump, and if there is not too great a difference in level between this sump and the entrance, a siphon may be used to remove it. Where the siphon cannot be used, pumps are installed to remove the water. When the tunnel is excavated by shafts the condition of one high and one low front, as compared with the level at the shaft, is had at each shaft. Generally, therefore, a sump is constructed at the bottom of the shaft; the culvert from the high front drains directly to the shaft sump, while the water from the low-front sump is either siphoned or pumped to the shaft sump. From the shaft sump the water is forced up the shaft to the surface by pumps.

THE PILOT METHOD.

The pilot system of tunneling has been successfully employed in constructing soft-ground sewer tunnels in America by the firm of Anderson & Barr, which controls the patents. The most important work on which the system has been employed is the main relief sewer tunnel built in Brooklyn, N.Y., in 1892. This work comprised 800 ft. of circular tunnel 15 ft. in diameter, 4400 ft. 14 ft. in diameter, 3200 ft. 12 ft. in diameter, and 1000 ft. 10 ft. in diameter, or 9400 ft. of tunnel altogether. The method of construction by the pilot system is as follows:

Shafts large enough for the proper conveyance of materials from and into the tunnel are sunk at such places on the line of work as are most convenient for the purpose. From these shafts a small tunnel, technically a pilot, about 6 ft. in diameter, composed of rolled boiler iron plates riveted to light angle irons on four sides, perforated for bolts, and bent to the required radius of the pilot, is built into the central part of the excavation on the axis of the tunnel. This pilot is generally kept about 30 ft. in advance of the completed excavation, as shown by Fig. 111. The material around the exterior of the pilot is then excavated, using the pilot as a support for braces which radiate from it and secure in position the plates of the outside shell which holds the sand, gravel, or other material in place until the concentric rings of brick masonry are built. Ribs of T-iron bent to the radius of the interior of the brick work, and supported by the braces radiating from the pilot, are used as centering supports for the masonry. On these ribs narrow lagging-boards are laid as the construction of the arch proceeds, the braces holding the shell plates and the superincumbent mass being removed as the masonry progresses. The key bricks of the arches are placed in position on ingeniously contrived key-boards, about 12 ins. in width, which are fitted into rabbeted lagging-boards one after another as the key bricks are laid in place. After the masonry has been in place at least twenty-four hours, allowing the cement mortar time to set, the braces, ribs, and lagging which support it are removed. In the meantime the excavation, bracing, pilot, and exterior shell have been carried forward, preparing the way for more masonry. The top plates of the shell are first placed in position, the material being excavated in advance and supported by light poling-boards; then the side-plates are butted to the top and the adjoining side-plates. In the pilot the plates are united continuously around the perimeter of the circle, while in the exterior shell the plates are used for about one-third of the perimeter on top, unless treacherous material is encountered, when the plates are continued down to the springing lines of the arch. This iron lining is left in place. The bottom is excavated so as to conform to the exterior lines of the masonry. The excavation follows so closely to the outer lines of the normal section of the tunnel that very little loss occurs, even in bad material; and there is no loss where sufficient bond exists in the material to hold it in place until the poling-boards are in position.

In the Brooklyn sewer tunnel work, previously mentioned, the pilot was built of steel plates ³/₈ in. thick, 12 ins. wide, and 37¹/₂ ins. long, rolled to a radius of 3 ft. Steel angles 4× 4¹/₂ ins. were riveted along all four sides of each plate, and the plates were bolted together by ³/₄-in. machine-bolts. The plates weighed 136 lbs. each, and six of them were required to make one complete ring 6 ft. in diameter. In bolting them together, iron shims were placed between the horizontal joints to form a footing for the wooden braces for the shell, which radiate from the pilot. The shell plates of the 15-ft. section of the tunnel were of No. 10 steel 12 ins. wide and 37 ins. long, with steel angles 2¹/₂× 2¹/₂× ³/₈ ins., riveted around the edges the same as for the pilot, and put together with ⁵/₈-in. bolts. These plates weighed 61 lbs. each, and eighteen of them were required to make one complete ring 15 ft. in diameter. The plates for the 12-ft. section were No. 12 steel 12 ins. wide with 2× 2× ¹/₄-in. angles. Seventeen plates were required to make a complete ring.

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