Fig. 98.—Sketch Showing Manner of Constructing the Lining Masonry, Austrian Method.
Fig. 98.—Sketch Showing Manner of Constructing the Lining Masonry, Austrian Method.
—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 byFig. 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 wallmasonry, 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.
—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 byFigs. 97and98, 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. 98shows 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.
—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 lateralpressures, 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.
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.
—The plan of excavation in the Italian method is shown by the diagram Fig. 99. Work is begun by drivingthe 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.
Fig. 99.—Diagram Showing Sequence of Excavation in Italian Method of Tunneling.
Fig. 99.—Diagram Showing Sequence of Excavation in Italian Method of Tunneling.
Fig. 99.—Diagram Showing Sequence of Excavation in Italian Method of Tunneling.
Fig. 100.—Sketch Showing Strutting for Lower Part of Section.
Fig. 100.—Sketch Showing Strutting for Lower Part of Section.
Fig. 100.—Sketch Showing Strutting for Lower Part of Section.
—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 byFig. 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 supportedby inclined props arranged as shown byFig. 103. When parts No. 2 are excavated the roof is strutted by inserting the transverse capsa,Fig. 100, the outer ends of which are carried by the system of strutsb,c,d, ande. The longitudinal poling-boards supporting the ceiling and walls are held in place by the capaand the side timbere. 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.
Figs.101 and 101A.—Sketches Showing Construction of Centers, Italian Method.
—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. 101and101Ashow two common types of center construction used with this method. The construction shown inFig. 101is a strong one where only pressures normal to the axis of the tunnel have to be withstood, but it is likely to twist underpressures parallel to the axis of the tunnel. In the construction shown byFig. 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.
Fig. 102.—Sketch Showing Invert and Foundation Masonry, Italian Method.
—The construction of the masonry lining begins with the invert, as indicated byFig. 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 byFig. 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 inTable II.
—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, aspreviously 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. 103is 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.
Fig. 103.—Sketch Showing Longitudinal Section of a Tunnel under Construction, Italian Method.
—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 fillinga 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 byFig. 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 byFig. 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 byFig. 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 byFig. 107, and in longitudinal section byFig. 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.
Fig. 104.—Sketch Showing Sequence of Excavation, Stazza Tunnel.
Fig. 104.—Sketch Showing Sequence of Excavation, Stazza Tunnel.
Fig. 104.—Sketch Showing Sequence of Excavation, Stazza Tunnel.
Fig. 105.—Sketch Showing Method of Strutting First Drift, Stazza Tunnel.
Fig. 105.—Sketch Showing Method of Strutting First Drift, Stazza Tunnel.
Fig. 105.—Sketch Showing Method of Strutting First Drift, Stazza Tunnel.
Figs. 106and107.—Sketches Showing Temporary Strutting Arch Construction, Stazza Tunnel.
Figs. 106and107.—Sketches Showing Temporary Strutting Arch Construction, Stazza Tunnel.
—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.
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 shouldbe 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.
—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 byFig. 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 methodby driving a top center heading, as shown byFig. 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 byFig. 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.
Fig. 108.—Sketch Showing Preliminary Drainage Galleries, Quicksand Method.Fig. 109.—Sketch Showing Construction of Roof Strutting, Quicksand Method.
Fig. 108.—Sketch Showing Preliminary Drainage Galleries, Quicksand Method.
Fig. 108.—Sketch Showing Preliminary Drainage Galleries, Quicksand Method.
Fig. 108.—Sketch Showing Preliminary Drainage Galleries, Quicksand Method.
Fig. 109.—Sketch Showing Construction of Roof Strutting, Quicksand Method.
Fig. 109.—Sketch Showing Construction of Roof Strutting, Quicksand Method.
Fig. 109.—Sketch Showing Construction of Roof Strutting, Quicksand Method.
—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 byFig. 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 longitudinalbars 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 ofFig. 110.
Fig. 110.—Sketch Showing Construction of Masonry Lining, Quicksand Method.
Fig. 110.—Sketch Showing Construction of Masonry Lining, Quicksand Method.
—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 byFig. 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 byTable II.
—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 thedescending 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 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 tunnelaltogether. 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 byFig. 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 cementmortar 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.
Bracing.ArchConstruction.Longitudinal Section.Fig. 111.—Sketch Showing Pilot Method of Tunneling.
Bracing.ArchConstruction.
Bracing.
Bracing.
ArchConstruction.
ArchConstruction.
Longitudinal Section.
Fig. 111.—Sketch Showing Pilot Method of Tunneling.
In the Brooklyn sewer tunnel work, previously mentioned, the pilot was built of steel plates3⁄8in. thick, 12 ins. wide, and 371⁄2ins. long, rolled to a radius of 3 ft. Steel angles 4 × 41⁄2ins. were riveted along all four sides of each plate, and the plates were bolted together by3⁄4-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× 21⁄2×3⁄8ins., riveted around the edges the same as for the pilot, and put together with5⁄8-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.
When a tunnel or rapid-transit subway has to be constructed at a small depth below the surface, the excavation is generally performed more economically by making an open cut than by subterranean tunneling proper. The necessary condition of small depth which makes open-cut tunneling desirable is most generally found in constructing rapid-transit subways or tunnels under city streets. This fact introduces the chief difficulties encountered in such work, since the surface traffic makes it necessary to obstruct the streets as little as possible, and has led to the development of the several special methods commonly employed in performing it.
Subways are usually constructed under and along important streets where electric cars are running. The engineers have taken advantage of the presence of these lines to facilitate the construction of subways. In New York, for instance, the tracks of the electric lines were supported by cast-iron yokes 4 or 5 ft. apart and were surrounded by concrete, leaving only a large hollow space in the middle for the wires and trolleys. The rails from 40 to 60 ft. long formed almost a solid concrete structure for their entire length. The tracks and the street surface were supported by horizontal beams inserted underneath the tracks. These were the caps of bents constructed underground whose rafters were finally resting on the subgrade of the proposed subway.
The various methods for constructing the subways may be classified as follows: (1) The single wide trench method; (2) the single narrow longitudinal trench method; (3) the parallel longitudinal trench method; (4) the slice method.
—The simplest manner by which to construct open-cut tunnels is to open a single cut or trench the full width of the tunnel masonry. This trench is strutted by means of side sheetings of vertical planks, held in place by transverse braces extending across the trench and abutting against longitudinal timbers laid against the sheeting plank. The lining is built in this trench, and is then filled around and above with well-rammed earth, after which the surface of the ground is restored. An especial merit of the single longitudinal trench method of open-cut tunneling is that it permits the construction of the lining in a single piece from the bottom up, thus enabling better workmanship and stronger construction than when the separate parts are built at different times. The great objection to the method when it is used for building subways under city streets is, that it occupies so much room that the street usually has to be closed to regular traffic. For this reason the single longitudinal trench method is seldom employed, except in those portions of city subways which pass under public squares or parks where room is plenty.
This method was followed in the construction of the New York subway, Section 2, along Elm St., a new street to be opened to traffic after the subway had been completed, and at other points where local conditions allowed it.
Fig. 112.—Diagram Showing Sequence of Construction in Open-Cut Tunnels.
Fig. 112.—Diagram Showing Sequence of Construction in Open-Cut Tunnels.
A modification of this method was used in Contract Section 6, on upper Broadway. The street at this point is very wide, so by opening a trench as wide as the proposed four-track line of the subway there still remained room enough for ordinary traffic. The electric car tracks were supported by means of trusses 60 or 70 ft. long, which were laid in couples parallel to the tracks and which rested on firm soil. The soil under the car tracks was removed, beginning with transversal cuts toreceive the needles which were tied to the lower chord of the trusses by means of iron stirrups. After the excavation had reached the subgrade, posts were erected to support the needles thus forming bents upon which the tracks rested. The trusses were removed and advanced to another section of the tunnel, and, in the clear space left, the subway was built from foundation up.
—This method was used on Contract Section 5, of the New York subway in order to comply with the peculiar conditions of the traffic along 42nd St. On this street, on account of the New York Central Station, there is a constant heavy traffic, while pedestrians use the northern sidewalks almost exclusively. A single longitudinal trench was then opened along the south side, and from this trench all the work of excavation and construction was carried on. At first the steel structure of the subway was erected in the trench and then a small heading was driven and strutted under and across the surface-car tracks. Afterward heavy I-beams were inserted, which rested with one end on top of the steel bents and the other end blocked to the floor of the excavation. These I-beams were located 5 ft. apart and they supported the surface of the street by means of longitudinal planks. The soil was removed from the wide space underneath the I-beams and the subway was constructed from the foundation up. When the structure had been completed, the packing was placed between the roof of the structure and the surface of the street, the I-beams withdrawn and the voids filled in.
—The parallel longitudinal trench method of open-cut tunneling consists in excavating twonarrow parallel trenches for the side walls, leaving the center core to be removed after the side walls have been built. The diagram,Fig. 112, shows the sequence of operations in this method. The two trenches No. 1 are first excavated a little wider than the side wall masonry, and strutted as shown byFig. 113. At the bottoms of these trenches a foundation course of concrete is laid, as shown byFig. 114, if the ground is soft; or the masonry is started directly on the natural material, if it is rock. From the foundations the walls are carried up to the level of the springing lines of the roof arch, if an arch is used; or to the level of its ceiling, if a flat roof is used. After the completion of the side walls, the portion of the excavation shown at No. 2,Fig. 112, is removed a sufficient depth to enable the roof arch to be built. When the arch is completed, it is filled above with well-rammed earth, and the surface is restored. The excavation of part No. 3 inclosed by the side walls and roof arch is carried on from the entrances and from shafts left at intervals along the line.
Fig. 113.—Sketch Showing Method of Timbering Open-Cut Tunnels, Double Parallel Trench Method.
Fig. 113.—Sketch Showing Method of Timbering Open-Cut Tunnels, Double Parallel Trench Method.
Fig. 114.—Side-Wall Foundation Construction Open-Cut Tunnels.
Fig. 114.—Side-Wall Foundation Construction Open-Cut Tunnels.
A modification of the method just described was employed in constructing the Paris underground railways. It consists in excavating a single longitudinal trench along one side of the street, and building the side wall in it as previously described. When this side wall is completed tothe roof, the right half of part No. 2,Fig. 112, is excavated to the lineAB, and the right-hand half of the roof arch is built. The space above the arch is then refilled and the surface of the street restored, after which the left-hand trench is dug and the side wall and roof-arch masonry is built just as in the opposite half. Generally the work is prosecuted by opening up lengths of trench at considerable intervals along the street and alternately on the left-and right-hand sides. By this method one-half of the street width is everywhere open to traffic, the travel simply passing from one side of the street to the other to avoid the excavation. When the lining has been completed, the center core of earth inclosed by it is removed from the entrances and shafts, leaving the tunnel finished except for the invert and track construction, etc.
Another modification of the parallel longitudinal trenches method was used in the construction of the New York subway. A narrow longitudinal trench was excavated on one side of the street near the sidewalk. Meanwhile the pavement of half of the street was removed and a wooden platform of heavy planks, supported by longitudinal beams which were buried in the ground, was substituted. Then small cuts underneath the car tracks were directed from the side trench and heavy beams or needles were placed in these cuts, which also reached the longitudinal beams of the wooden platform. The needles were wedged and blocked to the car track structure and the beams. They were temporarily supported by cribs built from underneath as the excavation progressed. When the subgrade was reached, vertical and batter posts were inserted to support the needles, thus forming regular timber bents underground. In the space thus left open the subway was constructed to the middle of the street. While the work was going on as described, another longitudinal narrow trench was excavated at some distance on the other side of the street. From this trench, the work of constructing the other half of the subway was carried on in the manner just described. After the work had beencompleted, the timbers removed, the voids filled in and the pavement of the street restored, another equal section was attacked on both sides of the street.
—The transverse trench or “slice” method of open-cut tunneling has been employed in one work, the Boston Subway. This method is described in the specifications for the work prepared by the chief engineer, Mr. H. A. Carson, M. Am. Soc. C. E., asfollows:—
“Trenches about 12 ft. wide shall be excavated across the street to as great a distance and depth as is necessary for the construction of the subway. The top of this excavation shall be bridged during the night by strong beams and timbering, whose upper surface is flush with the surface of the street. These beams shall be used to support the railway tracks as well as the ordinary traffic. In each trench a small portion or slice of the subway shall be constructed. Each slice of the subway thus built is to be properly joined in due time to the contiguous slices. The contractor shall at all times have as many slice-trenches in process of excavation, in process of being filled with masonry, and in process of being back-filled with earth above the completed masonry, as is necessary for the even and steady progress of the work towards completion at the time named in the contract.”
In regard to the success of this method Mr. Carson, in his fourth annual report on the Boston Subway work, says:
“The method was such that the street railway tracks were not disturbed at all, and the whole surface of the street, if desired, was left in daytime wholly free for the normal traffic.”
—It occasionally happens when filling-in is to take place in the future, or where landslides are liable to bury the tracks, that a railway tunnel has to be built on the surface of the ground. In such cases the construction of the tunnel consists simply in building the lining of the section on the ground surface with just enough excavation to secure the proper grade and foundation. Generally the liningis finished on the outside with a waterproof coating, and is sometimes banked and partly covered with earth to protect the masonry from falling stones and similar shocks from other causes. A recent example of tunnel construction of this character was described in “Engineering News” of Sept. 8, 1898. In constructing the Golden Circle Railroad, in the Cripple Creek mining district of Colorado, the line had to be carried across a valley used as a dumping-ground for the refuse of the surrounding mines. To protect the line from this refuse, the engineer constructed a tunnel lining consisting of successive steel ribs, filled between with masonry.
—From the fact that the open-cut method of tunneling consists first in excavating a cut, and second in covering this cut to form an underground passageway, it has been named the “cut-and-cover” method of tunneling. The cut-and-cover method of tunneling is almost never employed elsewhere than in cities, or where the surface of the ground has to be restored for the accommodation of traffic and business. When it is not necessary to restore the original surface, as is usually the case with tunnels built in the ordinary course of railway work, it would obviously be absurd to do so except in extraordinary cases. In a general way, therefore, it may be said that the cut-and-cover method of construction is confined to the building of tunnels under city streets; and the discussion of this kind of tunnels follows logically the general description of the open-cut method of tunneling which has been given.
The three most common purposes of tunnels under city streets are: to provide for the removal of railway tracks from the street surface, and separate the street railway traffic from the vehicular and pedestrian traffic; to provide for rapid transit railways from the business section to the outlying residence districts of the city; and to provide conduits for sewage or subways for water and gas mains, sewers, wires, etc. Within recent yearsthe greatest works of tunneling under city streets have been designed and carried out to furnish improved transit facilities.
—The construction of tunnels under city streets may be divided into two classes, which may be briefly defined as shallow tunnels and deep tunnels. Shallow tunnels, or those constructed at a small depth beneath the surface, are usually built by one of the cut-and-cover methods; deep tunnels, or those built at a great depth, beneath the surface are constructed by any of the various methods of tunneling described in this book, the choice of the method depending upon the character of the material penetrated, and the local conditions.
In building tunnels under city streets the first duty of the engineer is to disturb as little as possible the various existing structures and the activities for which these structures and the street are designed. The character of the difficulties encountered in performing this duty will depend upon the depth at which the tunnel is driven. In constructing shallow tunnels by the cut-and-cover method care has to be taken first of all not to disturb the street traffic any more than is absolutely necessary. This condition precludes the single trench method of open cut tunneling in all places where the street traffic is at all dense, and compels the engineer to use the methods employed in Paris and New York, as previously described, or else the transverse trench or slice method employed in the Boston Subway.
These methods have to be modified when the work is done on streets having underground trolley and cable roads, and in which are located gas and water pipes, conduits for wires, etc. Where underground trolley or cable railways are encountered, a common mode of procedure is to excavate parallel side trenches for the side walls, and turn the roof arch until it reaches the conduit carrying the cables or wires. The earth is then removed from beneath the conduit structure in small sections, and the arch completed as each section is opened. As fast as the arch is completed the conduit structure is supported on it. Wherepipes are encountered they may be supported by means of chains, suspending them from heavy cross-beams, or by means of strutting, or they may be removed and rebuilt at a new level. Generally the conditions require a different solution of this problem at different points.
Another serious difficulty of tunneling under city streets arises from the danger of disturbing the foundations of the adjacent buildings. This danger exists only where the depth of the tunnel excavation extends below the depth of the building foundations, and where the material penetrated is soft ground. Where the tunnel penetrates rock there is no danger of disturbing the building foundations. To prevent trouble of this character requires simply that the excavation of the tunnel be so conducted that there is no inflow of the surrounding material, which may, by causing a settlement of the neighboring material, allow the foundations resting on it to sink.
The Baltimore Belt tunnel, described in apreceding chapter, is an example of the method of work adopted in constructing a tunnel under city streets through very soft ground. This may be classed as a deep tunnel. Another method of deep tunneling under city streets is the shield method, examples of which are given in asucceeding chapter. Two notable examples of cut-and-cover methods of tunneling are the Boston Subway and the New York Rapid Transit Ry., a description of which follows.
—The Boston Subway may be defined as the underground terminal system of the surface street railway system of the city, and as such it comprises various branches, loops, and stations. The subway begins at the Public Garden on Boylston St., near Charles St., and passes with double tracks under Boylston St. to its intersection with Tremont St., where it meets the other double-track branch, passing under Tremont St. and beginning at its intersection with Shawmut Ave. From their intersection at Tremont and Boylston streets the two double-track branches proceed under Tremont St. with fourtracks to Scollay Square. At Scollay Square the subway divides again into two double-track branches, one passing under Hanover St., and the other under Washington St. At the intersection of Hanover and Washington streets the two double-track branches combine again into a four-track line, which runs under Washington St. to its terminus at Haymarket Square, where it comes to the surface by means of an incline. The subway, therefore, has three portals or entrances, located respectively at Boylston St., Shawmut Ave., and Haymarket Square. It also has five stations and two loops, the former being located at Boylston St., Park St., Scollay Square, Adams Square, and Haymarket Square, and the latter at Park St. and Adams Square. The total length of the subway is 10,810 ft.
—The material met with in constructing the subway was alluvial in character, the lower strata being generally composed of blue clay and sand, and the upper strata of more loose soil, such as loam, oyster shells, gravel, and peat. At many points the material was so stable that the walls of the excavation would stand vertical for some time after excavation. Surface water was encountered, but generally in small quantities, except near the Boylston St. portal, where it was so plentiful as to cause some trouble.