Masonry.

Fig. 78.—Sketch Showing Work of Excavating and Timbering Drifts and Headings.Fig. 79.—Sketch Showing Method of Roof Strutting.

Fig. 78.—Sketch Showing Work of Excavating and Timbering Drifts and Headings.

Fig. 79.—Sketch Showing Method of Roof Strutting.

Two methods of strutting the soffit of the excavation are employed, one being a modification of the longitudinal system employed in the English method of tunneling described in asucceeding chapter, and the other a modification of the Belgian systempreviouslydescribed.Fig. 79shows the method of employing the radial strutting of the Belgian system. At the beginning the center top heading is strutted with rectangular bents such as are employed for strutting the drifts. As this heading is enlarged by taking out the haunch sections, radial posts are inserted, as shown byFig. 79, which also indicatesthe method of strutting the side trenches when the excavation is carried downward from the center top heading instead of upward from bottom side drifts.

—Whatever plan of excavation or strutting is employed, the construction of the masonry lining in the German method of tunneling begins at the foundations of the side walls and is carried upward to the roof arch. The invert, if one is required, is built after the center core of earth is removed.

—Tunnel centers are generally employed in the German method of tunneling, a common construction being shown byFig. 80. It is essentially a queen-post truss, the tie beam of which rests on a transverse sill as shown by the illustration. The transverse sill is supported along its central portion by the unexcavated center core of earth, and at its ends either directly on the vertical posts or on longitudinal beams resting on these posts. The diagonal members of the queen-post truss form the bottom chords of small king-post trusses which are employed to build out the exterior member of the center to a closer approximation to the curve of the arch.

Fig. 80.—Sketch Showing Roof Arch Centers and Arch Construction.

—When the bottom side drift plan of excavation is employed, the spoil from the front of the drift is removed in narrow-gauge cars running on a track laid as close as practicable to the center core. These same cars are also employed to take the spoil from the drifts above, through holes left in the ceiling strutting of the bottom drifts. The spoil from the soffit sections may be removed by the same car lines used in excavating the drifts, or a narrow-gauge track may be laid on the top of the center core for this special purpose. In the latter case the soffit tracks are usually connected by means of inclined planes withthe tracks on the bottoms of the side drifts. Generally, however, the separate soffit car line is not used unless the material is of such a firm character that the headings and drifts can be carried a great distance ahead of the masonry work. With the center top heading plan of beginning the excavation, the car track has, of course, to be laid on the top of the center core. The center core itself is removed by means of car tracks along the floor of the completed tunnel.

—Like the Belgian method of tunneling, the German method has its advantages and disadvantages. Since the excavation consists at first of a narrow annular gallery only, the equilibrium of the earth is not greatly disturbed, and the strutting does not need to be so heavy as in methods where the opening is much larger. The undisturbed center core also furnishes an excellent support for the strutting, and for the centers upon which the roof arches are built. Another important advantage of the method is that the construction of the masonry lining is begun logically at the bottom, and progresses upward, and a more homogeneous and stable construction is possible. The great disadvantage of the method is the small space in which the hauling has to be done. The spoil cars practically fill the narrow drifts in passing to and from the front, and interfere greatly with the work of the carpenters and masons. Another objection to the method is that the invert is the very last portion of the lining to be built. This may not be a serious objection in reasonably compact and stable materials, but in very loose soils there is always the danger of the side walls being squeezed together before the invert masonry is in position to hold them apart. Altogether the difficulties are of a character which tend to increase the expense of the method, and this is the reason why to-day it is seldom used even in the country where it was first developed, and for some time extensively employed. For repairing accidents, such as the caving in of completed tunnels, the German method of tunneling is frequently used, because of the ease with which thetimbering is accomplished. In such cases the cost of the method used cuts a small figure, so long as it is safe and expeditious.

In the last few years a modification of the German method was used in this country for the construction of several railroad tunnels. The modification consists in excavating the two-side drifts up to the springing line of the arch of the proposed tunnel. Then a central heading, which is afterward enlarged to the whole section of the tunnel, is excavated close to the crown. At the same time the masonry is constructed from the foundation up in the side drifts. From the floor of the upper section already excavated and strutted, the top of the masonry of the drifts is reached by means of small side cuts; thus the lining is made continuous up to the keystone. The central nucleus or bench is removed after the tunnel has been lined.

The most important tunnel excavated by this method was the Baltimore Belt Line tunnel described as follows:

The Baltimore Belt Ry. Co. was organized in 1890 by officials of the Baltimore & Ohio, and Western Maryland railways, and Baltimore Capitalists, to build 7 miles of double track railway, mostly within the city limits of Baltimore. This railway was partly open cut and embankment, and partly tunnel, and its object was to afford the companies named facilities for reaching the center of the city with their passengers and freight. To carry out the work the Maryland Construction Co. was organized by the parties interested, and in September, 1890, this company let the contract for construction to Ryan & McDonald of Baltimore, Md. The chief difficulties of the work centered in the construction of the Howard-street tunnel, 8350 ft. long, running underneath the principal business section of the city.

—The soil penetrated by the tunnel was of almost all kinds and consistencies, but was chiefly sand of varying degrees of fineness penetrated by seams of loam, clay,and gravel. Some of the clay was so hard and tough that it could not be removed except by blasting. Rock was also found in a few places. For the most part, however, the work was through soft ground, furnishing more or less water, which necessitated unusual precautions to avoid the settling of the street, and consequent damage to the buildings along the line. A large quantity of water was encountered. Generally this water could be removed by drainage and pumps, and the earth be prevented from washing in by packing the space between the timbering with hay or other materials. At points where the inflow was greatest, and the earth was washed in despite the hay packing, the method was adopted of driving 6-in. perforated pipes into the sides of the excavation, and forcing cement grout through them into the soil to solidify it. These pipes penetrated the ground about 10 ft., and the method proved very efficient in preventing the inflow of water.

—The excavation was carried out according to the German method of tunneling. Bottom side drifts were first driven, and then heightened to the springing line of the roof arch. Next a center top heading was driven, and the haunch sections taken out. The object of beginning the excavations by bottom side drifts, was to drain the soil of the upper part of the section. The center core was removed after the side walls and roof arch were completed, its removal being kept from 50 ft. to 75 ft. to the rear of the advanced heading. The dimensions of the side drifts proper were about 8 × 8 ft., but they were often carried down much below the floor level to secure a solid foundation bed for the side walls.

—The side drifts were strutted by means of frames composed of two batter posts resting on boards, and having a cap-piece extending transversely across the roof of the drift. These frames were spaced about 4 ft. apart. The excavation was advanced in the usual way by driving poling-boards at the top and sides, with a slight outward and upward inclination, so that the next frame could be easily inserted leaving spaceenough between it and the sheeting to permit the next set of poling-boards to be inserted. These poling-boards were driven as close together as practicable so as to prevent as much as possible the inflow of water and earth.

Fig. 81.—Sketch Showing Method of Excavating and Strutting Baltimore Belt Line Tunnel.Larger illustration

Fig. 81.—Sketch Showing Method of Excavating and Strutting Baltimore Belt Line Tunnel.

Larger illustration

The center top heading was strutted in the same manner as were the side drifts. The arrangement of the strutting employed in enlarging the center top heading is shown clearly byFig. 81, which also shows the manner of strutting the side drifts and face of the excavation, and of building the masonry.

—Both wood and iron centers were employed in building the roof arch. The timber centering was constructed of square timbers, as shown byFig. 82. This construction of the iron centers is shown byFig. 83. Each of the iron centers consisted of two 6 × 6 in. angles butted together, and bent into the form of an arch rib. Six of these ribs were set up 4 ft.apart. They were made of two half ribs butted together at the crown, and were held erect and the proper distance apart by spacing rods. The rearmost rib was held fast to the completed arch masonry, and in turn supported the forward ribs while the lagging was being placed.

Fig. 82.—Roof Arch Construction with Timber Centers, Baltimore Belt Line Tunnel.

—The side walls of the lining were built first in the bottom side drifts, as shown byFig. 81. They were generally placed on a foundation of concrete, from 1 ft. to 2 ft. thick. As a rule the side walls were not built more than 20 ft. in advance of the arch, but occasionally this distance was increased to as much as 90 ft. The roof arch consisted ordinarily of five rings of brick, but at some places in especially unstable soil eight rings of brick were employed. The arch was built in concentric sections about 18 ft. in length. All thetimber of the strutting above the arch and outside of the side walls was left in place, and the voids were filled with rubble masonry laid in cement mortar. It required about 125 mason hours to build an 18-ft. arch section.Figs. 82and83show various details of the masonry arch work.

Fig. 83.—Roof Arch Construction with Iron Centers, Baltimore Belt Line Tunnel.

Owing to the very unstable character of the soil, considerable difficulty was experienced in building the masonry invert. The process adopted was as follows: Two parallel 12 × 12 in. timbers were first placed transversely across the tunnel, abutting against longitudinal timbers or wedges resting against the side walls. Short sheet piles were then driven into the tunnel bottom outside of these timbers, forming an inclosure similar to a cofferdam, from which the earth could be excavated without disturbing the surrounding ground. The earth being excavated, a layer of concrete 8 ins. thick was placed, and the brick masonry invert constructed on it. In less stable ground each of the above described cofferdams was subdivided by transverse timbers and sheet piling into three smaller cofferdams. Here the masonry of the middle section was first constructed,and then the side sections built. Where the ground was worst, still more care was necessary, and the bottom had to be covered with a sheeting of 11⁄4-in. plank held down by struts abutting against the large transverse timbers. The invert masonry was constructed on this sheeting. Refuge niches 9 ft. high, 3 ft. wide, and 15 ins. deep were built in the side walls.

—In this tunnel, owing to the quick striking of the centers, it was found that the masonry lining flattened at the crown and bulged at the sides. This was attributed to the insufficient time allowed for the mortar to set in the rubble filling. Earth packing was tried, but gave still worse results. Finally dry rubble filling was adopted, with satisfactory results. There was necessarily some sinking of the surface. This resulted partly from the necessity of changing and removing of the timbers, and from the compression and springing of the timbers under the great pressures. The crown of the arch also settled from 2 ins. to 6 ins., due to the compression of the mortar in the joints. The maximum sinking of the surface of the street over the tunnel was about 18 ins.; it usually ran from 1 to 12 ins. Some damage was done to the water and gas mains. This damage was not usually serious, but it of course necessitated immediate repairs, and in some instances it was found best to reconstruct the mains for some distance. At one point along the tunnel where very treacherous material was found, the surface settlement caused the collapse of an adjacent building, and necessitated its reconstruction.

The English method of tunneling through soft ground, as its name implies, originated in England, where, owing to the general prevalence of comparatively firm chalks, clays, shales, and sandstones, it has gained unusual popularity. The distinctive characteristics of the method are the excavation of the full section of the tunnel at once, the use of longitudinal strutting, and the alternate execution of the masonry work and excavation. In America the method is generally designated as the longitudinal bar method, owing to the mode of strutting, which has gained particular favor in America, and is commonly employed here even when the mode of excavation is distinctively German or Belgian in other respects.

Fig. 84.—Diagram Showing Sequence of Excavation in English Method of Tunneling.

—Although, as stated above, the distinctive characteristic of the English method is the excavation of the full section at once, the digging is usually started by driving a small heading or drift to locate and establish the axis of the tunnel, and to facilitate drainage in wet ground. These advance galleries may be driven either in the upper or in the lower part of the section, as the local conditions and choice of the engineer dictate. Whether the advance gallery is located at the top or at the bottom of the section makes no difference in the mode of enlarging the profile. This work always begins at the upper part of the section. A center top heading is driven and strutted by erecting posts carrying longitudinal bars supporting transverse poling-boards. This heading is immediatelywidened by digging away the earth at each side, and by strutting the opening by temporary posts resting on blocking, and carrying longitudinal bars supporting poling-boards. This process of widening is continued in this manner until the full roof section, No. 1,Fig. 84, is opened, when a heavy transverse sill is laid, and permanent struts are erected from it to the longitudinal bars, the temporary posts and blocking being removed. The excavation of part No. 2 then begins by opening a center trench and widening it on each side, temporary posts being erected to support the sill above. As soon as part No. 2 is fully excavated, a second transverse sill is placed below the first, and struts are placed between them. The excavation of part No. 3 is carried out in exactly the same manner as was part No. 2. The lengths of the various sections, Nos. 1, 2, and 3, generally run from 12 ft. to 20 ft., depending upon the character of the soil.

—The strutting in the English method of tunneling consists of a transverse framework set close to the face of the excavation, which supports one end of the longitudinal crown bars, the other ends of which rest on the completed lining. The transverse framework is composed of three horizontal sills arranged and supported as shown byFig. 85. The bottom sillAis carried by vertical posts resting on blocking on the floor of the excavation. From the bottom sill vertical struts rise to support the middle sillB. The top sill, or miners’ sillC, is carried by vertical posts or struts rising from the middle sillB. The vertical struts are usually round timbers from 6 ins. to 8 ins. in diameter; and the sills are square timbers of sufficient section to carry the vertical loads, and generally made up of two posts scarf-jointed and butted to permit them to be more easily handled. In firm soils the struts betweenthe sills are all set vertically, but those at the extreme sides of the roof section are inclined. In loose soils, however, where the sides of the excavation must be shored, the V-bracing shown byFig. 85is employed between one or more pairs of sills as the conditions necessitate. The manner of holding the transverse framework upright is explained quite clearly byFig. 85; inclined props extending from the completed masonry to the sills of the framework being employed. Two props are used to each sill. Sometimes, in addition to the props shown, another nearly horizontal prop extends from the crown of the arch masonry to the middle piece of the strutting.

Fig. 85.—Sketches Showing Construction of Strutting, English Method.

Referring toFig. 85, it will be observed that the longitudinal crown bars are above the extrados of the roof arch. When, therefore, the lining masonry has been completed close up to the transverse framework, the latter is removed, leaving the crown bars resting on the arch masonry; and excavation, which has been stopped while the masonry was being laid, is continued for another 12 ft. to 20 ft., and the transverse framework is erected at the face, and braced or propped against the completed lining as shown byFig. 85. The next step is to place thecrown bars, and this is done by pulling them ahead from their original position over the masonry of the completed section of the roof arch. It will be understood that the crown bars are not pulled ahead their full length at one operation, but are advanced by successive short movements as the excavation progresses, their outer ends being supported by temporary posts until the transverse framework is built at the face of the excavation.

—Two standard forms of centers are employed in the English method of tunneling, as shown byFigs. 86and87. Both consist of an outer portion, constructed much like a typical plank center, which is strengthened against distortion by an interior truss framework. The elemental members of this truss framework take the form of a queen-post truss, as is shown more particularly byFig. 86. InFig. 87the queen-post truss construction is less easily distinguished, owing to the cutting of the bottom tie-beam and other modifications, but it can still be observed. The possibility of cutting the tie-beam as shown inFig. 87, without danger, is due to the fact that the lateral pressures on the haunches of the center counteract the tendency of the center to flatten under load, which is usually counteracted by the tie-beam alone. The object of cutting the tie-beam is to afford room for the props running from the completed masonry to the transverse framework of the strutting as shown byFig. 85.

Figs.86 and 87.—Sketches of Typical Timber Roof-Arch Centers, English Method.

Generally four or five centers are used for each length of arch built. They are set up so that the tie-beams rest ondouble opposite wedges carried by a transverse beam below. This transverse beam in turn rests on another transverse beam which is supported by posts carried on blocking on the invert masonry. It is usually made with a butted joint at the middle to permit its removal, since it is so long that the masonry has to be built around its extreme ends. The lagging is of the usual form, and rests on the exterior edges of the curved upper member of the centers.

—In the English method of tunneling, the masonry begins with the construction of the invert, and proceeds to the crown of the arch. The lining is built in lengths, or successive rings, corresponding to the length of excavation, which, as previously stated, is from 12 ft. to 20 ft. Each ring or length of lining terminates close to the transverse strutting frame erected at the face of the excavation. Work is first begun on the invert at the point where the preceding ring of masonry ends, and is continued to the transverse strutting frame at the front of the excavation. As fast as the invert is completed, work is begun on the side walls. In very loose soils the longitudinal bars supporting the sides of the excavation are removed after the side walls are built; but in firmer soils they may be taken out one by one just ahead of the masonry, or in very firm soils it may be possible to remove them entirely before beginning the side walls. In all cases it is necessary to fill the space between the masonry and the walls of the excavation with riprap or earth. To build the roof arch the centers are first erected as described above, and the crown bars are removed as previously described by pulling them ahead after the arch ring is completed. As with the side walls, the vacant space between the arch ring and the roof of the excavation must be filled in. Usually earth or small stones are used for filling; but in very loose soils it is sometimes the practice not to remove the poling-boards, but to support them by short brick pillars resting on the arch ring and then to fill around these pillars.

—To haul away the material and take in supplies, tracks are laid on the invert masonry. Generally the permanent tracks are laid as fast as the lining is completed. A short section of temporary track is used to extend this permanent track close to the work of the advanced drift.

—The great advantage of the English method of tunneling is that the masonry lining is built in one piece from the foundations to the crown, making possible a strong, homogeneous construction. It also possesses a decided advantage because of the simple methods of hauling which are possible: there being no differences of level to surmount, no hoisting of cars nor trans-shipments of loads are necessary. The chief disadvantage of the method is that the excavators and masons work alternately, thus making the progress of the work slower perhaps than in any other method of tunneling commonly employed under similar conditions. This disadvantage is overcome to a considerable extent when the tunnel is excavated by shafts, and the work at the different headings is so arranged that the masons or excavators when freed from duty at one heading may be transferred to another where excavation or lining is to be done as the case may be. Another disadvantage of the English method arises from the excavation of the full section at once, which in unstable soils necessitates strong and careful strutting, and increases the danger of caving. The fact also that the arch ring has to carry the weight of the crown bars, and their loading at one end while the masonry is green, increases the chances of the arch being distorted.

—The English method of tunneling in its entirety is confined in actual practice pretty closely to the country from which it receives its name. A possible extension of its use more generally is considered by many as likely to follow the development of a successful excavating machine for soft material. The space afforded by the opening of the full section at once, especially adapts the method to the use of excavatorslike, for example, the endless chain bucket excavator used on the Central London Ry., and illustrated inFig. 11. The method also furnishes an excellent opportunity for electric hauling and lighting during construction.

The English method of tunneling has been used in building the Hoosac, Musconetcong, Allegheny, Baltimore and Potomac, and other tunnels in America. The names of the European tunnels built by this method are too numerous to mention here.

In this country tunnels through loose soils are excavated according to the “Crown Bar” or American Method. This consists in opening the whole section of the tunnel before the construction of the lining as in the English Method. It differs from the English method, however, in that many timber structures are erected for the support of the roof, and that the excavation and construction of the lining are far apart, so allowing the miners and the masons to work continuously and without interfering with each other.

Section A-B.

Fig. 88.—Sequence of Excavation in the American Method.Fig. 89.—Strutting the Heading in the American Method.

Fig. 88.—Sequence of Excavation in the American Method.

Fig. 89.—Strutting the Heading in the American Method.

Section C-D.Fig. 90.—Temporary Timbering of the Roof in the American Method.

Section C-D.

Fig. 90.—Temporary Timbering of the Roof in the American Method.

Section E-F.Fig. 91.—Showing Crown Bars Supported by Segmental Arches.

Section E-F.

Fig. 91.—Showing Crown Bars Supported by Segmental Arches.

—The diagram inFig. 88shows the sequence of excavation. The work begins by driving a central heading usually 7 × 8 ft., strutted by means of vertical or batter posts and cap-piece.Fig. 89,[11]the props resting on foot blocks. Between the cap-pieces of the consecutive frames are placed planks driven upward at a slightly inclined angle. After the heading has been excavated and strutted, the floor is lowered by removing the part marked 2 in the figure. The two batter posts supporting the cap-piece are now substituted by two longer ones resting on the floor of part 2 and abutting againstlongitudinal beams which are inserted underneath the cap-pieces. These longitudinal beams are called crown bars. The new batter posts are resting either on foot blocks or sills according to the quality of soil and they are strongly wedged to the crown bars. On each side of these crown bars are inserted poling-boards or planks close to each other, which are driven downward. The part marked 3 in the figure is removed by enlarging the cut 1 × 2 on both sides. The plank, inserted above the crown bar, is driven in either preceding or following the excavation and another crown bar is inserted at the end of this plank. This second crown bar is supported by a prop whose other end abuts against the foot of the rafter strutting the heading. Between this crown bar and the roof of the excavation, other planks are placed transversally to the axis of the tunnel and are driven in until they are supported by a new crown bar, etc. The various props supporting the crown bars are placed radially or in a fan-like manner, similar to the characteristic arrangement of the timbering in the Belgian method. Bracers to strengthen the timbering and the roof of the excavation are inserted longitudinally between the various posts and transversally between the crown bars,Fig. 90. As a rule, only three or four of these radial structures are temporarily erected. A trench is excavated at the side of the part marked 3 in the figure to receive the wall plate which is a heavy timber laid on the floor parallel to the longitudinal axis of the tunnel. On the wall plates are erected the arched timber sets composed of five or seven segments of hewn timbers so as to form a polygonal frame which is wedgedto the crown bars and which will support the arch of the roof. After one of these segmental timber sets is erected the temporary radial structure is removed and the upper section of the tunnel is cleared of any obstruction as the pressures are transferred to the wall plates,Fig. 91. The bench marked 4 in the figure is taken away and the vertical props inserted under the wall plates,Fig. 92.

[11]Figs. 89to91are taken from a paper by S. W. Hopkins inHarvard Engineering Journal, April, ’03, on the Fort George tunnel.

Section G-H.Longitudinal Section.Fig. 92.—Transversal and Longitudinal Section of a Tunnel Excavated and Strutted According to the American Method.

Section G-H.

Longitudinal Section.

Fig. 92.—Transversal and Longitudinal Section of a Tunnel Excavated and Strutted According to the American Method.

—The longitudinal strutting is used in connection with the American method of tunneling. In fact, the strutting consists of a series of longitudinal bars supporting planks laid transversally to the axis of the tunnel and abutting against the roof of the excavation. These crown bars during the excavations and immediately after are temporarily supported by radial timbers forming almost a fan-like structure, but this is soon substituted by a permanent one composed of a polygonal timber frame of five or seven segments which are cut to dimensions. The batter posts of the heading, the radial posts of the temporary timber structure and the crown bars are all round timbers from 10 to 12 ins. in diameter. All the other timbers are square edged, the usual dimensions being 10 × 10 ins. or 12 × 12 ins. with the exception of the wall plates which are 14 × 14 ins. The dimensions of the various members of the strutting and the distance apart of the different frames vary with the quality ofthe soil. For instance, in ordinary loose soils the frames are placed between 4 to 6 ft., but in very soft soils they are erected only 3 or 31⁄2ft. apart.

Chiefly in the southwest, in tunnels excavated according to the American method, the timbering has been left as regular lining and it was only after many years when this temporary structure had decayed or was burned down, that the tunnels were lined with masonry. But in many instances the whole timber structure was left in place even when the tunnel was lined with masonry immediately after the excavation had been made. This was usually done when the tunnel was lined with concrete masonry. In such a case the timbering was left to support the pressures of the roof while the concrete was plastic and before it hardened.

—In the American method the whole section of the tunnel is open before the construction of the lining, thus the masonry can be built from the foundations up. The centers are designed so as to support only the weight of the masonry during its construction and not the pressures of the tunnel as in the other methods and consequently they are of light construction. The centers described in the Murray Hill tunnel, page 123, may be advantageously used in building the concrete lining in tunnels through loose soils excavated by the American method.

—The excavation of the heading and the upper section of the tunnel is usually far ahead of the bench, consequently the hauling of both the débris and the building materials is made at two different levels, viz., on the bench and on the floor of the tunnel. When the face of the heading and the excavation of the bench are not more than 50 ft. apart, the hauling can be conveniently done on the tunnel floor, while the materials and débris on the upper section of the tunnel are hauled by wheelbarrows or light cars propelled by handpower. For a greater distance, however, it is more convenient to use light cars running on narrow-gauge tracks all through the tunnel. In this case the tracks on the tunnel floor and on top of thebench are connected by means of an inclined platform where the cars may ascend and descend without interfering with the excavation of the bench. Here, as a rule, tunnels have been excavated in soils considered good, generally through rock, while loose soils have been encountered only in small sections. The same method of excavation for whatever material is encountered is certainly very convenient, as it affords a great regularity in the work; hence its extensive use. A great disadvantage of this method is the double strutting, viz., the polygonal and the longitudinal strutting succeeding each other, whereas one of them could be easily spared. Another defect is that it requires a larger amount of excavation, in case the strutting is left in place.

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.

—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 sectionat 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 byFig. 94which is used in firm soils.

Figs.93 and 94.—Diagrams Showing Sequence of Excavation in Austrian Method of Tunneling.

—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 byFig. 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 byFig. 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 nexttask 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 ata,Fig. 96. The next operation is to place the timberb, having one end resting on the cap-piece of the top heading frame and the other beveled and resting on the top of the sillanear the end. The timberbis laid tangent to the curve of the roof arch, and to support it against flexure the strutcis inserted as shown. To support the thrust of this strut the additional postdis inserted and the original bottom heading frame is reinforced as shown. The next step is to insert the strute, 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 andstrutted by extending the bottom drift cap-piece by a timber similar to timberaabove, and then by inserting a side strut between the outer ends of these two timbers, as indicated byFig. 97. As the final parts. Nos. 7 and 8, are removed, the inclined propa,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 byFig. 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 atb,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 beamc, 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.

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