CHAPTER XIV.

THE FULL SECTION METHOD OF TUNNELING: ENGLISH METHOD; AMERICAN METHOD; AUSTRIAN METHOD.

ENGLISH METHOD.

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.

=Excavation.=--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 immediately widened 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.

=Strutting.=--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 by Fig. 85. The bottom sill _A_ is carried by vertical posts resting on blocking on the floor of the excavation. From the bottom sill vertical struts rise to support the middle sill _B_. The top sill, or miners’ sill _C_, is carried by vertical posts or struts rising from the middle sill _B_. 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 between the 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 by Fig. 85 is employed between one or more pairs of sills as the conditions necessitate. The manner of holding the transverse framework upright is explained quite clearly by Fig. 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.

Referring to Fig. 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 by Fig. 85. The next step is to place the crown 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.

=Centers.=--Two standard forms of centers are employed in the English method of tunneling, as shown by Figs. 86 and 87. 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 by Fig. 86. In Fig. 87 the 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 in Fig. 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 by Fig. 85.

Generally four or five centers are used for each length of arch built. They are set up so that the tie-beams rest on double 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.

=Masonry.=--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.

=Hauling.=--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.

=Advantages and Disadvantages.=--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.

=Conclusion.=--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 excavators like, for example, the endless chain bucket excavator used on the Central London Ry., and illustrated in Fig. 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.

AMERICAN METHOD.

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.

=Excavation.=--The diagram in Fig. 88 shows 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 against longitudinal 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 wedged to 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. 89 to 91 are taken from a paper by S. W. Hopkins in _Harvard Engineering Journal_, April, ’03, on the Fort George tunnel.

=Strutting.=--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 of the 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 3¹⁄₂ ft. 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.

=Centers.=--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.

=Hauling.=--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 the bench 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.

AUSTRIAN METHOD.

The Austrian full-section method of tunneling through soft ground was first used in constructing the Oberau tunnel on the Leipsic and Dresden R.R., in Austria in 1837. It consists in excavating the full section and building up the lining masonry from the foundations as in the English, but with the important exception that the invert is built last instead of first in all cases except where the presence of very loose soil requires its construction first. A still more important difference in the two methods is that the excavation is carried out in smaller sections and is continuous in the Austrian method instead of alternating with the mason work as it does in the English method.

=Excavation.=--The excavation in the Austrian method begins by driving the bottom center drift No. 1, Fig. 93, rising from the floor of the tunnel section nearly to the height of the springing lines of the roof arch. When this drift has been driven ahead a distance varying from 12 ft. to 20 ft. or sometimes more, the excavation of the center top heading No. 2 is driven for the same distance. The next operation is to remove part No. 3, thus forming a central passage the full depth of the tunnel section at the center. This trench is enlarged by removing parts Nos. 4, 5, 6, 7, and 8 in the order named until the full section is opened. A modification of this plan of excavation is shown by Fig. 94 which is used in firm soils.

=Strutting.=--Each part of the section is strutted as fast as it is excavated. The center bottom drift first excavated is strutted by laying a transverse sill across the floor, raising two side posts from it, and capping them with a transverse timber having its ends projecting beyond the side posts and halved as shown by Fig. 95. The top center heading No. 2, which is next excavated, is strutted by means of two side posts resting on blocking and carrying a transverse cap as also shown by Fig. 95. Sometimes the side posts in the heading strutting-frames are also carried on a transverse sill as are those of the bottom drift. This construction is usually adopted in loose soils. When the sill is employed, the middle part, No. 3, is strutted by inserting side posts between the bottom of the top sill and the cap of the frame in the drift below. When, however, the posts of the top heading frame are carried on blocking, it is the practice to replace them with long posts rising from the cap of the bottom drift frame to the cap of the top heading frame. Further, when the intermediate sill is employed at the bottom level of the top heading it projects beyond the side posts and has its ends halved.

After the completion of the center trench strutting the next task is to strut parts Nos. 4 and 5. This is done by continuing the upper sill by means of a timber having one end halved to join with the projecting end of the sill in position. This extension timber is shown at _a_, Fig. 96. The next operation is to place the timber _b_, having one end resting on the cap-piece of the top heading frame and the other beveled and resting on the top of the sill _a_ near the end. The timber _b_ is laid tangent to the curve of the roof arch, and to support it against flexure the strut _c_ is inserted as shown. To support the thrust of this strut the additional post _d_ is inserted and the original bottom heading frame is reinforced as shown. The next step is to insert the strut _e_, and when this and the previous construction are duplicated on the opposite side of the tunnel section we have the strutting of the parts Nos. 1 to 5; inclusive, complete. Part No. 6 is then removed and strutted by extending the bottom drift cap-piece by a timber similar to timber _a_ above, and then by inserting a side strut between the outer ends of these two timbers, as indicated by Fig. 97. As the final parts. Nos. 7 and 8, are removed, the inclined prop _a_, Fig. 97, is inserted as shown. When the soil is loose some of the members of the framework are doubled and additional bracing is introduced as shown by Fig. 97.

The frames just described are placed at intervals of about 4 ft. along the excavation, and are braced apart by horizontal struts. Some of the longitudinal bearing beams, as at _b_, Fig. 97, also extend through two or three frames, and help to tie them together. Finally, the longitudinal poling-boards extending from one frame to the next along the walls of the excavation serve to connect them together. The short transverse beam _c_, Fig. 90, located just above the floor of the invert, serves to carry the planking upon which the train car tracks are laid. Besides the timber strutting peculiar to the Austrian method, the Rziha iron strutting described in a previous chapter is frequently used in tunneling by the Austrian process.

=Centers.=--The two forms of centers used in the English method of tunneling are also used in the Austrian method. One of the methods of supporting these centers is shown by Fig. 98. The tie-beam of the center rests on longitudinal timbers carried by the strutting frames and intermediate props. In single-track tunnels it is the frequent practice also to carry the ends of the tie-beams in recesses left in the side wall masonry, with intermediate props inserted to prevent flexure at the center. When the Rziha iron strutting is employed, it also serves for the centering upon which the arch masonry is built.

=Masonry.=--In the Austrian system of tunneling, the lining is built from the foundations of the side walls upward to the crown of the roof arch in lengths in consecutive rings equal to the lengths of the consecutive openings of the full section, or from 12 ft. to 20 ft. long. Except in infrequent cases in very loose materials the invert is the last part of the masonry to be built, since to build it first requires the removal of the strutting which cannot easily or safely be accomplished until the side walls and roof arch are completed. As the side wall foundations are built, however, their interior faces are left inclined, as shown by Figs. 97 and 98, ready for the insertion of the invert, and are meanwhile kept from sliding inward by the insertion of blocking between them and the bottom of the strutting. Fig. 98 shows the nature of this blocking, and also the manner in which the side wall and roof arch masonry is carried upward. Finally when the roof arch is keyed and the centers are struck, the strutting is taken down and the invert is built.

=Advantages and Disadvantages.=--The principal advantages claimed for the Austrian method of tunneling are: (1) The excavation being conducted by driving a large number of consecutive small galleries, which are immediately strutted, there is little disturbance of the surrounding material; (2) the polygonal type of strutting adopted is easily erected and of great strength against symmetrical pressures; (3) the masonry, being built from the foundations up, is a single homogeneous structure, and is thus better able to withstand dangerous pressures; (4) the excavation is so conducted that the masons and excavators do not interfere, and both can work at the same time. The disadvantages which the method possesses are: (1) The strutting while very strong under symmetrical pressures, either vertical or lateral, is distorted easily by unsymmetrical vertical or lateral pressures, and by pressure in the direction of the axis of the tunnel; (2) the construction of the invert last exposes the side walls to the danger of being squeezed together, causing a rotation of the arch of the nature discussed in describing the Belgian method of tunneling.