CHAPTER V.

METHODS OF TIMBERING OR STRUTTING TUNNELS.

The purpose of timbering or strutting in tunnel work is to prevent the caving-in of the roof and side walls of the excavation previous to the construction of the lining. As the strutting has to resist all the pressures developed in the roof and side walls, which may be exceedingly troublesome and of great intensity in loose soils, its design and erection call for particular care. The method of strutting adopted depends upon the method of excavation employed; but in every case the problem is not only to build it strong enough to withstand the pressures developed, but to do this as economically as possible, and with as little hindrance as may be to the work which is going on simultaneously and which will come later. Only the latter general problems of strutting peculiar to all methods of tunnel work will be considered here. For this consideration strutting may be classified according to the material of which it is built, under the heads of timber structures and iron structures.

TIMBER STRUTTING.

Timber is nearly always employed for strutting in tunnel work. So long as it has the requisite strength, any kind of timber is suitable for strutting, since, it being only temporarily employed, its durability is a matter of slight importance. Timber with good elastic properties, like pine or spruce, is preferably chosen, since it yields gradually under stress, thus warning the engineer of the approach of danger; while oak and other strong timbers resist until the last moment, and then yield suddenly under the breaking load. Soft woods, moreover, are usually lighter in weight than hard woods, which is a considerable advantage where so much handling is required in a restricted space. Round timbers are generally employed, since they are less expensive, and quite as satisfactory in other respects as sawed timbers. In the English and Austrian methods of strutting, which are described further on, a few of the principal struts are of sawed timbers.

The various timbers of the strutting are seldom attached by framed joints, but wedges are used to give them the necessary bearing against each other. Where framed joints are employed they are made of the simplest form usually by halving the joining timbers, as shown by Fig. 17. Fig. 18 shows a form of joint used where round posts carry beams of similar shape. The reason why it is possible to do away with jointed connections to such a great extent, is that the strains which the timbers have to resist are either compressive or bending strains, and because the timbers are so short that they do not require to be spliced.

=Strutting of Headings.=--The method of strutting the heading that is employed depends upon the material through which the heading is driven. In solid rock strutting may not be required at all, or only for the purpose of preventing the fall of loose blocks from the roof, then vertical props are erected where required, or horizontal beams are inserted into the side walls, as shown by Fig. 19. These horizontal beams may be used singly at dangerous places, or they may be placed from 2 ft. to 3 ft. apart all along the heading. In the latter case they usually carry a lagging of planks, which may be placed at intervals or close together, and filled above with stone in case the roof of the excavation is very unstable. Planks used in this manner are usually called poling-boards. Where the side walls as well as the roof require support, vertical side posts are employed to carry the roof beams, as shown by Fig. 20; and, when necessary, poling-boards are inserted between these posts and the walls of the excavation.

_Frame Strutting._--In very loose soils not only the roof and side walls, but also the floor of the heading require strutting. In these cases frame strutting is employed, as shown by Fig. 21. It consists simply of a rectangular frame; at the top there is a crown bar supported by two vertical side posts setting on a sill laid across the bottom of the heading. These frames are spaced at close intervals, and carry longitudinal planks or poling-boards. The sill of the frame is sometimes omitted when the soil is stable enough to permit it, and in its place wooden footing blocks are substituted to carry the side posts. In soils where the pressures are great enough to bend the crown bar, a secondary frame is employed, as shown by Fig. 22, the two inclined roof members, or rafters, of which support the crown bar at the center.

It is the more common practice in driving headings through soft soils to use inclined poling-boards to support the roof. Fig. 23 shows one method of doing this. The method of operation is as follows: Assuming the poling-boards _a_ and _b_ to be in place, and supported by the frames _A_, _B_, _C_, as shown, the first step in continuation of the work is to insert the poling-board _c_ over the crown bar of frame _C_, and under the block _m_. Excavation is then begun at the top, and as fast as the soil is removed ahead of it the poling-board _c_ is driven ahead until its rear end only slightly overhangs the crown bar of frame _C_. The remainder of the face of the heading is then excavated nearly to the front end of the poling-board _c_, and another frame is set up. By a succession of these operations the heading is advanced. The poling-boards at the sides of the heading are placed in a similar manner to the roof poling-boards. A second method of using inclined poling-boards is shown by Fig. 24. Here the poling-boards run transversely, and are supported by the arrangement of timbering shown. The chief advantage of using these inclined poling-boards, particularly in the manner shown by Fig. 23, is that the excavators work under cover at all times, and are thus safe from falling fragments or sudden cavings.

_Box Strutting._--In very treacherous soils, such as quicksand, peat, and laminated clay, box strutting is commonly employed. The method of building this strutting is to set up at the face of the work a rectangular frame, and use it as a guide in driving a lagging or boxing of horizontal planks into the soft soil ahead. These planks have sharp edges, and are driven to a distance of 2 ft. or 3 ft. into the face of the heading, so as to inclose a rectangular body of earth. This earth is excavated nearly to the ends of the planks, and then another frame is inserted close up against the new face of the excavation, which supports the planks so that the remainder of the earth included by them may be removed. These two frames, with their plank lagging, constitute a “box;” and a series of these boxes, one succeeding another, form the strutting of the heading.

=Strutting the Face.=--In some cases it is found necessary to strut the face of the heading in order to prevent it from caving in. This is generally done by setting plank vertically, and bracing them up by means of inclined props whose feet abut against the sill of the nearest cross frame. This strutting is erected while the workmen are placing the side and roof strutting, and is removed to permit excavation.

=Full Section Timber Strutting.=--For strutting the full section two forms of timbering are employed, known as the polygonal system and the longitudinal system.

Longitudinal strutting consists of a timber structure so arranged as to have all the principal members supporting the poling-boards parallel to the axis of the tunnel. This system of strutting is peculiar to the English method of tunneling. The longitudinal timbers rest on this finished masonry at one end, and are carried on a cross frame or by props at the other end. At intermediate points the longitudinals are braced apart by struts in planes transverse to the tunnel axis. This construction makes a very strong strutting framework, since the transverse struts act as arch ribs to stiffen the longitudinals; but the use of transverse poling-boards requires the excavation of a larger cross-section than is necessary when longitudinal poling-boards are employed, and this increases the cost both for the amount of earth excavated and the greater quantity of filling required.

In polygonal strutting the main members are in a plane normal to the axis of the tunnel. They form a polygon whose sides follow closely the sectional profile of the excavation. These polygonal frames are placed at more or less short intervals apart, and are braced together by short longitudinal struts lying close to the sides of the excavation, and running from one frame to the next, and also by longer longitudinal members which extend over several frames. The polygonal system of strutting is peculiar to the Austrian method of tunneling, and is fully described in a succeeding chapter. One of its distinctive characteristics is that the poling-boards are inserted parallel to the tunnel axis. Polygonal strutting is generally held to be stronger than longitudinal strutting under uniform loads, but it is more liable to distortion when the loads are unsymmetrical.

=Strutting of Shafts.=--Tunnel shafts are strutted both to prevent the caving-in of the sides and to divide them into compartments. When the material penetrated is very compact, and caving is not likely, a single series of transverse struts, one above the other, running from the top to the bottom of the shaft, as shown by Fig. 25, is used to divide it into two compartments. In softer material, where the sides of the shaft require support, Fig. 26 shows a form of strutting commonly employed. It consists of vertical corner posts braced apart at intervals by four horizontal struts placed close to the walls of the shaft. The longer side struts are also braced apart at the center by a middle strut which divides the shaft into two compartments. A lagging of vertical plank is placed between the walls of the shaft and the horizontal side struts. In very loose soils the form of strutting shown by Fig. 27 is employed. This is practically the same construction as is shown by Fig. 26, with the addition of an interior polygonal horizontal bracing in each half of the shaft. Referring to Fig. 27, the timbers _a_, _a_, etc., are vertical and continuous from the top to the bottom of the shaft; and the horizontal timbers, _b_, _b_, etc., are spaced at more or less close intervals vertically. The lagging planks may be laid with spaces between them, or close together, or, in case of very loose material, with their edges overlapping. The manner of constructing the strutting is also governed by the stability of the soil. In firm soils it is possible to sink the shaft quite a depth without timbering, and the timbering can be erected in sections of considerable length, which is always an advantage, but in loose soils the timbering has to follow closely the excavation.

The solid wall shaft struttings which have been described are discontinued at the point where the shaft intersects the tunnel excavation; and from this point to the floor of the tunnel an open timbering is employed, whose only duty is to support the weight of the solid strutting above. This timbering is made in various forms, but the most common is a timber truss or arch construction which spans the tunnel section.

=Quantity of Timber.=--The quantity of timber employed in strutting a tunnel varies with the character of the material through which the tunnel is excavated: it is small for solid-rock tunnels, and large for soft-ground tunnels. In the Belgian method of excavation a smaller quantity of timber is used than in any of the other ordinary methods. For single-track tunnels excavated by this method there will be needed on an average about 3 to 3¹⁄₃ cu. yds. of timber per lineal foot of tunnel. Practical experience shows that about four-fifths of the timber once used can be employed for the second time. In any of the methods in which the whole tunnel section is excavated at once, the average amount of timber required per lineal foot is about 8.7 cu. yds. Of this amount about two-thirds can be used a second time. In the Italian method, in which the upper half and the lower half are excavated separately, about 5 cu. yds. of timber are required per lineal foot of tunnel, about one-half of which can be employed a second time. For quicksand tunnels the amount of timbering required per lineal foot varies from 3 to 5 cubic yds. Shaft strutting requires from 1 to 1¹⁄₂ cu. yds. of timber per lineal foot.

=Dimensions of Timber.=--The dimensions of the principal members composing the strutting of headings, full section, and shafts, are given in Table I. The planks used for lagging or the poling-boards are usually from 4 ins. to 6 ins. wide, with a length depending upon the method of strutting employed.

TABLE I.

Showing Sizes of Various Timbers Used in Strutting Tunnels Driven Through Different Materials.

+---------------------------------+-----------+----------------------+ | | ROCK. | SOFT SOILS. | | +-----+-----+--------+------+------+ | |Hard.|Soft.|Compact.|Loose.| Very | | | | | | |loose.| | +-----+-----+--------+------+------+ | | ins.| ins.| ins. | ins. | ins. | |Headings: | | | | | | | Cap-pieces and vertical struts | 6 | 8 | 10 | 12 | 14 | | Sills | | | 8 | 10 | 12 | | Struts | 5 | 5 | 6 | 7 | 8 | | Distance apart of the frames in | | | | | | | feet | 6 | 4.5| 3 | 2.6 | 2.6 | | | | | | | | |Strutting of the tunnel, | | | | | | |longitudinal strutting: | | | | | | | Crown bars | 12 | 14 | 14 | | | | Props vertical or inclined | | | | | | | supporting the crown bars | 10 | 12 | 14 | | | | Sills | 8 | 8 | 10 | | | | Cap-pieces or saddles | 10 | 12 | 14 | | | | Struts to stiffen the structure | 6 | 8 | 10 | | | | Distance apart of the frames (in| | | | | | | feet) | 4.5 | 4 | 3 | | | | | | | | | | |Polygonal strutting: | | | | | | | Cap-pieces and contour pieces | 8 | 10 | 12 | 14 | 16 | | Vertical struts on top | 10 | 12 | 14 | 16 | 18 | | Vertical struts below | 12 | 14 | 16 | 20 | 24 | | Intermediate sills | 12 | 14 | 16 | 20 | 24 | | Lower sills | | | 12 | 16 | 18 | | Raking props | 10 | 10 | 10 | 12 | 12 | | Distance apart of the frames (in| | | | | | | feet) | 6 | 4.5 | 4 | 3 | 3 | | | | | | | | |Shafts: | | | | | | | Horizontal beams forming the | | | | | | | frame | 8 | 8 | 10 | 12 | 14 | | Transverse beams | 8 | 8 | 8 | 10 | 12 | | Vertical struts between the | | | | | | | frames | 8 | 8 | 10 | 12 | 12 | | Struts to reënforce the frame | | 6 | 8 | 8 | 8 | | Distance apart of the strutting | | | | | | | (in feet) | 6 | 4.5 | 4 | 3 | 2.6 | +---------------------------------+-----+-----+--------+------+------+

IRON STRUTTING.

In 1862 Mr. Rziha employed old iron railway rails for strutting the Naensen tunnel, and his example was successfully followed in several tunnels built later where timber was scarce and expensive. The advantages which iron strutting is claimed to possess over the more common wooden structure are: its greater strength; the smaller amount of space which it takes up; and the fact that it does not wear out, and may, therefore, be used over and over again.

=Iron Strutting in Headings.=--In strutting the headings the cross frames have a crown bar consisting of a section of old railway rail carried either by wood or iron side posts. When wooden side posts are used their upper ends have a dovetail mortise, and are bound with an iron band, as shown by Fig. 28. The base of the rail crown bar is set into the dovetail mortise and fastened by wedges. When iron side posts are employed they usually consist of sections of railway rails, and the crown bar is attached to them by fish-plate connections, as shown by Fig. 29. The iron cross frames are set up as the heading advances, and carry the plank lagging or poling-boards, exactly in the same manner as the timber cross frames previously described.

=Full Section Iron Strutting.=--The iron strutting devised by Mr. Rziha for full section work is shown by Fig. 30. Briefly described, it consists of voussoir-shaped cast-iron segments, which are built up in arch form. Fig. 31 shows the construction of one of the segments, all of which are alike, with the exception of the crown segment, which has a mortise and tenon joint which is kept open by filling the mortise with sand. The segments are bolted together by means of suitable bolt-holes in the vertical flanges, and when fully connected form an arch rib of cast iron. This arch rib, A, Fig. 30, carries a series of angle or T-iron frames bent into approximately voussoir shape, as shown at B, Fig. 30. Above these frames are inserted the poling-boards, running longitudinally, and spanning the distance between consecutive arch ribs. By removing the bent iron frames the cast-iron rib forms a center upon which to construct the masonry. Finally, to remove the cast-iron rib itself, the sand is drawn out of the mortise and tenon joint in the crown segment, which allows the joint to close, and loosen the segments so that they are easily unbutted.

The illustration, Fig. 30, shows longitudinal poling-boards; more often longitudinal crown bars of railway rails span the space between connective arch ribs, and support transverse poling-boards. In building the masonry, work is begun at the bottom on each side, the bent iron frames being removed one after another to give room for the masonry. As each frame is removed, it is replaced with a sort of screw-jack to support the poling-boards until the masonry is sufficiently completed to allow their removal. The interior bracing of the arch rib shown at _a a_ and _b b_ consists of railway rails carried by brackets cast on to the segments. A similar bracing of rails connects the successive arch ribs. These lines of bracing serve to carry the scaffolding upon which the masons work in building the lining.

=Iron Shaft Strutting.=--In soft-ground shaft work, the use of an iron strutting, consisting of consecutive cast-iron rings, has sometimes been employed to advantage. Fig. 32 shows the construction of one of these rings, which, it will be seen, is composed of four segments connected to each other by means of bolted flanges. The holes shown in the circumferential web of the ring are to allow for the seepage from the earth side walls. The method of placing this cylindrical strutting is to start with a ring having a cutting-edge. By means of excavation inside the ring, and by ramming, the ring is sunk into the ground a distance equal to its height. Another ring is then fastened by special hooks on top of the first one, and the sinking continued until the second ring is down flush with the surface. A third ring is then added, and so on until the entire shaft is excavated and strutted. As in timber shaft strutting, the solid iron ring strutting is carried down only to the top of the tunnel section, and below this point there is an open timber or iron supporting framework.