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.

Fig. 19.—Ceiling Strutting for Tunnel Roofs.

Fig. 20.—Ceiling Strutting with Side Post Supports.

Fig. 21.—Sill, Side Post and Cap Cross Frame Strutting.

Fig. 22.—Reinforced Cross Frame Strutting for Treacherous Materials.

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.

Fig. 23.—Longitudinal Poling-Board System of Roof Strutting.

Fig. 24.—Transverse Poling-Board System of Roof Strutting.

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.

Fig. 25.—Shaft with Single Transverse Strutting.

Fig. 26.—Rectangular Frame Strutting for Shafts.

Fig. 27.—Reinforced Rectangular Frame Strutting for Shafts in Treacherous Materials.

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.	Com- pact.	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