—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 |