50 



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 desciibed 

 .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 timber- 

 ing 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 Bel- 

 gian 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 3J 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 

 <juicksand tunnels the amount of timbering required per lineal 

 foot varies from 3 to 5 cubic yds. Shaft strutting requires 

 from 1 to l cu. yds. of timber per lineal foot. 



Dimensions of Timber. The dimensions of the principal mem- 

 bers 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. 



