68 TUNNELING 



CHAPTER VIII. 

 METHODS OF LINING TUNNELS. 



TUNNELS in soft soils and in loose rock, and rock liable to 

 disintegration, are always provided with a lining to hold the 

 walls and roof in place. This lining may cover the entire 

 sectional profile of the tunnel, or only a part of it, and it may 

 be constructed of timber, iron, iron and masonry, or, more 

 commonly, of masonry alone. 



Timber Lining. Timber is seldom employed in lining 

 tunnels except as a temporary expedient, and is replaced by 

 masonry as soon as circumstances will permit. In the first 

 construction of many American railways, the necessity for 

 extreme economy in construction, and of getting the line open 

 for traffic as soon as possible, caused the engineers to line 

 many tunnels with timber, which was plentiful and cheap. 

 Except for their small cost and the ease and rapidity with 

 which they can be constructed, however, these timber linings 

 possess few advantages. It is only the matter of a few years 

 when the decay of the timber makes it necessary to rebuild 

 them, and there is always the serious danger of fire. In 

 several instances timber-lined tunnels in America have been 

 burned out, causing serious delays in traffic, and necessitating 

 complete reconstruction. Usually this reconstruction has con- 

 sisted in substituting masonry in place of the original timber 

 lining. In a succeeding chapter a description will be given of 

 some of the methods employed in replacing timber tunnel 

 linings with masonry. Various forms of timber lining are 

 employed, of which Fig. 44 and the illustrations in the chapter 



