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and roof arch. Rubble-Btone masonry is usually employed, 

 except at the entrances, where the masonry is utpoaad to view. 

 Here ashlar masonry is usually used. The stone adapted for 

 tunnel lining should be of a durable quality which weathers 

 well. Where bricks are used they should be of good qual- 

 ity. Owing to the comparative ease with which brick arches 

 can be built, thev are generally used to form the roof arch, even 

 where the side walls are of stone masonry. Masonry lining 

 may l>e built in the form of a series of separate rings, or in the 

 form of a continuous structure extending from one end of the 

 tunnel to the other. The latter method of construction pro- 

 duces a stronger structure; but in case of failure by crush- 

 ing, the<dam.ix r f done is likely to be more widespread than 

 where separate rings are employed, one or two of which 

 in iv fail without injury to the others adjacent to them. The 

 instruction is also somewhat simpler where str INI rate rings are 

 employed, since no provision has to l>e made for 1 Minding the 

 whole lining into a continuous structure. Where a series of 

 separate rings is employed, the length of each ring runs from 

 ."> ft. up to 20 ft., it depending upon the character of the 

 rial penetrated, and the method of construction employed. 

 the purpose of detailed discussion the construct inn of 

 >:iry lining may l>e divided into four parts, the side-wall 

 foundations, the side walls themselves, the roof arch, and the 

 invert. 



Foundation*. In tunnels through rook of a hard and dur- 

 able character the foundations for th side walls aro usually 

 laid directly on the rock. In loose rock, or rock liable to dis- 

 integration, this method of construction is not generally a safe 

 one, and the foundation excavation should lie sunk to a depth 

 at which the atmospheric influences cannot affect the founda- 

 tion bed. In either case the foundation masonry is made 

 thicker than that of the side walls proper, so as to distribute 

 the pressure over a greater area, and to afford more room for 

 adjusting the side- wall masonry to the proper profile. In 



