174 RAILWAY CONSTRUCTION. 



drills, but they are costly in their installation, and also in their 

 working and maintenance. 



In some tunnels, where the material has been firm and dry, 

 the upper portion of the excavation has been first removed, and 

 the masonry and brickwork lining built in position down to 

 about the springing of the arch, the remainder of the excavation 

 being afterwards taken out, and the side walls built by means of 

 shoring and underpinning. 



In other tunnels the complete section has been excavated 

 and timbered, and the work of building commenced from the 

 foundation of the side walls. A strong continuous invert from 

 side wall to side wall is necessary where passing through soft 

 swelling clay or loose strata intersected with small streams of 

 water. Where the material is very solid and dry, it is not 

 necessary to introduce inverts, but the foundations of the side 

 walls should be laid at such a depth below rail-level as not to be 

 affected by drain- water running through the tunnel. 



The side walls and arching may be either of masonry or 

 brickwork, but should be of the best description, especially for 

 the facework. For brick arching only the best hard-burnt 

 bricks should be used, and the inner or exposed ring should 

 consist of selected hard fire-bricks to withstand the heat and 

 gases escaping from the funnels of the locomotives. The thick- 

 ness of the side walls and arching will depend upon the descrip- 

 tion of material to be supported. In some places a comparative 

 thin lining may be sufficient, while in others extra thickness 

 must be given to resist the great pressure exerted by expanding 

 clay and loose wet strata. 



Weeping -holes, or small drain-pipes, placed low down must be 

 left in the side walls every three or four yards, or closer in very 

 wet places, to allow the water collected at the back of the walls 

 to escape into the side drains of tunnel In building the arch 

 portion every effort should be made to have close solid work 

 without any open joints or spaces through which the water may 

 run, and the crown of the arch and a few feet down on each side 

 should be coated with cement or asphalte to lead all water away 

 from the top to the sides. Water dripping from the under side 

 of the arch on to the line is a great destructor of the permanent 

 way materials, especially the fastenings; and bolts, nuts, fish- 

 plates, and spikes placed in a wet dripping tunnel will not last 

 half the time they would out in the open line, where they would 

 have the sun and wind to dry them. 



