RAIL W 'AY CONSTRUCTION. 179 



erection of centering and building of the arching, and afterwards 

 the remaining portion of earthwork can be removed at con- 

 venience. In this manner a tunnel-way may be constructed 

 under streets, gardens, and even under buildings. Being nearly 

 all done in the open, the work is more under control than in an 

 ordinary tunnel, but it is usually very costly. Temporary or 

 diverted roads must be arranged ; the excavated material must 

 generally all be removed by carts, sometimes to long distances ; 

 and provision must be made for diverting the network of sewers, 

 gas, and water pipes which are intercepted along the route. 



Fig. 224 is a sketch of covered-way with brick arching. 

 Fig. 225 illustrates another type where cast-iron girders and 

 jack-arches of brickwork were introduced on account of the 

 small headway. In soft yielding clay it is necessary to construct 

 strong inverts, as indicated in the sketches. Recesses for the 

 platelayers should be provided every ten or fifteen yards. 



The above systems of covered- way were largely adopted in 

 the construction of the underground portions of the Metropolitan 

 Eailway and District Railways in and around London. 



In addition to the ordinary type of tunnel formed by first 

 excavating the material and then lining the opening with brick- 

 work or masonry, tunnels of moderate size have been constructed 

 of cast-iron tubes, similar in section to Fig. 226. The tubes 

 were cast in short segments, bolted together inside, the outer 

 circumference, or surface in contact with the earth or clay, 

 being left free from projections of any kind. By making the 

 segments with bolt-holes exact to template, they were readily 

 fitted together in the work, and a thin layer of suitable packing 

 material placed between the bolting-flanges sufficed to render 

 the tubes water-tight. The tunnelling was carried on by means 

 of a short length of slightly larger tube, or cap, made of plate- 

 iron or steel, which fitted over the leading end of the main tube. 

 The front end of this cap was made very strong, and provided 

 with doors through which the miners could work. A series of 

 hydraulic presses attached to the cap were brought to bear on 

 the bolting-flange of the last completed ring, and as the exca- 

 vated matter was removed by the miners from the front the 

 cap was forced forward by the hydraulic presses, and another 

 ring of cast-iron segments inserted. On the City and South 

 London Railway, constructed on the above system, the small 

 annular space formed round the cast-iron tube by the operation 



