i;6 RAILWAY CONSTRUCTION. 



Small arched recesses or niches should be formed in the side 

 walls at convenient distances to serve as refuges for platelayers 

 or others working in the tunnels. 



It is most essential that the space between the masonry and 

 brickwork lining and the facework of the excavation should be 

 carefully filled in and hard packed, so as to prevent the possi- 

 bility of pieces of rock or other material falling on to the top of 

 the arch. The neglect of this precaution may lead to a casualty 

 years after the tunnel has been completed. 



It would be impossible to over-rate the importance of a 

 constant faithful supervision of the building of the lining, 

 especially the arching. The work has to be carried on by 

 workmen in cramped positions, with imperfect light, and sur- 

 rounded by all kinds of obstacles and inconveniences, and unless 

 a detailed inspection be rigidly maintained, a carelessness in the 

 selection of the materials, and a laxity in the workmanship, will 

 be the inevitable result. 



Figs. 204 to 219 are sections of tunnels which have been 

 constructed for double and single line railways. The sections 

 give the normal form and dimensions adopted in each case, 

 although there may have been many portions of the work where 

 unfavourable or treacherous material necessitated an increase in 

 the thickness of the side walls, or of the arching, or of both. 

 The types vary in accordance with the opinions of the designers 

 as to the most suitable section for the purpose, and range from 

 the comparatively thin lining and vertical side walls shown on 

 Fig. 207, to the almost circular form and very thick lining shown 

 on Fig. 216. The latter is the section which experience has 

 proved to be the best to sustain the enormous all-round pressure 

 exerted by certain descriptions of swelling clay. 



Careful judgment will be required to decide which parts of 

 a rock tunnel may be left unlined. The apparently solid-looking 

 portions are oftentimes deceptive, and numbers of instances are 

 on record of large pieces of rock falling down in tunnels which 

 for many years had been considered as thoroughly secure. 

 Where there is any doubt it is better and safer to put in a 

 lining, even if only to the extent of an arching springing from 

 side walls of solid rock, as shown on Fig. 206. A moderate 

 additional expenditure at the time of construction may prevent 

 a serious catastrophe afterwards. 



The faces or entrances to tunnels may be constructed with 



