70 TUNNELING 



interior of the ring is covered with a cement plaster lining 

 deep enough thoroughly to embed the interior joint flanges. 

 In the succeeding chapter describing the methods of driving 

 tunnels by shields several forms of iron tunnel lining are fully 

 described. 



Iron and Masonry Lining. During recent years a form of 

 combined masonry and iron lining has been extensively em- 

 ployed iii constructing city underground railways in both 

 Europe and America. Generally this form of lining is built 

 with a rectangular section. Two types of construction are 

 employed. In the first, masonry side walls carry a flat roof 

 of girders and beams, which carry a trough flooring filled with 

 concrete, or between which are sprung concrete or brick arches. 

 Sometimes the roof framing consists of a series of parallel 

 I-beams laid transversely across the tunnel, and in other cases 

 transverse plate girders carry longitudinal I-beams. In the 

 second type of construction the roof girders are supported by 

 columns embedded in the side walls. Where the tunnel pro- 

 vides for two or four tracks, intermediate column supports are 

 in some cases introduced between the side columns. In this 

 construction the roofing consists of concrete filled troughs or of 

 concrete or brick arches, as in the construction first described. 

 Examples of combined masonry and iron tunnel lining are 

 illustrated in the succeeding chapter on tunneling under city 

 streets. 



Masonry Lining. The form of tunnel lining most commonly 

 employed is brick or stone masonry. Concrete masonry lining 

 has been employed in several tunnels built in recent years. The 

 masonry lining may inclose the whole section or only a part of 

 it. The floor or invert is the part most commonly omitted; 

 but sometimes also the side walls and invert are both omitted, 

 and the lining is confined simply to an arch supporting the- 

 reof. The roof arch, the side walls, and the invert compose 

 the tunnel lining; and all three may consist of stone or brick 

 alone, or stone side walls may be employed with brick invert 



