160 TUNNELING 



double opposite wedges carried by a transverse beam below. 

 This transverse beam in turn rests on another transverse beam 

 which is supported by posts carried on blocking on the invert 

 masonry. It is usually made with a butted joint at the middle 

 to permit its removal, since it is so long that the masonry has 

 to be built around its extreme ends. The lagging is of the 

 usual form, and rests on the exterior edges of the curved upper 

 member of the centers. 



Masonry. In the English method of tunneling, the masonry 

 begins with the construction of the invert, and proceeds to the 

 crown of the arch. The lining is built in lengths, or successive 

 rings, corresponding to the length of excavation, which, as pre- 

 viously stated, is from 12 ft. to 20 ft. Each ring or length of 

 lining terminates close to the transverse strutting frame erected 

 at the face of the excavation. Work is first begun on the 

 invert at the point where the preceding ring of masonry ends, 

 and is continued to the transverse strutting frame at the front 

 of the excavation. As fast as the invert is completed, work is 

 begun on the side walls. In very loose soils the longitudinal 

 bars supporting the sides of the excavation are removed after 

 the side walls are built ; but in firmer soils they may be taken 

 out one by one just ahead of the masonry, or in very firm soils 

 it may be possible to remove them entirely before beginning 

 the side walls. In all cases it is necessary to fill the space 

 between the masonry and the walls of the excavation with rip- 

 rap or earth. To build the roof arch the centers are first 

 erected as described above, and the crown bars are removed as 

 previously described by putting them ahead after the arch ring 

 is completed. As with the side walls, the vacant space be- 

 tween the arch ring and the roof of the excavation must 

 be filled in. Usually earth or small stones are used for filling ; 

 but in very loose soils it is sometimes the practice not to 

 remove the poling-boards, but to support them by short brick 

 pillars resting on the arch ring and then to fill around these 

 pillars. 



