138 TUNNELING 



dimensions to the tie beams, contrary to the usual practice in 

 constructing centers, is that it has to serve as a sill for distrib- 

 uting the pressure to the foundation of unexcavated soil 

 which supports the center. Sometimes a sub-sill is used to 

 support the center upon the soil ; and in any case wedges are 

 employed to carry it, which can be removed for the purpose of 

 striking the center. After the arch is completed, the centers 

 may be removed immediately, or may be left in position until 

 the masonry has thoroughly set. In either case the leading 

 center over which the arch masonry terminates temporarily is 

 left in position until the next section of the arch is built. 



Masonry. The masoniy of the roof arch, which is the first 

 part built, is of necessity begun at the springing lines, and the 

 first course rests on short lengths of heavy planks. These 

 planks, besides giving an even surface upon which to begin the 

 masonry, are essential in furnishing a bearing to the struts 

 inserted to support the arch while the earth below them, part 

 No. 5, Fig. 61, is being excavated. As the arch masonry 

 progresses from the springing lines upward, the radial posts 

 of the strutting are removed, and replaced by short struts rest- 

 ing on the lagging of the centers, which support the crown 

 bars or longitudinals until the masonry is in place, when they 

 and the poling-boards are removed, and the space between the 

 arch masonry and walls of the excavation is filled with stone 

 or well-rammed earth. 



Considering now the side wall masonry, it will be re- 

 membered that in excavating the part No. 5, Fig. 61, of the 

 section, frequent side trenches were excavated, and struts 

 inserted to take the weight of the masonry. These struts are 

 inserted on a batter, with their feet near the center of the 

 tunnel floor, so that the side wall masonry may be carried up 

 behind them to a height as near as possible to the springing 

 lines of the arch. When this is done the struts are removed, 

 and the space remaining between the top of the partly fin- 

 ished side wall and the arch is filled in. This leaves the arch 



