Mi:ril"I oF LINING TUNNELS 73 



which rest on the lagging of the centers and support the poling- 

 boards. These props are removed in succession as the arch 

 masonry ris^s along the curve of the center, and the space 

 between tin- top of tin- aivh masonry and tlie ceiling of die 

 excavation is tilled with small stones packed closely. The key- 

 stone section of the aivh is built last, by inserting the stones or 

 bricks from the front edge of the arch ring, there being no 

 room to set them in from the top, as is the practice in ordinary 

 open-arch construction. The keying of the arch is an espe- 

 cially difficult operation, and only experienced men skilled in the 

 work should l>e employed to perform it. The task becomes 

 one of unusual difficulty when it becomes necessary to join the 

 arches coming from opjxwite directions. 



Invert In all but one or two methods of tunneling, the 

 invert is the last portion of the lining to \te built In the 

 English method of tunneling, the invert is the first portion of 

 the lining to )>e built, and the same practice is sometimes neces- 

 sary in soft soils where there is danger of the bottom of the 

 side walls being squeezed together by the Intend pressures 

 unless the invert masonry is in place to hold them apart. The 

 ground molds previously descriljcd are employed to direct the 

 construction of the invert masonry. 



General Observations. In describing the construction of the 

 roof arch, mention was made of the stone filling employed 

 between the back of the masonry ring and the ceiling of the 

 excavation. The spaces liehind the side walls are filled in a 

 similar manner. The object of this stone filling, which should 

 tie closely packed, is to distribute the vertical and lateral press* 

 ures in the walls of the excavation uniformly over the lining 

 masonry. As the masonry work progresses, the strutting 

 employed previously to sup|H>rt the walls of the excavation has 

 to be. removed. This work requires care to prevent accident, 

 ami should IKJ placed in charge of experienced mechanics who 

 aiv familiar with its construction, and can remove it with the 

 least .la mage to the timbers, so that they may !* used again. 



