282 



TUNNELING 



were used, one for each side wall, and the work on both walls 

 was carried on simultaneously. 



The arch was built upon a centering, the ribs of which were 

 oi ins. less in diameter than the distance between the side 

 walls, so as to permit the use of 2f ins. lagging. Each center 

 had three ribs, made in 1-in. or 2-in. board segments, 10 ins. thick 

 and 14 ins. deep. These ribs were mounted on frames, which 

 followed the opposite walls, and were 4 ft. apart, making the 

 total length of the center out to out about 9 ft. The frames, 

 upon which the ribs were supported, are shown in Fig. 143. 

 As will be seen, they were mounted on dollys to enable the 

 center to be moved from one section to another. Jacks were 



used to raise and lower 

 the center into its proper 

 position. 



The arch was built up 

 from the springing lines 

 on both sides at the same 

 time, four masons being 

 employed. The rings 

 were built beginning with 

 the intrados, which was 

 brought up, say, a dis- 

 tance of about 2 ft. from the springing line. Then the back of 

 the ring was well plastered with from | in. to in. of mortar, 

 and the second ring brought up to the same height and 

 plastered on the back, and so on until the last ring was laid. 

 After bringing the full width of the arch up some distance, 

 new laggings were placed on the ribs for an additional height 

 of 2 ft. and the same process was repeated. All the space 

 between the extrados of the masonry arch and the old lining 

 was compactly filled with dry rubble. When high enough 

 so that the hip segments had a foot or more bearing on the 

 masonry the segments were securely wedged and blocked up 

 against the brickwork, and the longitudinal 4 X 6 in. timbers 



Cross Section . longitudinal Sectfc 



FIG. 143. Relining Timber-Lined Tunnel, 

 Great Northern Ry. 



