278 TUNNELING 



ance, and from 2J to 4 ft. in length, were removed. Near the 

 sides, the earth above the arch was a stiff clay, which was self- 

 sustaining; but near the center there occurred a stratum of 

 gravel and clay saturated with water. This gave considerable 

 trouble, falling through almost continuously until timbering 

 could be placed. One end of this timber rested on the old 

 arch, the other on the adjoining section of the new work. As 

 the new work was to be set 6 to 13 ins. back from the old, it 

 was necessary to block up this distance on top of the old arch, 

 to carry the end of the lagging timl>er, in order that the timber 

 should be clear of the new arch. 



Owing to the small clearance between the car roof and the 

 arch, a special form of centering was required, one that would 

 occupy as small space as possible. Bar iron 1 in. thick, 4 ins. 

 wide, and 20 ft. long was curved to a radius of 6| ft., and on 

 the underside of this was riveted a 6-in. plate i in thick. This 

 plate projected 1 in. on the sides of the centering, and carried 

 the ends of the 1 in. boards used for lagging. The rivets were 

 counter-sunk on the outside of the centering to present a smooth 

 surface next the arch. 



In keying up a section of the new work, a space about 18 ins. 

 square had to be left open for the use of the workmen. As 

 soon as the next section had been torn out, this space was built 

 up. In building up the last section, this space had to be filled 

 from below, which proved to be a tedious undertaking. The 

 opening was gradually reduced to a size of 10 x 18 in., and the 

 top ring then completed and keyed up, the adhesion of mortar 

 holding the bricks in place until the key could be driven home. 

 The next ring was treated in a similar manner, and so on to the 

 face ring. Altogether 412 lin. ft. of the walls and 178 lin. ft. 

 of the arch were taken down and rebuilt, amounting in all to 

 607 cu. yds. of masonry at the total cost of $ 7,440, or about 

 $12.25 per cu. yds. 



The regular trains arrived so frequently at the tunnel that 

 slightly over two hours was the longest working-time between 



