286 



TUNNELINd 



The average progress per working-day was 30 ft. of side 

 wall, or about 45 cu. yds. ; and the average cost, including all 

 work required in removing the timber work, train service, lights 

 and tools, engineering and superintendence, and interest on 

 plant, was $8 per cubic yard. 



The centering used for putting in the brick arches is shown 

 in Fig. 147. From 3 ft. to 9 ft. of arch was put in at a time, 

 the length depending upon the nature of the ground. To re- 

 move the old timber arch, one of the segments was partly sawed 

 through; and then a small charge of giant powder was exploded 



in it, the resulting debris, 

 cordwood, rock, etc., being 

 caught by a platform car ex- 

 tending underneath. From 

 this car the debris was re- 

 moved to another car, which 

 conveyed it out of the tunnel. 

 The center was then placed 

 and the brickwork begun, the 

 cement car shown in Fig. 146 

 being used for mixing the 

 mortar. The size of the 

 bricks used was 2 + 2| + 9 

 ins., four rings making a 20- 



in. arch and giving 1.62 cu. yds. of masonry in the arch per 

 lin. ft. of tunnel. The bricks were laid in rowlock bond, two 

 gangs, of three bricklayers and six helpers each, laying about 12 

 lin. ft. per day. The brickwork cost about $17 per cu. yd. 

 The total cost of the new lining averaged about $50 per lin. ft. 

 Little Tom Tunnel. The tunnel has a total length of 1,902 

 ft., but only 1,410 ft. of it were originally lined with timber. 

 This old timber lining consists of bents spaced 3 ft. apart, and 

 located as shown by the dotted lines in the cross-section, Fig. 

 148. Instead of renewing this timber, it was decided to replace 

 it with a brick lining. Although the tunnel was constructed 



FIG. 147. Centering Mullan Tunnel. 



