280 TUNNELING 



CHAPTER XXIII. 



RELINING TIMBER LINED TUNNELS WITH 

 MASONRY. 



THE original construction of many American railway tunnels 

 with a timber lining to reduce the cost and hasten the work has 

 made it necessary to reline them, as time has passed, with some 

 more permanent material. In most cases the work of removing 

 the old lining and replacing it with the new masonry has had 

 to be done without interfering with the running of trains, and a 

 number of ingenious methods have been developed by engineers 

 for accomplishing this task. Three of these methods which 

 have been employed, respectively, in relining the Boulder 

 tunnel on the Montana Central Ry., in Montana, the Mullan 

 tunnel on the Northern Pacific Ry., in Montana, and the Little 

 Tom tunnel on the Norfolk & Western R. R., in Virginia, have 

 been selected as fairly representative of this class of tunnel 

 work. 



Boulder Tunnel. This tunnel penetrates a spur of the main 

 range of the Rocky Mountains, at an elevation at the summit 

 of grade of 5,454 ft., and is 6,112 ft. in length. Its alinement is 

 a tangent, with the exception of 150 ft. of 30' curve at the 

 north end. The material penetrated is blue trap-rock with 

 seams for 4,950 ft. from the north end, and syenitic boulders 

 with the intervening spaces filled with disintegrated material 

 for the remaining 1,160 ft. The dimensions and character of 

 the old timber lining and of the new masonry lining replacing 

 it are shown in Figs. 141 and 142. 



The form of masonry adopted consisted of coarse rubble side 

 walls of granite, 13 ft. 8 ins. high, and generally 20 ins. thick, 



