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 alignment 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. 159] and [160].
The form of masonry adopted consisted of coarse rubble side walls of granite, 13 ft. 8 ins. high, and generally 20 ins. thick, with a full center circular arch of four rings of brick laid in rowlock form. When greater strength was needed the thickness of the side walls was increased to 30 ins. and that of the arch to six rings of brick.
Cross Section.
Longitudinal Section.