=Mullan Tunnel.=--This tunnel is 3,850 ft. long, and crosses the main range of the Rocky Mountains, about 20 miles west of Helena, Mont. The tunnel is on a tangent throughout, and has a grade of 20% falling toward the east. The summit of the grade, west of the tunnel, is 5,548 ft. above sea level, and the mountain above the line of the tunnel rises to an elevation of 5,855 ft. Owing to the treacherous nature of the material through which the tunnel passed, it had been a constant menace to traffic ever since its construction in 1883, and numerous delays to trains had been caused by the falls of rock and fires in the timber lining. For these reasons it was finally decided to build a permanent masonry lining, and work on this was begun in July, 1892. [Illustration: ~_With Wall Plates._~ ~_Without Wall Plates._~ ~Old Timber Sections.~ ~_Minimum Section._~ ~_Average Section._~ ~Permanent Work.~ FIG. 163.--Relining Timber Lined Tunnel, Great Northern Ry.] The original timbering consisted of sets spaced 4 ft. apart _c._ to _c._, with 12 × 12 in. posts supporting wall plates, and a five-segment arch of 12 × 12 in. timbers joined by 1¹⁄₂-in. dowels. The arch was covered with 4-in. lagging, and the space between this and the roof was filled with cordwood. Except where the width had been reduced by timbering placed inside the original timbering to increase the strength, the clear width was 16 ft., and the clear height 20 ft. above the top of the rail. Fig. 163 shows the timbering and also the form of masonry lining adopted. The side walls are of concrete and the arch of brick. This new masonry, of course, required the removal of all the original timbering. The manner of doing this work is as follows: A 7-ft. section, _A B_, Fig. 164, was first prepared by removing one post and supporting the arch by struts, _S S_. After clearing away any backing, and excavating for the foundation of the side wall, two temporary posts, _F F_, were set up, and fastened by hook bolts. Fig. 146, _L_, and a lagging was built to form a mold for the concrete. Several of these 7-ft. sections were prepared at a time, each two being separated by a 5-ft. section of timbering. [Illustration: ~Section, with Concrete Car.~ ~With Wall Plate.~ ~Without Wall Plate.~ ~Longitudinal Section.~ FIG. 164.--Construction of Centering Mullan Tunnel.] The mortar car was then run along, and enough mortar (1 cement to 3 sand) was run by the chute into each section to make an 8-in. layer of concrete. As the car passed along to each section, broken stone was shoveled into the last preceding section until all the mortar was taken up. The walls were thus built up in 8-in. layers, and became hard enough to support the arches in about 10 to 14 days. The arches were then allowed to rest on the wall, and the posts of the remaining 5-ft. sections were removed, and the concrete wall built up in the same way as before.