METHOD OF RELINING HODGES PASS TUNNEL, OREGON SHORT LINE RY.—The centers and side wall forms and the methods of work adopted in relining the Hodges Pass tunnel on the Oregon Short Line Ry. are explained in the accompanying illustrations. This tunnel is 1,425.8 ft. long and when constructed in 1882 was lined with timber. The new lining consists of concrete side walls carrying a brick roof arch. Both the old and the new linings are shown in the drawings. The tunnel is through a variety of rock and clay strata, and through the soft strata an invert was required. Altogether about one-third of the length of the tunnel was provided with an invert. It will be noted also that the new lining occupies materially more space than the old; this made necessary considerable excavation in enlarging the section.
Fig. 133.—Method of Placing Invert Concrete, Hodges' Pass Tunnel.
The work of relining consisted of three operations, viz., the invert construction, the construction of the side walls and the arch construction.
Fig. 134.—Method of Constructing Concrete Side Walls, Hodges' Pass Tunnel.
The form of the invert is shown in Fig. 136. It, of course, had to be constructed without entailing a break in the track, and the method adopted was as follows: The ties and ballast were removed from a section of track about 12 ft. long and in their place was substituted the timber frame shown in Fig. 133. Under the middle portion of this frame a trench reaching clear across the tunnel and having a width of 6 to 7 ft. in the direction of the track was excavated to sub-grade of the invert. The concrete was filled into this trench, formed to shape on top, and allowed to harden. The bridging frame was then taken out and the ties and ballast were replaced. Another section of track was then bridged, trenched and concreted and so on until the length of invert required was constructed.
Fig. 135.—Side Wall Forms for Plans A and B, Fig. 134.
The side wall construction was a more complex operation. It comprised first the removal of the old lining, the enlarging excavation and the form erection and concreting. Two methods of performing this task were employed. Both are illustrated in Fig. 134. By the first method, designated as Plan A, the concreting was done continuously in sections of considerable length. The forms used are shown in detail by Fig. 135. By the second method, the concreting was done in alternate short panels. This method is designated Plan B on the drawings, Fig. 134. The forms used are shown in detail by Fig. 135. The only difference in the form construction for the two plans is in the connection of the posts at the top.