. Railroad structures and estimates . Fig. 5a. Ordinary Single Track Tunnels. TUNNEL CONSTRUCTION. 69 Construction. — For ordinary tunnel work, Figs. 5a and 9, thetimbers generally consist of 12 X 12 upright posts at varyingcenters usually not over 3 ft., with 12 X 12 caps and archbeams, 4 sills and 4 lagging the space behind being filled withwood or stone packing. The concrete lining may consist of1:2:5 material for side walls, and 1 : 2 : 3 for m Proaie of Grade(FoundatloD to be made to take inoUnedreaction of botizontal forces 2-^^li*\ Uae sill or wedgesas req^uired Fig. 6. Concrete
. Railroad structures and estimates . Fig. 5a. Ordinary Single Track Tunnels. TUNNEL CONSTRUCTION. 69 Construction. — For ordinary tunnel work, Figs. 5a and 9, thetimbers generally consist of 12 X 12 upright posts at varyingcenters usually not over 3 ft., with 12 X 12 caps and archbeams, 4 sills and 4 lagging the space behind being filled withwood or stone packing. The concrete lining may consist of1:2:5 material for side walls, and 1 : 2 : 3 for m Proaie of Grade(FoundatloD to be made to take inoUnedreaction of botizontal forces 2-^^li*\ Uae sill or wedgesas req^uired Fig. 6. Concrete Tunnel Lining with Steel Rib Reinforcement, C. C. & O. Ry. The steel reinforcement used on the C. C. & 0. Ry., Fig. 6,consists of 12 I beam ribs at 2 to 3 ft. centers, in two curvedsections spliced together just above the springing line of the arch,at the most dangerous points the ribs are placed 12 centers, theribs being carried down to the floor of the tunnel only on the sidefrom which the pressure occurs. 70 TUNNEL PORTALS. The A. R. E. A. recommend that concrete be used for the per-manent tunnel Uning except where local conditions will injurethe concrete before there is time for it to harden. In the event that a brick lining be used, that portion of thearch for a horizontal distance of five feet on each side of the centerhne of each track should be laid with vitrified brick in rich Port-land cement mortar. Copinj to hire 3 pitchtowud
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