Fig. 64.—Arrangement of Drill Holes in the Fort George Tunnel.

Longitudinal Section.

Fig. 65.—Longitudinal Section of the Heading and Bench Excavation at the Fort George Tunnel.

[Larger illustration]

Strutting.

—When the rock was of such a character as to be dangerous and required permanent timber support, until the masonry lining was in place, the method employed was as follows: a top heading was first excavated about 10 ft. deep and from 10 ft. to 12 ft. wide for some distance, 100 ft. to 500 ft., the dangerous rock being supported by 10 × 10 in. yellow pine plumb or raking posts and sometimes by timber bents (“caps and legs”). The next process was to widen the heading to the full width of 30 ft. for a length of about 20 ft., placing timber supports under the dangerous rock as the widening-out progressed. The excavation was deepened a little at the sides to 9.5 ft. below the roof grade (ordered line of excavation) or about 11 ft. below the roof grade, which was necessary when segmental timbering was to be used, to allow for placing a 12 × 12 in. “wall plate” (timber sill) along each side. These wall plates, generally 20 ft. long, were set to the correct elevation and were leveled by blocking and wedging. As soon as the wall plates were set, the work of erecting the segmental timber sets, one set at a time, was begun by starting from the wall plates and supporting the timber on scaffolding until keyed in, then it was blocked up to the rock at each joint and at other necessary points. When two or more sets were erected, lagging, made of boards 2 ins. thick by 6 to 10 ins. wide, was placed over the segmental timber “sets” and the space above the timber dry packed with small stone placed by hand. Sometimes there was enough room between the timber and the rock to do all the dry packing after the full number of sets, generally six, had been placed on the two wall plates. The temporary timber posts and braces were taken out as the segmental timber sets were erected.

The seven timbers that made up a timber set were of yellow pine each 10 × 10 ins., 5 ft. 2 ins. long at the intrados and 5 ft. 6 ins. at the extrados. The sets were spaced from 3 ft. to 5 ft. apart, but generally 3.5 ft. and braced to each other at each joint of the segmental timbers by 6 × 8 in. spreaders which were wedged against the joint splices.

When the timbers were all erected on a set of wall plates (20 ft.) and the lagging and dry packing were completed the work of taking out the bench, which had been partly drilled as the timber sets were erected, was resumed. The face of the bench, which had been left about 4 ft. from the end of the previous set of wall plates, was brought forward slowly by placing 10 × 10 in. plumb posts which extended below subgrade under the wall plates. These posts were generally spaced the same as the timber sets above and directly under them.

When the face of the bench had been brought to within 3 or 4 ft. of the forward end of the wall plate, the process of widening out and timbering another 20 ft. length of heading was begun. In some places the rock, though needing permanent support, was such that the work of taking out the bench and widening the heading was carried on simultaneously without increasing the danger; but the greater portion of the work, when strutting was required, was done as has been described.