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Abstract

Two parallel, essentially horizontal, 50-foot-diameter tunnels spaced 250 feet apart were excavated in the horizontally stratified rock of the Niagara area, about 200 feet below the rock surface and 300 feet below the surface of the ground. Surface-rock cuts up to 60 feet deep were also made for canals and intake structures. Measurements for a few months after excavation showed a decrease in the horizontal diameter of the first tunnel of 1-2 inches. Corresponding figures for the second tunnel excavated some months later were about one-third of this. Some crushing of the sandstone occurred below the tunnel. Inward movements of canal walls have been about 1 inch. All rock movements approximate straight lines when plotted against the logarithm of time; thus much the greater part of the total probable unrestrained movement for, say, 100 years took place before any concrete structures were placed.

Owing to the cooling contraction of the tunnel-lining concrete no rock-movement load has come on the lining. A stress of some 300 psi occurred in a slab at the bottom of a surface-rock cut when an adjacent rock plug was removed from the cut.

Steel ribs supporting the tunnel roof during construction showed stresses from zero to 60 per cent of their design load.

The various structures associated with these excavations appear, as designed, to be fully capable of withstanding any rock movements likely to occur.

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