Abstract

On May 3, 2003, the Old Man of the Mountains natural rock profile collapsed, resulting in the unfortunate loss of the official emblem of the state of New Hampshire. A systematic reconnaissance of its stability had been performed in 1976 by the New Hampshire Highway Department as part of the environmental impact statement for Interstate 93. This reconnaissance estimated the Profile's in-place stability and its capacity to withstand blasting vibration from below. The work showed that 1) the dead weight of the Profile's blocks cantilevered at their combined point of bearing created a delicate stability; 2) the Profile from the nose up was relatively more stable than from the upper lip and chin down; 3) the Profile was subject to toppling collapse if natural processes or dynamic stress disturbed it; and 4) blasting could take place beneath it if no vibration in excess of those in the ambient natural environment was allowed to reach the rock mass. Careful blast monitoring during construction of Interstate 93 between 1985 and 1986 showed this vibration objective was achieved. Most recently, estimates of the mechanism and cause of the Profile's collapse suggest it was a progressive toppling failure initiated by a sudden loss of intact compressive strength in the granite immediately beneath the point of bearing of the cantilevered chin. The granite's intact strength had been naturally compromised over time by kaolinization decomposition and freeze–thaw degradation.

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