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World Environmental and Water Resources Congress 2017 381

1639 ft. The last step is clearly observed in construction photos at the time (Outland, 1977), five
feet below Outlet No. 1, at an elevation of 1645 ft. The cross section presented by BWWS to all
of the investigative panels after the failure was not an accurate portrayal of the dam’s maximum
section, especially with regards to estimating the factor of safety with regards to cantilever
overturning (even ignoring uplift).
These discrepancies resulted in erroneous evaluations of the dam’s static stability by most
of the investigative panels in 1928 (Rogers, 1992; 1995). Rogers and McMahon (1993) showed
that the dam’s maximum section became unstable in cantilever overturning when the reservoir
rose to elevation 1830 ft, seven feet below the spillway sill. Although ignored in the original
design, the arch stresses began exceeding 7000 pounds per square foot (psf) at elevation 1822 ft,
increasing to 10,000 psf at elevation 1830, five feet below spillway crest. The reservoir had been
brought up to within 4 ft of the spillway for 17 days in mid-May 1927, but was not filled to
capacity until March 2, 1928, 10 days prior to the failure (Fig. 2).

Figure 2. Daily record of reservoir elevations between March 1, 1926 and March 13, 1928.
Note the reservoir was not filled to capacity until March 2, 1928, 10 days before the failure
(Committee Report for the State, 1928).

ASSUMED COEFFICIENTS OF FRICTION

One of the most vexing aspects of the St. Francis Dam failure was the sheer size of the dam’s
displaced blocks of concrete. Some weighing as much as 10,000 tons were transported more
than a kilometer downstream of the dam, and water was observed to be seeping from the dam’s
concrete monoliths for weeks after the failure, testifying to the fact that mass concrete was nearly

as impermeable as most had assumed.
For these reasons, a significant number of questions probed into the coefficient of friction
assumed by the dam’s designers. W.W. Hurlbut, the senior BWWS Office Engineer stated that
his office assumed the same coefficient of friction on St. Francis that had been employed at

World Environmental and Water Resources Congress 2017