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World Environmental and Water Resources Congress 2017 389

When asked who made the stability computations for St. Francis Dam, Hurlbut replied
that the computations were actually made for the Hollywood Dam, and then applied to the St.
Francis Dam. He then added “Those computations were made under Mr. Bayley’s instructions in
the design of that dam, that the same general design that was used on the Hollywood Dam was
used on the St. Francis, under the Chief Engineer’s directions” (referring to William
Mulholland). Bayley acknowledged that Mulholland had only built two [concrete] gravity dams
during his lengthy career, most of his experience being in embankment dams.
Hurlbut said that his office engineers designed the entire project in-house, without any
advice or review from external sources. He then went onto describe the physical similarities
between Hollywood and St. Francis Dams: St. Francis having a crest length of 668 feet, while
Hollywood’s was 890 feet; St Francis was designed with a constant radius of 500 feet, while
Hollywood employed 550 feet. These figures are at considerable variance with those cited by
Edgar Bayley, who stated that St. Francis was set out on an arched radius about 50 feet longer
than the Hollywood Dam, with a crest of 492 feet, as opposed to 542 feet for Hollywood Dam.
The same bearing pressures were used on both dams: 10 tons per square foot (tsf) on the toe of
the dam, and 12 tsf on the dam’s upstream heel.
When pressed if he were the engineer-of-record responsible for the St. Francis design,
Hurlbut replied “I did not, as I stated, make the design on that dam. The design of the
Hollywood Dam was made by Mr. Bayley, as I said, and I said that the same section was ordered
to be used, with minor modifications, by the Chief Engineer. I did not make the computations on
that.” Hurlbut then added that “there were no changes in the design to fit the different
conditions,” and that he considered the respective factors of safety employed in both dams to
“have been identical.” There was, therefore, no site-specific design input, other than the site’s
natural topography.
In his initial testimony Edgar A. Bayley asserted that the St. Francis Dam was designed
by Mr. William Wilkinson. Bayley said he only performed the “cross sectional transfer of the
Hollywood Dam.”
During his testimony Hurlbut was questioned about the differences in geology at the
Hollywood and St. Francis Dam sites. He said schist was the dominant material at St. Francis
and sandstone at Hollywood. He added that at Hollywood Dam the rock was “not so broken up.”
Hurlbut related that had visited the St. Francis Dam site “probably a dozen times” during its
construction.
In the initial round of questions posed to W.W. Hurlbut included queries about the
structural stability of vertical strips of the dam, to ascertain its safety with respect to overturning
or sliding, because of the prominent shrinkage cracks noted after the failure (three of the jurors,
Blaine Noice, Oliver Bowen, and Chester Waltz, became founding members of the Structural
Engineers Association of California in 1932). Hurlbut didn’t appear to understand what was
being asked of him.
In his second appearance on the witness stand Edgar Bayley stated that he verified a
Factor of Safety of 2 against overturning with the resultant thrust in the middle third of the
Hollywood Dam’s base. Bayley then summarized rough estimates of gravity dam dimensions
needed for overturning stability: height divided by square root of the dam material’s specific
gravity (Gs). The Gs was usually assumed to be > 2.0 for concrete.

Bayley felt that “arching should add between 6% and 10% to the safety factor,” but this
was ignored in the original calculations (the Trial Load Method of analyzing arch dams was not
formally introduced until 1929, after the dam’s collapse). When Bayley was on the witness stand

World Environmental and Water Resources Congress 2017