St. Francis Dam - Pushing the Limits: New Adventures in Engineering - page 128

sides. Mulholland saw that a relatively small dam built where the canyon narrowed would hold back
an enormous amount of water. He also recognized early on that the geology of the location called
for special caution, but these conditions did not keep him from designing a dam for the site. He
assumed that the buttressing effect of the dam would mitigate any slippage at the canyon walls.
Until 1923, all the dam designs that Mulholland had overseen were earthworks, large embank-
ments whose fine-grained silt and clay cores were more or less impermeable to water. The first
concrete dam built for the city of Los Angeles was the two-hundred-foot-high Weid Canyon dam,
which was designed to impound the Hollywood Reservoir. It has been speculated that Mulholland
decided to adopt a concrete-dam design over the clay-core type with which he was so familiar be-
cause of the limited supply of clayey materials in the sides of Weid Canyon. A year before the
unique concrete dam was completed, in 1925, it was christened Mulholland Dam, a testament to the
stature of the chief engineer.
St. Francis Dam was similarly designed to be made of concrete, because there was no suitable
clay or silt available at the San Francisquito Canyon site. The new dam would also be a stepped
concrete gravity-arch structure: Its downstream face was constructed like a wide set of steps; its ma-
terial was mass (unreinforced) concrete; and it held back the water through its sheer weight pressing
down on the ground, aided by an arched plan that took advantage of the water pressure behind it to
compress or wedge the dam between the sides of the canyon, which served as abutments.
The original design of the St. Francis called for a dam reaching 175 feet above the San Francis-
quito Creek bed, which would have given it a capacity of thirty thousand acre-feet of water—that is,
enough water to flood thirty thousand acres to a depth of one foot—enough to supply Los Angeles
for one year. But because of increased water use by Los Angeles, before the first concrete was
poured the dam's capacity was increased to thirty-two thousand acre-feet by raising its height and
adding a wing dike that extended from the west abutment. Almost one year after the beginning of
the placement of concrete, apparently in response to further growth in water usage, the reservoir ca-
pacity was once again increased by raising and extending the wing dike and by adding another ten
feet to the dam's height, to increase its capacity to more than thirty-eight thousand acre-feet—more
than 25 percent greater than the original design. The changes in its height had been made without
a proportionate widening of the dam's base, but Mulholland believed the original design was still
sufficiently safe. A gravity dam derives its ability to hold back water without tipping over from the
width of its base, however, and so the factor of safety of the dam as built was definitely reduced by
the design changes. Mulholland believed that St. Francis Dam still had a “factor of safety of three
or four,” meaning it was three or four times stronger than absolutely necessary.
Construction of the dam lagged that of its sister structure, Mulholland Dam, by about one year,
and the successful advance of that structure must have inspired plenty of confidence in the safety
and robustness of the basic design, despite some less than conservative design features. St. Francis
Dam contained 130,000 cubic yards of concrete but no reinforcing steel. The main structure also
lacked contraction joints, which allow concrete to crack in a controlled manner as it cures. (The
grooves in a concrete sidewalk cause it to crack at the base of these reduced sections, thus keep-
ing the predictable cracks more or less straight and hidden.) No doubt the arched nature of the dam
was expected to close as much as possible any cracks that did develop. St. Francis Dam was also