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earthquake of M = 5.8, [ARI 03] showed that a major earthquake with a base
acceleration of 0.5 g will lead to a maximum acceleration of 3.0 g at the crest
without compromising safety.
10.2.4.
Earthquakeperformanceofhydraulicfills
The hydraulic fill method of construction was a cheap but dangerous method.
Used in the first part of the 20th Century, it was abandoned in the 1940s, after the
failure of the Fort Peck Dam in 1938 [JAN 76]. The process of transporting pumped
fine soils, sands, silts, and their deposit by sedimentation had the advantage of being
economical before earth-moving machines existed and the disadvantage of not
compacting the soil. Thus, the soil was normally consolidated under its own weight
or compacted after seismic loading. As a result, a number of hydraulic fills failed
during construction or after an earthquake.
For instance, the Sheffield Dam is a 8 m high sandy hydraulic fill, built in 1917
on a sandy foundation. A layer of clay on the upstream face is extended by a cut-off
trench in the foundation, which acts as a seal. A breach of 30 m was caused by the
liquefaction of the sandy area under water during the 1925 Santa Barbara earthquake
(M = 6.3).
The lower Van Norman Dam, a 42 m hydraulic fill, built between 1912 and
1930, experienced instabilities and leaks after construction that warranted extra
downstream toe, although the top part was compacted. It was shaken by the San
Fernando earthquake on February 9, 1971 (M = 6.6), with a maximum acceleration
measured between 0.55 and 0.60 g. The downstream face moved downstream
around 1 m without failure, while the upstream face disappeared in a major slide of
42m. During the earthquake, the crest was wavering, liquefaction softening the
resistance of the lower part of the upstream shell by pore pressure rise, but the major
sliding started 26 s after the earthquake and developed during 50 s with pore
pressure dissipation of up to the residual strength. Overtopping of the crest and a
major failure was miraculously avoided because the reservoir water level was
relatively low when the earthquake occurred, leading to a final freeboard of 1.4 m.
The concerns regarding the vulnerability of hydraulic fills to liquefaction were so
clearly confirmed after the Van Norman Dam sliding that the Department of Water
Resources of the State of California [JAN 76] ordered a reassessment of all the 36
hydraulic fills in the state of California. Many of them had to be replaced or
rehabilitated. According to this study, the seismic threshold of liquefaction appeared
of the order of 0.2 g for a signal of long duration and low frequencies and 0.3 g for a
signal of short duration with high frequencies.
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