HomeMy WebLinkAboutSeismicThe analyses indicates that in spite of areas of high pore
pressure in the upstream shell, and the potential horizontal
�. displacement of the dam of about 3 feet, the dam would be
amply safe. There would be some likelihood of surface
sloughing or insignificant movement along slopes at shallow
depths near the crest. The minimum factor of safety with
the high pore pressures would be reduced to 1.4 from 3.1 for
normal operating conditions.
(c) Hypothetical Extreme Earthquake, Magnitude 8.25
This hypothetical study was made for the purpose of
developing a better understanding of the performance of
high embankment dams located near an epicentral region of
great earthquakes. The results of the study indicate:
o The relatively high pore pressure zone in the upstream
shell spreads over a significantly larger area within
the upstream shell when compared with the similar area
developed after a magnitude 6.5 earthquake.
o The minimum factor of safety with high pore pressure
development reduced to 1.12 for the critical circle
immediately after an earthquake of magnitude 8.25.
The dam is dynamically stable and would not develop
any massive slide in the upstream slope. The minimum
factor of safety of 1.12 would be of a transient
nature. The pore water pressure will dissipate in
time and the dam will regain its pre -earthquake
strength and stability factor of safety.
o The maximum horizontal displacements of the upstream
slope after an earthquake of magnitude 8.25 would be
_ in the order of 8 ft. The increase in strength caused
by aging would reduce it to half the computed amount.
;s
The conclusion was that a high dam, well -designed and built with
suitable materials like Oroville Dam, would be able to safely
withstand a near, extreme earthquake of 8.25 without significant
damage, or danger of reservoir release.
1.5.2 - Miboro Dam (***)
Miboro Dam, Japan (Seed et al., 1977)
Kita-Muto Earthquake, 1961; Magnitude 7;
a = 0.1 g to 0.25 g at 20 km from epicenter.
a max = 0.6 g at 10 km.
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uplift where drains are to be provided but are assumed to be
ineffective in reducing uplift. Safety factors in
accordance with extreme conditions will then apply.
Allowable tensile strength at the rock -concrete interface
shall be zero. If under earthquake loading conditions a
crack is considered to develop at the upstream heel, the
uplift pressure shall be taken as equal to the normal
distribution as described above over 100 percent of the base
area.
Under PMF conditions where cracking at the upstream heel
develops, uplift shall be considered to be equal to full
headwater within the full depth of the crack, reducing to
the values at the line of drains and downstream toe as
proportioned above.
Apron and chute slabs and slab walls against rock shall be
designed against uplift resulting from sudden changes in
water level.
Y Uplift from centrifugal forces shall be considered where
contraction joints occur on the concave floor of chutes.
Toe curve pressures on the interior face of training walls
at concave chute surfaces shall be calculated in accordance
WLt11 Plate 21 of Hydraulic Design of Spiiiways Eii illy-2-
1603 by U.S. Army Corps of Engineers (COE 1981).
Hydraulic loads due to earthquakes are given in the
following section on seismic loads.
3.2.8 - Seismic Loads (o)
The largest mean peak horizontal ground acceleration that could
affect the sites is 0.5g with a duration of 6 seconds (Acres
1982c).
(a) Watana (o)
Design of critical concrete structures wiii use an 80th
percentile response spectrum from the "Safe Evaluation
- Earthquake" (SEE) with a 10 percent damping ratio scaled
down by a factor of 80 percent.
- (b) Arch Dam at Devil Canyon (o)
The arch dam is to be checked under seismic loading by
dynamic analysis based on trial load method and the ADSAS
program developed by the Department of the Interior.
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The arch dam will be designed for a base ground acceleration
Of 0.8 x SEE = 0.57g.
Arch dam system damping ration - 0.10 of critical*.
Acceleration response spectrum - See Figure F.3.2.1.
For final design, a time -history finite element analysis
will be carried out.
Concrete Retaining Structures (other than arch dam)
Mass concrete retaining structures will be designed for
0.8 x SEE using static analysis.
Other Major Structures
Non -reservoir retaining major structures will be designed
for the 100/110-year return earthquake corresponding to
0.2g.
Hydrodynamic Pressure
The hydrodynamic pressure due to horizontal earthquake on
water -retaining surfaces shall be computed using the
theory of Westergaard for the dynamic change in pressure:
1/2
P = a.51.25 (hy) lbs/ft2
Where h = total height of structure (ft)
y = depth below reservoir surface (ft)
a = ground acceleration/acceleration due to
gravity
The distribution of pressure is parabolic; hence, the
total force and moment at a section y feet below water
level are given by:
F = 2/3. P.y
M = 0.4. F.y
^This damping ratio is similar to ones used at Swan Lake,
E1 Cajon and Salinas Dams.
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