SL31
•
Rapid Drawdown
o
Shortterm, effective stress, stability
analysis using c' and
φ
' by
Morgenstern, 1963 (using a computer!)
o
Useful for dams and dikes when the
reservoir level is reduced
o
Condition just after drawdown is
often more critical than submerged
or nonsubmerged conditions
o
Immediately after drawdown the soil is
still saturated (and heavy), seepage forces begin
to develop, and effective stresses are still low
o
Based on simplified method of slices:
o
homogenous soil, fully submerged initially
o
embankment on rigid,
impermeable base
o
circular failure surface
o
seepage parallel to rigid base,
equipotential lines vertical,
∴
u =
γ
w
h where h is the height
of the fill above the point
o
soil remains saturated
o
saturated soil weight is twice the weight of water (
γ
sat
= 2
γ
w
)
o
Morgenstern stability charts provide the overall FS
for various drawdown ratios (L/H)
for a range of c’/
γ
H = 0.0125, 0.025, 0.05 and for different slope and
φ
angles
o
FS in the charts is based on a critical circle tangent to the base
used for the chart (a slope
circle  not toe circle).
Note: the "base" is at the depth H used to get (c’/
γ
H).
The charts
can be used for approximate solution with any H, i.e. H does not have to be the physical
depth to the rigid, impermeable base of the embankment (see flow net above).
o
For
complete drawdown
(L = H), the charts provide the minimum FS directly using (c’/
γ
L)
o
For
partial drawdown
a trial procedure is necessary because the least FS may have a
failure circle which is above the base, not tangent to it.
In this case we must investigate
various circle depths using different effective heights (H
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 Fall '08
 Staff
 drawdown, circle tangent, complete drawdown, minimum FS

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