Lecture 23 - SL31 Rapid Drawdown Drawdown Water Level...

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SL31 Rapid Drawdown o Short-term, 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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Lecture 23 - SL31 Rapid Drawdown Drawdown Water Level...

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