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Multiple Time Step Rapid Drawdown Stability AnalysisStability analysis during rapid drawdown is an important consideration in the design of embankment dams. During rapid drawdown, the stabilizing effect of the water on the upstream face is lost, but the pore-water pressures within the embankment may remain high. As a result, the stability of the upstream face of the dam can be much reduced.The dissipation of pore-water pressure in the embankment during rapid drawdown is largely influenced by the permeability and the storage characteristic of the embankment materials. Instead of doing a stability analysis of the embankment with an assumed piezometric line after the rapid drawdown, a rigorous approach is to model the dissipation of pore-water pressure in the embankment with SEEP/W first and then use SLOPE/W to compute the factor of safety of the embankment based on the computed pore-water pressures. SLOPE/W reads in the SEEP/W pore water pressure of all time steps automatically to compute the factor of safety versus time after the rapid drawdown. The following figure shows the critical slip surface and factor of safety before drawdown when the reservoir is at full supply level. The light blue region is the reservoir. The factor of safety before drawdown is 1.977.  The next figure shows the SEEP/W computed piezometric lines at different time steps after the rapid drawdown. As you can see, the piezometric surface in the embankment is dropping with time due to pore-water dissipation when the reservoir is empty.  The subsequent figure shows the critical slip surface and factor of safety of the embankment when the pore water pressure at 18 hours after rapid drawdown was used. The factor of safety is 1.144.  This last figure shows the computed factor of safety of the embankment versus time after the rapid drawdown. The factor of safety dropped quickly from 1.977 to below 1.0 shortly after the rapid drawdown and gradually increased to 1.737 after 58 days.  The advantage of this approach is that the hydraulic properties of the materials can be considered, and time can be included in the analysis. Therefore rapid drawdown is not just an instance in time, but is a process. A prerequisite is that a finite element transient seepage analysis is required, however in the design of an embankment dam, the hydraulic properties of the materials are usually available and a seepage analysis is likely necessary. GeoStudio 2007 allows easy integration between SLOPE/W and SEEP/W, making this rigorous approach an attractive alternative.
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Volume 5, Issue 4 |
In this issue:
• Rapid Drawdown Stability
• Windows 7 Certified
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Banff Workshop October 19-22, 2009. Banff, Alberta, Canada. Seats fill up quickly, so please register early. |
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Grokking GeoStudio Check out Nate's latest blog entry that delves into sensitivity studies, probabilistic analyses, and other ways of generating large numbers of similar analyses. |