Steve asked me a few weeks ago to write about multi-stage rapid drawdown, and how it has changed in version 7.1. This is a bit outside my comfort zone as a non-engineer, but I’ve taken a rapid drawdown crash course from a couple of our in-house experts, and here’s what I’ve found.
Background
The concept is simple, but the feature has evolved over the last few point releases. The solver simply needs to know the water level before the drawdown, and the water level after the drawdown.
Pre-Version 7
My understanding is that it is not possible to do staged rapid drawdown with SLOPE/W version 6 and earlier. If there was some way to do it, it would have involved multiple analyses and manually changing material properties between runs. I’m not familiar enough with the theory to know if that would even be possible though.
Version 7.00-7.02
Beginning with version 7.00, SLOPE/W adds direct support for staged rapid drawdown analyses, using the 3-stage undrained strength method proposed by Duncan, Wright and Wong (1990).
The feature was first implemented with a very simple conceptual model: draw a single piez line identifying the “full” water level, and tell SLOPE/W how far down the level will drop (the “drawdown distance”). The solver will compute the lower level by simply subtracting the drawdown distance from the x coordinates of the piez line.
There are only two steps you needed to take:
1. In KeyIn Analyses, select the “Staged Rapid Drawdown” option on the Settings tab, and enter a Drawdown Distance.
2. Use Draw Pore-Water Pressure to draw the “full” piezometric line.
Version 7.03
We were quickly told by clients that this approach was too simplistic. It covered some simple cases, but often times the “empty” position was not a uniform drawdown of the “full”.
So in version 7.03 you no longer enter a Drawdown Distance. Instead you define two piez lines. The first is the full level, the second is the empty level. (Careful! In 7.03 it’s important that the full is piez line #1 and the empty is line #2. Switch them around and you’ll get weird answers.)
This time there are three steps to take:
1. In KeyIn Analyses, select the “Staged Rapid Drawdown” option on the Settings tab.
2. Use Draw Pore-Water Pressure to draw the “full” piezometric line.
3. Use Draw Pore-Water Pressure again to draw the “empty” piezometric line.
Version 7.10
Version 7.10 does not change much from 7.03. The only difference is that it’s no longer important in what order you draw your piez lines, so long as you have exactly two. The higher of the two will be the “full” level, and the lower will be the “empty” level.
Further Reading
Several examples of staged rapid drawdown ship with GeoStudio. If you have version 7.1 and you’ve installed the Resource CD you’ll get five matches if you search the examples for the keywords “staged rapid drawdown”. Read the PDF with the “Rapid Drawdown With Multi-stage” example for a description of how staged rapid drawdown is implemented.
If you don’t have 7.1 but you’ve installed the examples distributed with earlier versions, look in the slopew folder for files with “drawdown” in the name. (There may be one named “rapid drawdown with multi-stage.gsz”, but we also may have renamed it so I can’t be sure.)
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