Rapid Drawdown / Filling Transient Effective Stress Method

A feature of SVFlux and SVSlope

July 14, 2009

Rapid Drawdown Pore-water Pressures
(click to animate)
Rapid drawdown is one of the most severe loading conditions that can occur along drainage channels. During spring flooding, the levels in canals can be high. Once the floodwaters recede, the levels in these drainage channels can drop quite rapidly and result in shallow slope failures. These shallow failures can lead to larger slope deterioration if they are left unattended.

Analysis of rapid drawdown is commonly required for any earth levee design. There are two commonly used methods:

It is commonly viewed that a total stress analysis is most useful for a rapid draw-down case while an effective stress analysis is most useful for a long-term draw-down scenario. This is a misconception and the latest version of SVFlux and SVSlope allows for easy comparisons between the two. The effective stress analysis has traditionally been avoided due to the complexity of performing such an analysis in software. It remains a more rigorous analysis than the total stress analysis and has the benefit of providing a detailed picture of the performance of pore-water pressures during a draw-down scenario.

An example of the rapid draw-down analysis may be found in SVSlope_Group_3 > RapidDrawdownEffective.svm, which is part of our distributed public models (available with the download of SVOffice 2009). Pictured above is the effective stress method in SVFlux, showing the pore-water pressures. Below are the final results from SVSlope.

Rapid drawdown - Factor of Safety versus Time
(click to animate)
Rapid drawdown - Factor of Safety versus Time - Critical Slip Surface (click to animate)

Ease of Use

Factor of Safety versus Time (click to enlarge)
Significant work has resulted in ease-of-use improvements in SVFlux / SVSlope when performing a transient effective stress analysis. In such a scenario, the model is set up as a combined / coupled model in our software. The seepage geometry, boundary conditions, and material properties are then added to the model and it is run for the designated time period. The user may then "flip" to SVSlope and pick the time periods which need to be analyzed. SVSlope then performs a stability analysis for all time periods and the resulting factor of safety may be plotted vs. time as shown to the right.

A movie illustrating the ease of these calculations is available. There is also an animated tutorial available, demonstrating the entry of geometry.

Rapid Filling

SVSlope can also easily be used for the analysis of rapid filling. The example below illustrates the use of the software for the filling scenario. It is interesting to note the use of mesh refinement to improve the accuracy of the filling contours.

An example of the filling analysis may be found in our distributed public models as SVSlope_Group_3 > ReservoirFilling (available with download of the software).

Raising Water Table - Factor of Safety versus Time (click to animate)

Factor of Safety versus Time (click to enlarge)

Raising Water Table Flow Pore-water Pressures (click to animate)

For more information on these features, feel free to contact us directly, or download our student version and try it out for yourself!