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Slope Stability Analysis under an Earthquake

Slope Stability Analysis under an Earthquake

(OP)
Hello Engineers,

I am trying to analize the global stability of a Retaining Wall.

Basically, I will run 3 models:

1- Undrained Condition. (during Construction)
2- Drained Condition (long term condition)
3- Under an Earthquake with PGA 0.3g.

I am using Slope W (2004) to work on these analysis.

I was wondering how do I perform a seismic analysis using slope W. I have been checking all the options but realized that I have no option to input any earthquake parameter (e.g Magnitud, PGa, etc).

This project is located on an active seismic area.

The soil profile consist bassically of preconsolidated low plasticity silt (ML), qu ranges between 2 and 4 kg/cm2 (using a pocket penetrometer). I know PP gives higher values than the Unconfined compressive strength (UCS) test, but PP results are what we have available.

If any one knows, please let me know.

RE: Slope Stability Analysis under an Earthquake

Should be in the user's manual.

RE: Slope Stability Analysis under an Earthquake

you have also compute excess pore pressure if ater is present  

RE: Slope Stability Analysis under an Earthquake

SlopeW will only be able to model the post earthquake stability of the retaining wall. If you know how the earthquake will affect the soil then you can put post earthquake strengths and pore pressures into the model.

However, you will have to do a dynamic analysis to fully understand the interaction of the soil and wall structure during the earthquake. This will require a 2D finite element modelling program. This would estimate strains during and immediately after the earthquake, but requires inputs such as a design earthquake time history.

 

RE: Slope Stability Analysis under an Earthquake

In slope/w should be somehting like Key In -> Seismic Load.

With this you can complete a psuedo-static analysis. USBR or US Army Corps provides recommendation on this for embankment dams. Might be a worthwhile  starting point.

Apply half the PGA, reduce material strength by 20%, check if FOS<1.0 as a screening test.  

RE: Slope Stability Analysis under an Earthquake

Without seeing your structure, I will guess that it bears upon soil, and you are concerned with sliding on or below the base, rather than the loading on the wall structure.  Otherwise, you would probably not be using Slope/w.

From my experience with earth slopes (primarily dams), it may well be the case that applying a seismic coefficient of 0.3 will cause yield of the soil.  However, unless the soil is brittle, sensitive, or liquefiable, that may only be momentary yield occurring for a few tenths of a second during the whole earthquake.  If that is the case, the deformations could be very minor (depending on what the word "minor" means for your structure).  Commonly people analyze a slope with seismic coefficient assumed to be half of the peak ground acceleration, and if the FS is found to be greater than 1.0, they are comfortable that the deformation will be very small.

About 15 or 20 years ago, a research report by Gus Franklin and Mary Ellen Hynes of the Corps of Engineers indicated that, for well built embankment dams, deformation would be less than 1 m for any earthquake with PGA less than three times the yield acceleration.  The yield analysis should be performed with consolidated undrained strengths for any saturated material, i.e., undrained strengths as governed by the state of stress after construction, before the earthquake.

All bets are off, however, if there are brittle, sensitive, or liquefiable materials involved.

Bon chance!

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