rapid drawdown slope stabilty analysis

[drt]

I'm performing a rapid drawdown analysis for a 2:1 slope, 25 feet tall, phi = 26 and c=300 psf. I used the charts contained in Morgenstern's "Stability Charts for Earth Slopes During Rapid Drawdown" (Geotechnique, 1960?) and was able to extrapolate to find FS=1.49.

However, for my own piece of mind I tried using GSTABLE7 (StedWin) to do an effective stress analysis (I realize that Duncan and other suggest an undrained analysis, but I'm trying to do it with effective stresses). Assuming the critical failure circle would pass through the toe of the slope, I divided the slope up into eight sections and calculated the total stress and pore pressure at the center of each slice prior to drawdown (ignoring seepage). Assuming the soil to be saturated, I calculated the change in total stress and pore water pressure at the base of each slice as a result of water above the slope being "removed”. I then calculated Ru for each slice after drawdown. Using Stedwin I created eight different soils and assigned them the appropriate Ru values. I then ran the analysis.

The factor of safety obtained using this method was approximately 1.2. I realize the accuracy of my calculation depends on the number of slice, etc , however these numbers are further apart than I would like.

Is my methodology correct ? What other suggestions can you give me ? Are there computer programs that will evaluate it rapid drawdown directly ? Please help.


Thanks
DRT

 

[ishvaaag]

You can still make the analysis of the stability of the slope by classical methods; the more critical case is where you consider the soil entirely saturated and some diminished angle of inner friction due to the complete saturation of the soil. It is on your judgement to assign what cohesion is assumed to be standing in such saturated condition.

I have seen common the practice of assuming the same angle of inner friction for dry soils than for saturated and submerged. Even if technically correct if entirely static (and only if the grains remain quite unchanged in the presence of water action) it doesn't consider the increased liquefaction risk and lubrication once any minor relative displacement occurs, so I prefer for saturated and submerged conditions to use reduced angles of inner friction.

With this approach you may use diverse worksheets downloadable form the mathcad collaboratory site to establish the safety of the slope, some constrained to toe failure and others not.

Other than that you should go for more specialized geotechnical programs.

I have a reference of one TALUDEF program made in Denver by Wang, Sun and Ropchan that might be suitable to your case but the reference is old and was for VAX 11/370

Maybe other is FEDAR of Taylor, Brown and Christian for IBM 43XX and PRIME 2XXX

Since I don't have used them, I can't give full assurance.

 

[geoserver]

On my opinion, reducing the angle of internal friction is an empihirical way to face the problem; I think that it should be more correct to consider the forces induced by seepage, with flow parallel to the slope.

 

[isa]

The program that you need is PLAXIS. Look at

http://www.plaxis.com.

Regards.
Antonio Mondadori