Earth dam under floods or wave loads 1

[geotechniqa]

If you have earth dam composed of low permeability soils and this dam is exposed to flood , it should shear under undrained case. However, in most practice earth dams and levees are usually analyzed considering undrained shear only when they are under rapid drawdown case. Why the undrained analysis is not considered when such dams and levees are subjected to floods ( during the flood loading).

 

[cvg]

I believe all load cases are generally considered - full reservoir, rapid drawdown, seismic. It just happens that rapid drawdown is often the most severe loading case.

 

[fattdad]

+1 - cvg

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

Geotechniqa,

Most dams do not have large changes in water levee that occur very quickly, however, it can happen. Levees on the other hand often have quick changes in water level.

If the undrained strength of the embankment, dam or levee, is low enough, then it will fail during a rapid increase in loading. However, most embankments have a high undrained strength relative to the water load, so they don't fail.

As has been stated, drained conditions generally control for most embankments.

 

[geotechniqa]

If, for the low permeability embankment, the dam is safe under rapid drawdown (unloding) , it will be safe under the the rapid loading condition.

 

[BigH]

As I indicated in your other post on the same subject in another forum, there is a big difference in the two conditions. The rapid unloading means that the embankment is in excess pore pressure state - high internal phreatic surface - zero at the face and "high" just inside the face. In the rapid loading condition, the internal water pressure cannot develop until the water has a chance to penetrate into the embankment. With a low permeable material, this will take some time - a long time - and the flood event will be on the "downward" slide before any rise of the internal phreatic pressures can develop.

 

[geotechniqa]

It is not a mattter of phreatic surface only, but the load of the flood water exerts excess pore pressure in the dam (assuming ,at least, part of the dam is saturated)

 

[BigH]

geotechniqa - unless I don't see something, it cannot exert any additional pore pressures in your dam unless the pore pressures have a chance to build up due to the wave - this needs time (think of it as the reverse of consolidation theory). You can always analyze both cases using effective stresses AND the pore pressures applicable at a specific time. Undrained analyses are usually done because one doesn't have a good handled on the build-up or drop-off of pore pressure.

 

[dgillette]

What failure mode is being discussed here? If it's upstream stability, the high reservoir generally increases FS. If it's downstream translation, the effect should be very minor. Where in the dam is the strength of concern? I suspect Big H's answer covers it, but I can't figure out where you are talking about.

And finally, is this a real dam being analyzed, or a hypothetical case?