Earth dam under floods

[geotechniqa]

If you have earth dam containing low permeability soil layer 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 asr subjected to floods.

 

[BigH]

In my view, the flood condition would be a short term situation whereas the embankment itself, being low permeability, would not see a change of phreatic surface within. Hence, the dam material would be still under the same internal effective long term stress conditions - presumed to be fully or steady state saturated where the phreatic surface is well known. This of course, is not true if there are permeable layers beneath that may be affected by the short term duration (increase in phreatic pressure) and hence the W*tan(phi) strength would be decreased by "uplift".

 

[DRC1]

For dams, flooding is slow and thus the soil is in a bouyant condition. During rapid drw down, the soil is still nearly saturated , but no longer bouyant, thus unit weigt will rise dramatically. Althoug all possible modes of failure should be investigated, rapid draw down often, but not always, controls.

 

[cvg]

in reality it is hard to determine if flooding is "slow" or "fast". Does the flood pool rise at a rate faster or slower than the embankment saturates? Does it recede faster or slower than the embankment drains? This depends on the shape of the flood hydrograph, capacity relationship of the reservoir and the nature of the embankment soils. Since we can rarely answer this question with any certainty, all load cases should be investigated including first fill, flood stage, rapid drawdown, seismic.