Rapid Draw Down 1

[ONENGINEER]

RDD anaylses are conducted on an old 15 m high embankment dam made of one zone fat clay with 2.3:1 slopes. The factor of safety for 2.5 days drawdown is ~ 0.25, using soil parameters obtained from a recent site investigation. This low fos seems to be very critical for a dam that has been there for 65 years with no known failure history. One assumption is that there was never a RDD need and hence no failure was experienced. Another assumption is that the analysis has overlooked an item or criteria that led to calculate this[tt][/tt] low fos. In the latter case, I would appreciate feedbacks for what might have gone strategically wrong in the analysis to obtain such a low fos. Thank you.

 

[geotechguy1]

How were the soil parameters derived? What testing was done? What seepage modelling was done?

 

[ONENGINEER]

Thank you for the reply. I understood that the analysis had been carried out with drained parameters instead of undrained. The undrained conditions provided a FoS of 1.4. So this seems to have been the reason.

 

[ONENGINEER]

Another question about RDD analysis: Could one use the C[sub]u[/sub] value obtained from the UU test? Or will that be the C value obtained from CU test. Not quite sure about the application of the C obtained from the following tests especially for RDD.

- UCS
- UU
- CU​

Thanks for shedding light on these.

 

[PlaxisUser]

I would watch the 1997 Buchanan Lecture by Lambe available on You Tube and the companion paper is on Google