Dam Slope Stability Analysis-Sat. and Unsat. soil parameters

[jmurphy]

I have completed a slope stability analysis on a water storage dam (40' fill, 3:1 side slopes, silty clay) using Geoslope, which uses finite element analysis and accounts for pore water pressure based on a seepage analysis. I have recieved a response back from reviewers stating that the soils should have different parameters based depending on saturation.

Therefore, here is my question:

Should a slope stability analysis on a dam with a consistant fill have different soil paramters (c and phi) for the saturated and unsaturated fill, based on the position of the phreatic line?

Thanks,

 

[oldestguy]

Did you do any tests? If so, were the soils damp, wet? I'd get back to them and explain tests and applicability. Better yet ask them to cite typical examples of where tests were done that showed a difference if any. What safety factor is involved? Does not this allow for some variations

 

[fattdad]

I wonder whether you can attribute confusion over the terms "drained" and "undrained" strength on the reviewer's concerns on degree of saturation? Otherwise, I have no clue what this is about.

There may be some concern on the "rapid drawdown" stability, which you can likely model in Geoslope.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

First, you might try to explain what parameters should be different in the saturated or unsaturated conditions. Is it the unit weight (unsaturated soil has a different unit weight than when saturated as the air pores become water pores). I assume that you used effective stress parameters for your analysis - total stress is a form of effective stress but the later has to model the porewater pressures that develop during loading and shear to "equate" to the total. Effective stress with correct porewater pressures are always the best - whether positive or negative (suction) pressures. They should clarify their concerns and be a bit specific than just throwing out terms. I recently saw where shoring started to be placed at a non-trench site only because an untrained safety staff said to shore - shoring wasn't needed but an engineered understanding of the excavation was.

 

[VAD]

You may wish to read Fredlund's and Rahardjo book "Soil Mechanics for Unsaturated Soils" to get the answers that you need. More numbers to play around with to allow better judgement, perhaps.

 

[GeoPaveTraffic]

Unsaturated analysis of dams if very uncommon. I would expect different parameters to be used for drained and undrained loading conditions as stated previously. Are you sure that the reviewers have asked for unsaturated analysis? If they are in fact looking for unsaturated anslysis, ask them for the method that they would like to see.

 

[dgillette]

More often than not, the unsaturated material forms only a small part of the critical failure surface, so the differences in phi and c don't affect the results much. Typically, phi is lower than phi' and c is higher than c' because of capillarity in the unsaturated soil.

The most critical issue is usually how high the phreatic/piezometric surface is under the downstream slope. Allow for uncertainty in the prediction!