Using Slide 6.0 to analyse stability of an embankment

[jonesjack89]

Hi I am a student studing geotechnical engineering in the UK. I am currently involved in a class project where I have to assess the stability of a shallow clay slope (composed of glaciolacustrine deposits) after the rapid placement of fill material.

I understand that the lowest factors of safety associated with placing fill material on a fully saturated clay are associated with the short-term (undrained) condition, due to the generation of excess pore water pressure. I also understand that the factor of safety will increase with time as this excess pore pressure dissipates and effective stress increases i.e. in the long-term (drained) condition.

I have been using Slide 6.0 to carry out a back analysis of a failure that occured a short time after the placement of the fill material and notice that when I carry out a total stress analysis using undrained strength parameters for the clay (Cu = 60 kPa and phi = 0) I get very high factors of safety. Much higher than when using effective stress parameters (c' = 5 kPa and phi = 23) that represent long-term drained conditions.

This seems wrong as I expected to get my lowest FOS associated with the undrained condition.

 

[moe333]

For a low embankment height, the undrained strength will be higher. Look at the vertical stress range of your problem to see whether drained or undrained strenght is the critical condition. Plot both strenghts to see what stress level they cross over at.

 

[jonesjack89]

Hi Moe, thanks for getting back to me.

I may have been incorrect by using the word "embankment." The site is simply a shallow slope with a load of fill material placed on top, although the thickness of the fill isn't that large.

To simulate the short-term conditions in Slide i am now using an effective stress analysis and then using the "B Bar" method to calculate excess pore-water pressure. Carrying out my short-term analysis this way gives me a much lower factor of safety compared to when i simply carry out a total stress analysis to simulate short-term conditions. Is this because a total stress analysis simply does not include excess pore-water pressure?

Thanks again,

 

[moe333]

The forum policy doesn't allow discussion of homework but I will provide a final comment. I'm not familiar with SLIDE, but sounds like you are using Su/P' analysis. This is different than just using a constant Cu from your first post, but you need to determine if a Su/P' analysis is appropriate for your consitions. There is a lot of free literature on these analysis methods and I'm sure your text covers it as well. Good Luck.

 

[jonesjack89]

Ok I will look into that. It is not homework, it is my University dissertation. Thanks again