While performing slope stability analysis for a project we are stuck at the selection of soil parameters to be used for short-term and long-term analysis. The embankment mainly comprises of normally consolidated clay. We intend to use undrained soil parameters (i.e. phi=0, cu>0), and drained soil parameters (i.e. phi>0, cd=0) for assessing the stability of slope under short-term, and long-term conditions respectively. Theoretically, the undrained parameters have to be determined from UU or CU triaxial test while CD test has to be performed for the determination of drained clay parameters.
In our case, only UU triaxial test could be performed and hence we are only available with the undrained clay parameters which in our case are phi=0, and cu=35 kPa. Now is there any guideline through which we can find appropriate soil parameters for the same clay under drained conditions (i.e. any guideline through which we can convert the undrained soil parameters into drained parameters)?
Dear Muhammad,
You can calculate the results of CD test from results of CU test only if you measured the pore water pressure during the CU test. If you did that, all you have to do is to subtract the pore water pressure from both confining and vertical pressures of CU test ( on other wards, shift mohr circles backward by the pore water pressure value) as shown in the figure. the tangent line of the shifted circles is the failure surface of CD test. you can calculate or measure the effective c & phi.
By the way, phi of clay is not zero in CU condition, it should have some value. your case (phi=0, and cu=35 kPa) is UU test not CU one.
Thank you Ahmed. I got your point but my question is more from the application point of view. I am actually looking for some references through which I can correlate values of c and phi of clays under drained/undrained conditions with other clay properties (like LL, PI, etc).
For all those interested in this phenomenon thread, I found this quite useful reply at some other forum. I am sharing this for common benefit.
"For drained friction angle of clays, most widely used chart is by Terzaghi, Peck and Mesri (1996) from Soil Mechanics in Engineering Practice. However, Sorensen (2013) is the most recent publication on this issue and I prefer to use that. You can find it online easily. Both of these charts correlate drained friction angle with PI. It is important to understand that drained friction angles of clayey materials are not expected to increase with increasing undrained strength.
For drained cohesion, Lunne et al (1997) published a table that correlates clay type and drained friction angle with drained cohesion. Please, for NC clays, use c' 0 to 1 kPa. For OC clays do not exceed 10 kPa. Especially for excavation problems. Also, important point on this issue is that drained cohesion is not a physical parameter. It is just a curve fitting parameter."
Slope stability concerns firstly with FOS which should be more than 1. Than the soil parameters determined after study of slope inclination. Have you covered all this aspects.
Empirical correlations are always affected by uncertainties. Sometimes large uncertainties. I strongly recommend not to use such correlations, to do always the appropriate lab tests. Always! You spend a little for the tests, but you will be more confident in using the input parameters, and will be able to use smaller safety coefficients in your design. You will save money at the end!
Hello guys,
Need some clarity of the aspect of short term slope stability (phi=0). I have come across write up that suggests that one can directly equate the undrained shear strength obtained from the vane shear test to the undrained cohesion. (Su=Cu).
can some one please explain to me why this is so. if not, i really need to find out how i can manupulate my obtained vaues of undrained shear strength to obtain the undrained cohesion. thanks Muhammad Irfan Gianvito Scaringi Ahmed Ebid
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