Discussion Topic : From Cu+Phi(u) to Cu; stability analysis

Dear all, I have been dealing with processes from Poland and I have a question that maybe the experience of some of you will be helpfull. For cohesive soils they don't use the undrained shear test - Cu, it is not commom at all to have this parameter as a "basic" one for clays. Instead of this they use Cu and Phi(u). My question is if any of you know how to estimate the Cu, from those parameters. I don't think they are drained parameters. It is also important to mention that those parameters are not obtained, most of the times, from tests but from tables which take into consideration the Liquidity Index. So it means that from the liquidity index they after deduce a Cu and phi(u). I will give you some examples: - A silty clay with Cu=8kPa and Phi=9? - a silty clay/clay with Cu=31kPa and Phi=9? Furthermore I was wondering how they will do to input parameters in softwares once the majority of softwares assume Mohr Coulomb for a drained state and Tresca for undrained. I don't think it will be correct to assume, for a short term (so undrained) the numbers given above in Mohr Coulomb criterion (once this way the software will assume a drained condition which is not the case). Does any of you have experience with those type of parameters and geotechnical design softwares? Thanks in advance.
Posted: 13/04/2016 13:25
Diana Martins Marques

2 Entries

Minh A Nguyen

Hi Diana I suggest you carry a check with your counterpart in Poland or the one provided these data that: These quoted values are not from actual tests for your site, but rather from correlations (i.e. local experience) with Atterberg's limits (in a way very similar to countries in Eastern Europe bloc which previously follow the SNiP code by Russia in the pre-90s period). These values have no direct relationship to the Su (undrained shear strength) as they mostly come from a fitting of failure envelop from quick (not completely undrained) direct shear test on clayey soil samples. They are therefore subjected to the limitations of direct shear test (e.g., unknow stress state, localised failure zone, drainage control and shear strain effect, etc) You haven't mentioned if there are available in-situ data such as CPT or PMT, or at least field vane shear test and SPT. The design should also consider the importance category of the foundation, as low important and not critical structures can be designed using established local design experience. Obviously, for critical structures the design requirements for input mechanical properties of the soils and design procedures are much more than those provided from simple correlations. Regards, Minh A Nguyen (London, UK)

Diana Martins Marques

Hello Minh A Nguyen, First of all thank you for your answer. Most of the times they don't provide any in situ or laboratorial tests. They just assume the parameters (Cu, Phi(u), IL, gama and M) directly from those corelations from atterberg limits. Regarding what you said: These values have no direct relationship to the Su (undrained shear strength) as they mostly come from a fitting of failure envelop from quick (not completely undrained) direct shear test on clayey soil samples. The point is if they are not undrained neither drained parameters ones so how could I use them directly? (In a accurate way of course). Could I use them in Mohr Columb criterion? I don't think it will be correct because they are represented as Cu and Phi(u). Thank you very much. BR Diana