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Hi dgillette and sixdegrees Many thanks for your replies which lead to further reading (mainly Manual of soil testing - Head 1980, a bit old but that's what I've got to hand) to answer sixdegrees: when a sample is taken of the ground it is often that the pore pressure becomes negative due to...

Dgillette I have reviewed another test which showed similar curve. again low plasticity CLAY tested under TOTAL stress. Water table is 130m above ground so the confining pressure was quite high. doing the calculation phi=tan-1((sigma1-sigma3)/)sigma1+sigma3)) as if there was no pore pressure...

It's a UU test so the cell was loaded to TOTAL stress pressure I did the calculation that dgilette suggested by pretending the cell pressure (total stress) was an effective stress Was I meant to ignore the cell pressure applied and do the calculation with a calculated effective stress FYI the...

dgillette Sorry if I confused you but that's offshore sample which was taken. Water table is 130m above ground and sample is 21m down

Water depth at the site in 130m, the sample is 21m down so total stress as confining pressure was 1700kPa The geology is definitely overconsolidated very stiff clay but also interstratified sand & clay Doing the calculation as drained a find phi=7 for the first curve on a similar material I...

Fattdad looking at the stress and the undrained shear strength profile the sample is not soft

Sorry I deleted the first version here it is with just the graphhttp://files.engineering.com/getfile.aspx?folder=bfc6c51e-c85f-4ba7-9359-e8010f324bf1&file=UU.pdf

When a UU result show a curve like the one attached what are the explanation possible? my suggestion is: - sample not fully saturated and therefore phi>0 the specimen is low plasticity very silty very sandy CLAY Is this behaviour on UU test curve frequent for this type of cohesive soil?

fattdad that's exactly what I'm talking about. So if I understand well, provided a very sandy clay or an overconsolidated clay is fully saturated the phi=0 condition is applicable?

The reason for this post is that after some soil mechanics reading I am still confused about the phi=0 condition or rather the phi>0 for certain cohesive material in undrained condition. Beeing in the offshore industry we follow API as recommendation for driven pile capacity. Cohesive material...