Hello everyone
I have some question about principal of soil's shear strength
What are the common values of cohesion and internal friction angle of Clayey Sand soil in saturated and unsaturated state, I mean in unsaturated state or natural water content the shear strength parameters are bigger than saturated state or not?
In saturated state, if drainage conditions could be adjustable, how strain rate (in strain control shearing test device) could influence the final result, for example for two same sample with exact degree of saturation and avoidance of excess pore water pressure built up and same normal stress under direct shear testing if for one sample the strain rate is less than the other one, what are the differences in final result of cohesion C and friction angle phi?
I know that if strain rate is too fast the soil won't drain and the C and Phi parameters are undrained but I am curious that if is it true that undrained C and Phi values are more than C and Phi drained condition because of total stress condition against effective stress condition?
Drained cohesion value or C' for a common clayey sand soil with 50 percent clay in natural water content but not saturated could be 120 kpa or not
And for the last question, above mentioned C' value for the sample, if it experiences moisture lost, become more or less
Thank you
Silty sand and silty gravel I have triaxial tested. They behaved like "slow sand". Meaning - draining very slow. Which can give the illusion of cohesion. Nonetheless, drained and undrained Phi were the same in all tests attempted thus far (I attach a picture, of drained and undrained triaxial test plotted).
A. Schofield in his book "Disturbed Soil Properties and Geotechnical Design" describes clay in a similar way - cohesion is temporary, not reliable, and dissipates quickly if soil is exposed to water.
From the test they run in the lab, I know clay needs a few days for the pore pressure to "saturate into deep layers". And a rule of thumb seems to apply here: if you can "liquefy" a specimen in your hands, by push-pulling on it (while holding in your hands) it will have undrained strength in it. Thus, pore water will provide extra strength during dilative phase. It may seem counter intuitive, but liquefy-able specimens have dilative strength to them. On the other hand, if a specimen squishes into mud, instead of gradually liquefying - no undrained strength to speak of. Take this rule of thumb with caution. I noticed it by playing with silt specimens. They were plausible to liquefy using only my hands. But I knew what I was looking for. Describing it in words, in non-miss-interpret-able manner is tricky. Nonetheless, drained and undrained soil had near identical effective stress limit.
Hope this is relevant. I will be following this question. It is interesting.
Thank you so much dear Thomas for sharing I was waiting so long for the answer and your response shed so much light on it
I was wondering if in an undrained unconsolidated triaxial test a heavily cemented sand being saturated for almost 4 days under 2 Bar atmospheric pressure by back-pressure but it didn't get fully saturated and I mean based on B values of about 64 percent, It is not fully saturated but in the article of Black and Lee. “Saturating laboratory samples under back pressure” a sample with approximately 70 percent is assumed to be saturated so keep that in your mind, under same strain rate I found that two samples with identical density and cementing percentage differ hugely in terms of cohesion in direct shear test but not as much as internal friction angle, It is known that because of compulsory failure plane that Is being applied in direct shear test the result must be higher or at least not that different but there is other way around, although I am aware of the influence of all around pressure in triaxial test but I want to know despite of higher degree of saturation which knowingly will reduce strength Is there another fundamental parameter that influence the results vastly or not?
Thanks again
Best Regards.
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