Any Correlation for Cohesion

[mxslab]

I have too many charts and tables for Undrained Shear Strength. I couldn't find any source for cohesion value for clays.
Is there any Direct correlation beetwen cu (Undrained Shear Strength) and c (Cohesion)or any information about cohesion table (or charts, graph... etc)
In my boring logs and lab test results; i have atterberg limits, water content, SPT-N values, sive analyse.
Thank you.

 

[fattdad]

Cohension and undrained shear strength are not the same thing. In stability analyses, you get to run analysis for drained strength and separate analysis for undrained strength. Phi is for drained strength and Su is for undrained strength. In my practice (and for long-term performance), I ignore drained cohesion.

f-d

ípapß gordo ainÆt no madre flaca!

 

[mxslab]

Thank you for your answer fattdad. I am really sorry about my english in advance.
I know that they are not same thing. Sometimes there is no lab experiment for clay's cohesion. (To use in vesic's bearing capacity as follows:

In my country you cannot say to some people "Run some tests for clay"
So all i wanted to know (if possible) can i find any Cohesion values for clay from literature. (Like from SPT or Cu or Plasticity Index etc...)
And as you answer for Phi for drained bearing capaticy;
if there is a lab test for clay (like DIRECT SHEAR TEST or Triaxial Shear Test on Soil) i use phi and c at same time in the given formulla.
Or else i assume phi=0 and only go with cohesion. As vesic suggests for Nc=5,14 or as Terzaghi suggests Nc=5,7 and Ngama=0 (Phi=0)

 

[fattdad]

If you are trying to run the rational bearing capacity equation, then the goal is bearing capacity.

I typically size footings for settlement performance.

I don't use drained cohesion - especially in slope stability!

I don't use drained cohesion in bearing capacity either - not that it would matter, that is. You see ultimate bearing capacity usually returns too much settlement.

If there is no testing, you can correlate to N-value or get a pocket penetrometer to guess at undrained shear strength. Many folks will tell you that's a difficult correlation!

f-d

ípapß gordo ainÆt no madre flaca!

 

[GeoEnvGuy]

For use in the bearing capacity formula I typically see corrected field vane, torvane/pocket pen, or Undrained unconsolidated triaxial results used as cohesion in cohesive soils. Correlations for undrained shear strength Su is stress dependent this is why it is commonly normalized by the vertical effective overburden stress, such as "A reevaluation of Su(mob) = 0.22sigma'p using laboratory shear tests" can be used as a general check of the results of field tests at varying depths of normally consolidated clays. As mentioned by fattdad settlement performance is more important than bearing capacity in most cases for cohesive deposits.

A lot of correlations can be found in the "Manual on estimating properties for foundation design" with respect to field and laboratory tests to estimate the undrained shear strength of cohesive soils.

https://www.google.com/search?q=man....69i57j0l2.12069j0j7&sourceid=chrome&ie=UTF-8

 

[Retrograde]

A correlation I have seen is that the drained cohesion is 4% of the undrained shear strength.

 

[mxslab]

Thank you all for your answers. I knew that designing a footing for ultimate bearing capacity is Illogical.
I need approval of "city hall" and they have a form that suggests to calculate ultimate bearing capacity. That's why i needed "c" value for.
GeoEnvGuy thanks for that manual. Almost every other chart, table and correlation i knew is in it.
I can share my calculations when i finished if you want to check it.
Thank you all again.

 

[Geosubhtech]

mxslab:

You can use Field SPT with energy correction to estimate the Cohesion by using Stroud's curve. the link attached.

 

[EireChch]

I think it’s drained cohesion they’re after

 

[mxslab]

suhb PMT Thank you for your answer i am already using it for mv and su (Cu) and as FireChch told it's for undrained shear strength. I am looking for drained cohesion for long term bearing capacitcy.
In normal cases i would go with Φu=0 and Cu= Something (From that graph with Plasticity Index f1 and N Value [or from lab tests]) because when earthquake hits the building (Photo 1 shows the building) my mat foundation will lean on the opposite direction and build apreessure to the clay (I made them put 30 cm of gravel fill under foundation) and this event happens for maximum of 25 seconds therefore clay cannot be drained so i have to use Cu (su). But the civil engineer of "city hall" don't understand this and he want me to calculate long term bearing capacitcy and compare it with building's average pressure(Photo 2 shows average pressure under mat footing).

I apologize in advance My english is not good enough to explain my problem and thank you all for your answers.

 

[EireChch]

mxslab

Your structure has an applied pressure of ~130kPa (13.65 ton/m2). Determine you phi' from your atterberg data and ignore c'.

A 20 x 20m foundation on a clay layer with a phi' of 20 to 23 degrees, unit weight of 19kN/m3, and groundwater at 1m depth achieves an allowable bearing capacity of 130 to 205kPa. However I am probably being conservative with some assumptions (i.e groundwater).

If you structure is smaller than that and on soft clay then i think you could have a long term bearing capacity issue.

Also, doesnt IBC allow you to increase you bearing capacity by 33% for earthquake loading?

 

[mxslab]

FireChch thank you for your answer. After all your answers i decided to do as you say. The building we are speaking is 12m x 22m
After we speak i did calculations like you told me and i am more conservative i guess so i get phi of 16 degrees(phi'=2/3phi) and
for c i took a very little value (0,75 t/m² and aplied c'=2/3c so C'=0,5 t/m²)- calculate the "Long Term" (Photo-1)
Then for short term capacity i took phi=0 and Cu=15,29 t/m² (150 kN/m²) and calculate for "Short Term" (Photo - 2)
Photos have foreign language (Turkish) but there is formulas all same.
Turkish Building Code has changed in 01.01.2019 and now we cannot increase bearing capacity by 50% (Previous Code's Limit was %50 for my country) for earthquake but
the bearing capacity safety factor now 1,4 and it was 2-3-4 (by choice 3).
Again thanks for your all answers.