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The illusive c’
Does anyone have any recommendations for a sensible c’ for a soft sandy clayey SILT?
I’m thinking I’ll probably assume something around 5, then bash out a quick sensitivity analysis to see how much difference it makes to my foundation design then make a decision based on risk. But I’d be very happy to hear if anyone has experience in this sort of thing? – Damo I’m looking at you, and John obviously!
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I’d ask Damian!
I’d never guess a c’
I think the idea of sensitivity analysis is decent
Here’s a method for a ‘cultured’ guestimate:
1) If you have a phi’ ( or perhaps you can guess that or estimate it from Atterberg limit data)
2) Then if you have an undrained strength ( correlated form ‘N’ tests and /or Atterberg)
3) Then since you know that cu is tau for a particular effective stress
4) Take a couple of values of cu at different depths in the strata, determine effective vertical stress at that depth
5), using your value of phi’ make the equation cu = sigma v’xtan(phi’) +c’
6) You know everything bar c’ solve for c’ and compare at a few depths
7) If they differ – don’t be surprised but you could check sensitivity to your guess of phi’
Don’t try this at home kidz …and don’t try it close to the surface where there will be dessication effects
Good point on the cu. I’ve got a phi’ and an undrained strength from corrected N values, so i’ll do that.
Thanks John.
For all of the work I am doing, c’ is counted as zero. C580 states: “In drained conditions, the shear strength of soils is principally frictional with c′=0”. I think if you know lots about the stresses you are interested in then there is more sense in using a >0 value. If not, then perhaps too risky to assume much. Also if you use a c’ that implies there can be tension generated in the soil – is this likely? I would be interested to know what you find out from the sensitivity analysis with 5 or zero.
It seems common to adopt a balance of phi’ based on plasticity index (form John above) and use c’ = 0. So I would go for zero!
Finally, need to be sure that the intercept on τ-σv’ does not tail off to zero, rather than continuing for y intercept which might not be easy to tell without lots of information, so again points to using zero.
Good points mate.
I’m in about 5m of silt over laying clay, so I’m hoping to keep the failure plane inside the silt so I only have to work within one material. If I can do it will c’=0 then I’ll consider it a win and go with that. So I think I’ll assume c’=0, see if it works then go form there.
Guz, let us know how you get on. I am having issues with taking an undrained strength, cu, with a phi’ value for a cohesive material in order to get a drained parameter… Therefore it would be interesting to see what answer you get out and what you decide to use. What phi’ are you using – crit?
Morning mate, The material on my last site and the subject of my thesis was described as ‘Soft and very soft alluvial clay with slightly gravelly organic inclusions’. Cu=10 and an assumed C’=0. I’ve identified during my thesis research that the designer has adjusted some of the tested results (Mv and Cv) based on their own in-house experience (I’ll reserve comment on the appropriateness of this) however, they accepted the Cu and C’ without change.
It is worth noting that increasing the strength of this material was critical on my last project, therefore, it was in the interest of the designer (reducing their risk) to underestimate which allowed them more time or the use of more surcharge to increase the strength beyond the required minimum…this contrasts what you’re trying to do!
So for those that care, I’ve got the design past my director using phi’=34, c’=0. That gave me a strip foundation, 6m long by 1m wide by 0.8m deep.
Now I just have to make sure it can accept the longitudinal tension since my “really simple RC slab” has become a composite bridge!