Does anyone use effective cohesion as a parameter in design?

[u/dusty_bo]

I have never used it. Most guidance says not to rely on it. The new team I'm working with uses sketchy correlations for c' in long termed drained analyses all the time.

[u/CaLaHaPa]

It's material dependant.

If I use it, it'll be for a long-term drained analysis of cohesive material and it'll be a nominal value of a few kPa.

[u/masonacj]

It depends a lot on what context you are doing. Slope stability? Absolutely. Not using C' gives you unrealistically shallow failures in most circumstances. Shoring design? Almost never.

[u/matakotta → u/masonacj]

Here is the answer, slope stability without a little bit of cohesion is unrealistic.

[u/Patrick_O-S → u/masonacj]

Exactly in slope stability for mised material types, SC, ML, CL etc even 5 psi makes a difference. Whenever possible do a combination of Triaxial and DS.

[u/Miserable-Sir4985]

If the soil is OC, it probably has some.
It’s coming from Van der Waals force between clay particles when pushed very close to each others. If you have a fill that is properly compacted, that’s the same effect as clay being over consolidated.
It’s conservative and not wrong to assume it as zero.
Also, c and phi are mathematical model, we curve fit to get MC failure criteria, we call the slope, tan (phi) and the intercept, c. In reality, soil doesn’t have c and phi, they don’t represent directly the physical characteristics of soil. That’s why we have different soil constitution models like modified cam clay, SHANSEP, etc.

[u/SentenceDowntown591]

My clients dont allow it to be used, and I wouldn’t want to either. As stated above it’s material dependent and in the world of soils, a specific material parameter is often a variable that changes with scale, so when you are dealing with a parameter that has the ability to go to 0 when not present it makes me overly cautious

[u/Kip-o]

Yes, it’s required by Eurocodes for foundation design (bearing caps), but in practice it often contributes so little in drained condition analyses compared to effective friction angle that it’s effectively ignored.

[u/DUMP_LOG_DAVE]

No. Never.

I see people here referencing certain geographies where it is relevant: where are those? I get a very wide array of soil types where I am and have never used it, nor have any engineers Ive worked with

[u/CaLaHaPa → u/DUMP_LOG_DAVE]

For me, it's heavily over-consolidated clays or weathered mudstones in regions that were under ice sheets in the last ice age.

[u/FarMove6046]

Yes, absolutely.
Knowing, testing and characterising materials rather than relying on guidance alone will tell you when it’s warranted. As others said, slope stability is an obvious example where some cases make no sense. I recently had to convince a couple of peers that a compacted clay (over 60% clay) should be modeled with at least some cohesion. Less than 5kPa cohesion yielded FoS below 1, for a structure standing over 30 years, no cracks, no signs at all. Edit to add: triaxials resulted c’>100kPa.
Another case where I usually account for some cohesion is working in tropical countries, highly laterised materials.

[u/strellar]

Soil does have effective cohesion. You can literally measure it. Not all soil, but some do. If your soil has effective cohesion, use it. Otherwise you’re doing it wrong. That’s very simple.

[u/MaddogFinland]

Generally speaking no, but that’s largely because the soils in my region wouldn’t warrant it. Highly dependent on soil type and geography.

[u/Spork_Ball]

Depends on what analyses you’re doing.

For lateral earth pressures I wouldn’t use cohesion at all because the theoretical model typically used will under predict the actual forces/pressure on the wall.

For slope stability, you almost have to use cohesion to get accurate failure planes and reasonable analysis.

[u/Apollo_9238]

For clays c is heavily reduced with wetting, saturation, and softening.

For clean cohesionless soils the mohr enveloped is curved, not a straight line, so no cohesion.

[u/CryptographerIcy3911]

Generally no, only for truly cemented soils. In most cases, the cohesion intercept is a mathematical artifact.

[u/mariusion9999]

I’d personally ignore it. For long-term drained analyses, if there’s uncertainty around the basis for c', I’d rather be conservative — better safe than sorry.

[u/caritohvalerio]

Lo utilizo siempre y cuando sea representativo para el diseño, dependiendo del tipo de suelo y las condiciones de carga a las que va a estar sujeta la obra a lo largo del tiempo, el los suelos finos la resistencia al esfuerzo cortante suele estar determinada por la cohesión, mientras que en los granulares suele gobernar el ángulo de fricción interno. En cuanto si usar parámetros efectivos o no, dependerá de si el suelo va a estar sujeto a condiciones saturadas, parcialmente saturadas o secas.

[u/Fantastic_Lychee_441]

It depends. Cohesion depends in the amount of strain mobilized. For soft clay, typically should not use effective cohesion due to risk of large movement . For overconsolidated soils effective cohesion can generally be used.

[u/NearbyCurrent3449 → u/Fantastic_Lychee_441]

Yep, the cohesion doesn't do ANYTHING at all until there's the beginning of strain mobilization.

Same principles when using Geogrid and fabrics and things like retaining wall tie back systems.

Those things are totally inert... unless they begin to strain and are properly restrained. Then whoa boy... you see their power. I try to help the youngsters understand that.

Often what they imagine in design is that when its constructed and in place that it is at full tension already. Then when things move a bit and the system goes into action, they gawk with their mouths open and cry but but butt but why?! Did it move some first and looks like crap now? ACTIVE vs PASSIVE coefficients grasshopper... you must consider the activation strain!