Hi Sina!

If you have a seabed with normally-consolidated marine clay, you can use:

• undrained parameters cu = 0.22 s'v (MESRI 1989)
• with s'v the vertical effective stress
• drained parameters c'=0 kPa & phi'=25~30°

For more information you can check those articles (links attached):

• Evaluation of undrained shear strength
• Geotechnical properties of soils

Regards

Philippe

http://www.civil.utah.edu/~bartlett/CVEEN6340/UndrainedShearStrength.pdf

https://www.researchgate.net/file.PostFileLoader.html?id=5785db9296b7e4c4923c80f1&assetKey=AS%3A383270811389952%401468390290874

Hi Philippe,

I appreciate your response. This is very helpful. 

However, I am a bit confused due to different labelings of different references.

I see there are a few parameters as follows:

c (cohesion or shear strength)

c' (?)

c_cu (consolidated, undrained cohesion?)

c_u (?)

But I am not sure as to use which where. 

I am using terzaghi's bearing capacity equations where parameter c is defined as cohesion. (Does it wotk for marine clay?)

In the case of marine clay, does this mean that I should simply replace c in terzaghi's equations with c_cu?

Is s'v same as sigma (normal stress=footing weight per unit area)?

Also, should I consider local failure or general failure?

Thanks a lot. 

Dear Sina,

The cohesive strength c and friction angle are functions of depth.

I suggest:

"The effect of increasing strength with depth on the bearing capacity of clays"

DOI: 10.1680/geot.1974.24.4.690

and

“Geophysical & Geotechnical Techniques for the Investigation of Near-seabed Soils & Rocks”

Thank you Sadegh,

I could not find/access these two articles. 

If I try to assume it is constant, my analysis would be considered conservative, right? 

My main problem is when I am analyzing seabed soil, do I need to just replace C in Terzaghi equations with the undrainded consolidated cohesions?

Thanks,

Sina

Dear Sina,

When cohesive strength C increases by depth, equation’s factors change for this condition. I attach one of them.

Regards

As the undrained shear strength of water-saturated soils shows a clear dependence on the consistency limits, the following relationship for normally consolidated clay soils can be made (Engel, 2002 cited in Prinz and Strauß, 2006)

Cu=σv.(0,11+0,37.IP). with σv the effective overburden pressure and Ip the plasticity index. The undrained shear strength cu can be measured in the laboratory in a uniaxial or triaxial compression test or in the field with the wing-probe and the penetrometer, respectively. Average values of shear strength parameters for cohesive soils are given in Table .