Undisturbed vs. remoulded shear strength

[sylvaninhoo]

Pipeline engineer with only limited knowledge in soil mechanics:

A seabed soil is described as clay with undrained shear strength of 5 kPa (21 psf).

I am assuming this is the undisturbed shear strength. What would be the typical upper bound and lower bound values for the ratio undisturbed to remoulded shear strength for this type of soil ?

Many thanks.

 

[GOTEK]

First, 5 kPa = 104.43 psf. Was Su reported from a vane shear test? I'm guessing probably not since no remolded value was given. Your reported undrained shear strength is low but not uncommon. The remolded shear strength is related to the Sensitivity of your clay...sensitivity is the ratio of the undisturbed shear strength to the remolded shear strength. Many of the clays that I have experience with the lower bound of remolded Su values can be anywhere from 0.1 to 0.25 of the undisturbed Su. Also,if remolding is being conducted in the laboratory then be sure that the remolded specimen's water content is equal to the undisturbed specimen's water content.

 

[jdonville]

Sylvaninhoo,

To expand a bit on GOTEK's reply, 100 psf for undrained shear strength is technically classified as VERY SOFT. More commonly known as "muck" or even "poo". Aside from the fact that you can't build with it, material this soft is very challenging to test as it pretty much deforms under any normal sampling pressure. I suspect that unless the material was tested in situ, you may have a remolded shear strength value. Again, echoing GOTEK, if there was an in situ test, both peak and remolded strength values should have been reported.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[BigH]

GOTEK - 104.43 psf??? One significant figure translates to 5?? I'd say that 5 kPa = 100 psf and leave it at that. Calculators can be deadly sometimes! Bring back the slip sticks!

In effect, he is dealing, at this strength, muck - or, as we Canuks would like to say - "Loon Shit". This is very low - doesn't really matter if peak or remoulded in practice. It is difficult to say anything about the sensitivity from this value. I have seen "not so sensitive" clays have these kind of values.

If he got the strength from vane, I would suppose that there would be some disturbance and this is also reflected in the vane test. You hit the right point of sensitivity and how significant it can be. Norweigan quick clays and those in Quebec can have sensitivities in the order of 500 or more. Your 10 to 25% of the peak being the remoulded indicates you are in sensitive clays - "normal" sensitivity is in the order of 1 to 2 - but many texts give the cut-offs and descriptors of sensitivity.

 

[fndn]

You mention seabed soil and such marine deposits have "apparent cohesion" which is very low. This cohesion depends on the depth of sample, however, the rate of strain and if the residual or peak strength were used can also dramatically change the cohesion value. Sensitivity as BigH mentioned ranges from 1 to 2. We need to know how that strength was obtained and what is it that you are trying to build?

 

[sylvaninhoo]

Thanks everybody for these profitable posts

The bottom of this is the analysis of subsea pipelines laid on seabed - and in particular how much they self-embed.

The soil is described as very soft clay with Su = 5 kPa (or 100 psf !) at suface, linearly increasing to 40 kPA at 33m depth. I have no idea what was the test protocol. It has to be the standard soil investigation for jack-up installation

 

[jdonville]

sylvaninhoo,

It's a tough call to advise you how to proceed. The self-embedment of the pipline will likely depend on some or all of the following:

-the unit weight of the pipeline with product inside
-the diameter of the pipeline, flexibility of the pipeline
-the depth of the sea floor (influences buoyant weights and effective weights)

I would assume, based on your limited data, that 5kPa is the unit cohesion of the surface.

Good luck with your analysis.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com