"mudmat" type foundation on a clay inclined slope 1

[nicolini]

I need to verify the stability of a "mudmat" type foundation of a structure to be placed at sea bottom.
The soil is there made by a very soft clay and its surface is inclined.
The base of foundation is inclined parallel to the slope and is provided with "skirts".
The problem is now how to compute the maximum shear force (parallel to the slope) sustained by the foundation during relatively short term loading (undrained conditions).
In fact, if I perform an undrained analysis, the initial shear stress at the base of the skirts is to be taken into account or not?
Thank You for helping.

 

[Focht3]

Hmmm,

Are you checking for the stability of a jack-up rig? If so, then you need to proceed with extreme caution -

And we will need a lot more information in order to help you: foundation size, depth, dimensions, soil properties, inclination, etc. And it would help to know where this is going (approximately.)



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[nicolini]

No, it is not a jack-up foundation; it is a small service structure to be placed at sea bottom.
Foundation size (B×L), as well as the height of the skirts, are the elements to be designed.
Available data from site investigation allows to assume an
increasing with depth Undrained Strength, with a rate equal to about Su=1.5×(depth below mudline) [in kPa] (i.e. Su=1.5kPa at 1.0m below mudline).
Now, the shear resistance of the mudmat is:
Sr = Suo×B×L (where Suo is Su at the skirt tip depth) if the seabed is horizontal.
My question is this.
If the same soil is sloping of about 10÷15 degrees and the mudmat is placed on it, being the foundation base parallel to the slope (installation by self-weight), does the same formula hold (for short term loading)? Or is the initial total shear stress at the skit tip level, produced by the sloping surface, to be added?

Any suggestion will be really welcome..........

 

[Focht3]

Hmmm,

To a certain extent the answer is, "It depends."

If you are using undrained analyses, then you only have to consider the forces added by the foundation and imposed loads. For an effective stress analysis, you pretty much have to consider everything. Unless you have a reason to suspect the soils can/will drain rapidly, I'd stick to an undrained analysis.

That's a pretty weak soil; at 5 meters (16 feet) the shear strength will be 7.5 kPa - about 157 psf. You will have significant settlement issues to address. And you won't get "decent" shear strengths until the structure penetrates at least 30 or 40 meters. I'm not sure that your current arrangement is practical as a "final" foundation...

Yet your most significant problem is really slope stability, not settlement or the lateral stability of the structure (i.e. its ability to resist downslope movement.) As I recall, a 15 percent grade (about 8.5[°] slope) in very weak soils has a significant risk of experiencing instability. Has anyone performed a bathymetric survey (using side scan sonar) to look at the bottom stability over a wide area around the site, say 260 km²? Have you carefully reviewed the results?

Is this site in the Mediterranean? North Sea? Gulf of Mexico? Some background on the geology of the area is needed in order to better understand the soils you are dealing with -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[nicolini]

Ok. thank You, Focht3,
actually, this is the approach I will follow.
As far as the "general" topic is concerned, that soil is really "a shame", but very often shallower material at sea bottom are so poorly resistant.
The site is actually not defined, we only know soil data and the basic geometry of the structures to be placed.
I reviewed the available soil data and, basing on results of CPT given by the Client, estimated the Su versus depth I told.
The good new is that such structures are very light, but as the soil is soft, we need to be sure of the calculation
method as the factors of safety we obtained until now are
relatively low and the height of the skirts not negligible.

So, really thank You for helping and suggestions.
Bye.