Is subsea soil ONLY undrained ? 4

[SpaceMonkey89]

Hello fellow members. This is my first post on the Eng-Tips forums so I would just like to say it's a pleasure.

I am verifying the stability of a subsea gravity foundation using API-RP-2A and was wondering if it is even applicable to use a Drained analysis.
As I understand we use a Drained analysis for coarse grain soils (sand/gravel) that have high permeability which allow the water to drain out when loaded. We use an Un-Drained analysis for fine grained soils (clay/silt) which don't allow (at least in the short term) water to drain out.

My question is if considering a soil that is hundreds of meters bellow the mean sea level, is it appropriate to consider a drained analysis. Since we now have massive water column above the foundation level, can water still drain out of the soil under the foundation ?

Much appreciated.

 

[oldestguy]

I suggest you look up how to run a tri-axial compression test. Similar boundary to your example.

 

[Ron]

SM89...yes

 

[Ron]

My "yes" answer was to your question, not the title of the post. If the foundation load is sufficient, water can be "squeezed" from below the foundation.

 

[L0k]

Hi SM89, the answer is yes.

 

[SpaceMonkey89]

Thank you very much for your help. Much appreciated!!

 

[BigH]

SM89 - may I suggest that you do some research on past geotechnical analyses for foundations in the seabed - including deep piles. In general, as others have posed, undrained analyses are typically used for foundation design. You might wish to see the papers by Robert G Bea and the OTC papers. Drained analysis could be done if one could model the developed pore pressures - but this is not easy nor accurate which is why undrained is normally used.

 

[killswitchengage]

I dont think the answer is Drained . You see the soil is in permanant contact with water so shear resistance is reduced to undrained situation for me , also the soil is saturated all the time where the water may dissipate ?

 

[BigH]

@killswitchengage - saturated soil will dissipate pore pressures due to loadings - even under water. Undrained analyses are typically used because it is difficult to determine exactly (or model) the porewater pressure responses to loadings - especially pile loadings. Effective stress (ultimately it becomes drained) response is the "proper" way to analyse soil analyses but, as indicated, may be difficult to do.

 

[killswitchengage]

Good point BigH
I just find it hard to use Drained situation at all in this case

 

[SpaceMonkey89]

My understanding was that drained is used for coarse grain soils where the drainage rate is high (porewater pressure dissipates quickly) whereas undrained is used for fine grain soils such as clay where immediately following loading the pore pressure cannot dissipate (quickly!!! over time it will!).

So to put it layman's (thats me ) terms drained for sand, undrained for clay.

Is this the case or am I mistaken ?

 

[BigH]

With time, SM89, all fine grained soils will drain. Please read my post (up two from yours) . . . undrained is used for fine grained soils because effective stress analysis is difficult to model IF you don't have the porewater pressure response for loadings. Pick up a good geo book showing porewater pressure response to loading . . . (Lambe and Whitman come to mind).

 

[SpaceMonkey89]

Thanks for the recommendation BigH. I guess I have some reading to do !!!

 

[Ron]

To follow up on BigH's comments, sometimes the classic geotechnical texts give a better explanation than the newer texts of the concepts that you need to review . I agree that Lambe and Whitman would be a good one for this subject. They "wrote the book" on lab testing, so their explanations are clearer and more relevant.