Hi, everybody
Currently, I'm running a pseudo-static analysis for the assesssment of a COMB-WALL (king pile + sheet pile). In the next figure you can see the model configuration
In green a very soft soil (MH)
In blue a sandy soil (SM)
In orange a the bearing stratum (GP)
In gray a quarry run (QR)
The fact is that when I'm running pseudo-static analysis (kh = +/- 0.24g) in the model the multiplier SumMstage not converge to 1 and I have to stop running because it is much time consumed. Then, I can't assess the behaviour of the COMB-WALL in this condition. Very soft soil is modelling with soft soil creep model. When I check out the deformations in output, the deformation in the very soft soil is extremely large, like the soil "fails". I have some questions:
1. Is it okay to model a very soft soil with SSC creep model when it is analyzing a retaining wall and not necessarily a settlement calculation or I should have used HS or HSS?
2. Could a very soft soil fail when acting a inertial force due to the dynamic loading? What can I do in order to allow the model runs completely? Without a complete running I can't assess the forces acting in the COMB-WALL.
3. I have tried to run the model eliminating the very soft soil when a pseudo-static analysis is performing, considering that the "soils fails" and then it does not contribute any passive resistance to the COMB-WALL. The assumption is conservative but, is this correct?
I hope you can help me with these questions
Ramón S.
Dear Ramón,
1) It is ok to use the SSC model, but the question is, why would you? It's a layer at the bottom of some body of water, why would it be interesting to calculate creep there if your're interested in the stability of the retaining structure? You could have used the Soft Soil model (it would save you a parameter and defining a time interval in the pseudo-static analysis). Or, as you already suggest under 3), maybe leave it out completely if it hardly contributes to the passive resistance.
2) Of course a very soft soil can fail under a pseudo-static loading (you're not applying a dynamic loading) just like any other soil. It doesn't matter if it's soft or hard, it matters what the strength is. If you really filled in the soil parameters correctly there is nothing you can do, the conclusion is that the soil cannot handle a constant acceleration of 0.24G. Now it could be the the failure occurs somewhere that is not interesting from engineering point of view - in that case you could decide to either make the material stronger, or leave it out.
3) How do you define correct? You leave something out that is in reality there, so in that respect it is not correct. On the other hand you say you know it will give a conservative answer, so it may not be correct but still acceptable. Do think about what part of the solution is conservative if you remove the very soft layer .... is it structural forces, factor of safety, deformations? Some results may be conservative while others aren't.
With kind regards,
Dennis Waterman
Dear Dennis thanks for your repply.
Totally understood.