Time dependent plastic analysis

Question

Hello, 
 I have two questions about the time dependent analyses in Plaxis 
 
 1. There is an option in "plastic" analyses to enter the time. But it also says it does not consider the soil consolidation. So what is this time input for? how is it being considered in the analyses? 
 
 2. For calculating the consolidation, can the "HS Small" model be used? 
 
 Cheers, 
 Farbod

Vasileios Basas · Answer

Hi Farbod,
Indeed a plastic calculation does not take time effects into account. But there are two exceptions: one is when we use the Soft Soil Creep model, and the other is when we use the viscoelastic behaviour for geogrids. For the latter case, you can also check November's coffee corner on the "mechanical behaviour of geosynthetics". You can find it here: event.on24.com/.../45EA334E1B5491C616B6374F8B81C028
For your second question, the answer is yes. You can use the "HS Small" model for consolidation calculations.

Stefanos Papavasileiou · Answer

Dear Farbod, 
 The phenomenon of soil creeping is directly linked to the time parameter. I would advise you to check our detailed article about the practical application of Soft Soil Creep model here: Practical application of the Soft Soil Creep model 
 Time property in a phase is not considered in a steady state groundwater flow calculation as we assume time to be infinitely long enough to reach a steady state condition. Note that pore pressures and deformations are not coupled when a Plastic analysis is used. That is the case only for the Fully coupled flow-deformation analysis. 
 You can find more details on this topic on our Reference manual.

Stefanos Papavasileiou · Answer

Dear Farbod, 
 Creep is a phenomenon that develops from the very first second of times. The SSC model does not differentiate the moment creep "steps in" because it is always there. 
 In SSC model the creep is there but the rate the model creeps is different at times. But that has to do with how the model is formulated so if you read the practical application it should be clear.

Stefanos Papavasileiou · Answer

Dear Farbod, 
 I recommend you check our Reference manual about undrained behaviour as it is very difficult to explain what undrained behaviour is in PLAXIS in a Forum post. 
 For SSC there is no specific behaviour, it is like any other model but only supports Undrained A in which stiffness and strength are defined in terms of effective values. A large bulk stiffness for water is automatically applied to make the soil as a whole incompressible, and (excess) pore pressures are calculated, even above the phreatic surface. 
 You mention long-term but typically in a consolidation analysis the Drainage type does not matter: After a consolidation analysis I have excess pore pressures in "drained" material sets. Why? 
 Also here's a nice article about SSC and undrained triaxial creep behaviour (our validation): Soft Soil Creep model - undrained triaxial creep behaviour of Haney clay

Dennis Waterman · Answer

Dear Edwin, 
 You're making the story a bit confusing by mixing concepts. So to clarify: 
 1) When doing "Plastic" analysis type one is NOT doing consolidation. Hence, the statement "I found a consolidation analysis carried out with a plastic analysis type" is contradictive and incorrect. 
 2) There is no such thing as elastic, primary and secondary consolidation. There is "consolidation" (aka primary compression), which is the time dependent generation of settlements due to dissipation of excess pore pressure and there is "creep" (aka secondary compression), which is the time dependent generation of settlements due to densification of the soil particle matrix. Those two are independent phenomena. 
 3) PLAXIS can calculate the effects of consolidation by doing either a "Consolidation" or "Fully coupled flow-deformation" analysis type for any constitutive soil model. 4) PLAXIS can calculate the effects of creep in any calculation analysis type under the condition that both: - one uses a constitutive model that can calculate creep. For instance Soft Soil Creep, Sekiguchi-Ohta Viscid, Creep-SCLAY1S. - one specify a time interval for the calculation phase With kind regards 
 Dennis Waterman

Dennis Waterman · Answer

Dear Farbod, 
 1) In a plastic analysis (or any other analysis) when time is specified the SSC will experience creep. Creep and consolidation are separate independent phenomena. Hence, time does matter. 
 2) If the material is drained no excess pore pressures will be generated, and if the material is undrained than excess pore pressures will be generated. Note that the setting drained / undrained is only used in a Plastic, Dynamics and Safety analysis. A Consolidation, Fully-coupled flow deformation and Dynamics with consolidation analysis will use the real permeabilities to simultaneously develop and dissipate excess pore pressures. With kind regards, 
 Dennis Waterman