SS vs SSC

Question

New user, in need of guidance trying to figure out what model is best for the analysis of fill under a building over deep, soft soils. 
 Does the SS model calculate only primary consolidation and the SSC only secondary creep settlement as a result of loading onto the soft soils? 
 OR 
 Does the SSC calculate both?

Dennis Waterman · Answer

Dear Callum, 
 Consolidation is not a material behaviour nor a material property, it is the phenomena that excess pore pressures are dissipated in time leading to settlements. In order to calculate consolidation one would need to define the type of calculation of the phase as "Consolidation". Whether you would use for the soil Linear Elastic material, Hardening Soil, Soft Soil Creep or something else doesn't matter - if there are excess pore pressures a Consolidation calculation will dissipate them. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

As I already explained, ANY model calculates consolidation settlements because consolidation is not a material property, it's a calculation type. The naming of the model already gives the hint: Soft Soil model = no creep, Soft Soil Creep model = creep. If you use equivalent parameters in Soft Soil and Soft Soil Creep model then the Soft Soil Creep model gives more settlements than the Soft Soil model because of the creep. But notice that the Soft Soil Creep model is not just the Soft Soil model + creep; it's a quite different model. Hence, one cannot just rely on having the same lambda* and kappa* for both models. Apart from that, another reason why the Soft Soil Creep model gives less settlements than the Soft Soil mode is if no time interval is specified when using the Soft Soil Creep model - this will make the model behave purely elastic only using the kappa*, which is a far stiffer behaviour than the Soft Soil model. The Soft Soil Creep model always needs a time interval. With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Hi Callum, 
 The equation for elasto-plastic deformation for Soft Soil and Soft Soil Creep is quite different. I'm not sure what version of the manual you're referring to, because I can't find the equations on the pages you mention in the 2D 2022 manuals, but referring to that version of the manual, if you check equations 226 for the plastic response of Soft Soil compared to equation 258 for the plastic response of the Soft Soil Creep model you see that they're quite different. In Soft Soil the plasticity is determined only by lambda*, whereas in Soft Soil Creep plasticity is determined by (lamda* - kappa*)/mu*. So indeed, you may determined lamda* and kappa* in the same way from a soil test, but then once put in the model you calibrate the parameters, you may end up having (slightly) different values of lambda* and kappa* in both models to get the best representation of the test results used to determine lambda* and kappa*. You have to be aware is that what you determine from the oedometer test are soil parameters representing the complex behaviour of the soil. What you put in PLAXIS are model parameters and a model is a simplifcation of the complex soil behaviour. Which means one should always check whether the model used with the parameters determined really respresent the test on the real soil (by simulating the test in PLAXIS) and possibly adjust/calibrate the paramters in PLAXIS to get the best match. And Soft Soil and Soft Soil Creep have a different way of simplifying soil behaviour, hence after calibration they may have slightly different lambda* and kappa* to get the best match between model behaviour and soil behaviour. With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Well, first of all, if you apply a load in phase with a time interval, the load is applied linear in time. So in your case the building load is gradually applied over a period of 10 years. I'm not sure that is really what you intended? Hence, you may want to insert a phase to model the application of the building load in what is more or less the construction time of the building, and then leave everything for 10 years to just consolidate and creep. Since you apply the load in 10 years, you probably never reach consolidated state as you keep developping excess pore pressures by the continuous increasing load. Hence, the soil behaves pretty much undrained and the so if you push it down in the middle, it heaves on the sides. 
 Third of all, your boundary conditions are way to close to your applied building load. The influence zone of an applied load on undrained soil is up to 3 times the width of the area where the load is applied. Hence, the left and right boundary should be at least a distance of 3 times the width of the load from the load.... Now the upheave is very concentrated in the narrow area between load and boundary condition, and therefore heaves unrealistically much. 
 Best regards, 
 Dennis Waterman

Dennis Waterman · Answer

Sorry, I forgot 
 Well, no, because also in a plastic calculation there is creep. So if would already make a difference whether you would specify a plastic phase of 100 days or 100 years, the latter would give a lot more deformation. Creep is something that is always there, whether there are pore pressures or not and whether there is consolidation or not. There will always be creep though it will go slower and slower in time. But there is no asymptotic value or so that would give it a maximum: at infinite time there is infinite settlement - or at least according to the Soft Soil Creep model there is. In reality at some point probably creep would really stop when the soil reaches it's minimum void ratio, but that is not taken into account by the Soft Soil Creep model 
 Best regards, 
 Dennis Waterman

SS vs SSC

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