Consolidation analysis

Question

Hello Plaxis Community,

I am currently making a 3D model for a 7 story building on one basement level founded in soft clays.
My project is surrounded by sheet piles driven into the clay to retain the water table which circulates in the sands in the upper part. So I have major settling issues. I chose to improve the soils with preloading associated with vertical drains to accelerate consolidation. As the floors are built, I carry out consolidations in staged construction. At the end of the construction, I carry out a consolidation analysis with "minimum exess pressure" in order to dissipate the pore pressures as much as possible and thus determine the settlements due to long-term consolidation. The soil model is in HSM with clay in undrained A. My questions are: - When carrying out several consolidations from phase to phase, the results of the consolidation time in the Output seem to add up. Is it possible to display the consolidation time only of the phase? - When we want to compare the consolidation time with the degree of consolidation U(%), the consolidation time does not seems correct in the Output... Is it possible to see the consolidation time when doing an analysis with the degree of consolidation? - During my modelling, experts in plaxis my adviser to simulate as close as possible to reality the pore pressures to do the following phases: 1. Initialization in k0 procedure 2. Initialization phase by checking "ignore undrained behavior" 3. Earthworks of the project 4. Installation of the foundation raft with small loads and reset of the displacements in order to measure the settlements from this phase 5. Consolidation mininum exess pressure: this seems to allow the interstitial pressures to be well balanced with reality 6. Construction of the building 7. Minimum consolidation exess pressure: used to determine the settlements after long-term consolidation. Could you explain to me why it is necessary to do all these manipulations in order to better calibrate the pore pressures in the clays? Thank you all in advance for your answers 
 Aur&eacute;lien PACE

Vasileios Basas · Answer

Hi Aur&eacute;lien, 
 When running a Consolidation analysis with a Staged Construction loading type, it is necessary to specify a value for the Time interval parameter, which has, in this case, the meaning of the total consolidation period applied in the current calculation phase. 
 Although the time interval of each phase adds up to the previous total Time, this is only reflected in graphs where you plot a quantity (e.g. settlements or excess pore pressures) against the Time. This idea is logical because it would only be possible to express these quantities' development throughout staged construction (Time). 
 Regarding your first question, you can extract data from those graphs and post-process them in a spreadsheet, thus being able to plot them in different time intervals. Alternatively, you could select the "Reset time" option from the "Deformation control parameters" of each calculation phase, which will reset the history curves where Time is used on one of the axes. 
 Regarding your second question, it would be prudent to ask the person who gave those suggestions, as they may have a better idea and understanding of your model. However, given the limited information that I have at my disposal, I can explain a couple of things: 
 
 The "Ignore undrained behaviour" option temporarily excludes the effects of undrained behaviour in situations where undrained material data sets (Undrained (A) or Undrained (B)) are used. The selection of this option is associated with the selection of the Plastic calculation type. When the option is selected, the water's stiffness is not considered. As a result, all undrained material clusters become temporarily drained. Existing excess pore pressures that were previously generated will remain, but no new excess pore pressures will be generated in that particular calculation phase. This option is usually essential to ensure that Gravity loading results in initial effective stresses when undrained materials are used. 
 On the other hand, the "Reset displacements to zero" option should be selected when irrelevant displacements of previous calculation steps are to be disregarded at the beginning of the current calculation phase so that the new calculation starts from a zero displacement field. For example, deformations due to gravity loading are physically meaningless. Selecting this option is unnecessary because you can check your phase displacements instead of total displacements. However, if the deformations of Phases 3 and 4 are irrelevant to your analysis, then you can select it. 
 
 I hope this clarifies things a bit.