Reasons for pairing Plastic and Consolidation analyses with each excavation phase?

Hello Max,

I would first ask the consultant as there might be a specific reason why to follow this approach. 

Note that a Consolidation phase can also compute Steady-state pore pressures as shown below:

In principle, a Plastic calculation considers instant (in no time) loading and activation (or deactivation) of body weights Additionally, in a Plastic phase excess pore pressures may develop depending on the Drainage type option (i.e., undrained). The steady-state groundwater flow is in any case solved separately.

A Consolidation is a phase where the main purpose is to dissipate excess pore pressures based on permeabilities and boundary conditions. Moreover, a Consolidation analysis considers staged construction loading type (as Plastic), but in connection to the time specified. Good read: https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45973/load-activation-in-time
Check here about excess pore pressures in consolidation analysis: https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45958/after-a-consolidation-analysis-i-have-excess-pore-pressures-in-drained-material-sets-why

I hope that this helps as the decision is for the geotechnical engineer in the end.

Thanks for the detailed response Stefanos.  I suppose my main question is whether I am introducing "artificial" additional deformation by having both a plastic phase and a consolidation phase and summing the combined ux from both phases.  For instance:

[1]: End of plastic phase: Px=50 mm

[2]: End of consol phase: Px=25 mm

implied Ux at end of staged excavation: Ux = [1]+[2] = 75 mm

If the consolidation calculation considers staged construction/excavation as deactivation of body weights (as per Plastic), then is the Plastic calc redundant?

If you wanted to get the most accurate picture of lateral deformations would you consider the results of both plastic and consolidation or just one? 

Dear Max,

There is no artificial additional deformation, the deformation from the plastic phase is real.
A reason to split the construction phase in a plastic phase + consolidation phase is time. If a load is applied in a 100 days consolidation phase then the load is applied linearly in time, hence in 100 days. If in reality the load is applied very fast and after that it remains constant, this should be modelled with a short phase in which the load is applied + a long phase in which there is only consolidated. Now that short phase could still be consolidation, but if it is really short (let's say 1 day or less) then the behaviour will be almost perfectly undrained and a Plastic phase would do. In fact, a very short consolidation phase may have the problem that the duration of the phase is smaller than the critical time step for consolidation and then one has to refine the mesh to overcome that - in which case a plastic phase is easier.

With respect to excavation, please note when soil deactivated in PLAXIS it is not excavated like in reality (removed from top to bottom), but the best physical equivalent from what happens in PLAXIS is that we reduce the weight of the soil gradually until it is zero. This means that the stress distribution against the wall during the excavation process in PLAXIS reduces to zero in a different way then in reality, I've tried to show that in the picture below:


As you can imagine, if you do this gradually in a consolidation analysis the time dependent generation and dissipation of excess pore pressures in PLAXIS may also be slightly different from reality. That is why some users prefer to do the excavation in a plastic phase and separate the consolidation by putting it in a next phase. Be aware that by doing so in an excavation one would generate maximum excess pore tension (since it won't dissipate during the duraction of the excavation) and this may overestimate the short time factor of safety of the excavation.

With kind regards,

Dennis Waterman