About PLAXIS Calculation Optimizations

Question

hi sir: I have several questions about the "SIG Workshop: Geotechnical Analysis &ndash; PLAXIS Calculation Optimizations" lesson. The lesson shows: Stiffness changes are applied at the beginning of phase and unbalance is applied gradually during phase, change of stiffness without change of load has no effect. My question is : 1.When I use a node-to-node anchor with prestress force at a phase, when the anchor element is applied? when the prestress force is applied? beginning of phase or applied gradually during phase? 2.When I numerical a deep excavation, the first phase is excavation (deactivate soil element ) and the second phase is to install the slab (use activate plate element with vertical deformation fixed) . when the slab (plate element) is applied? beginning of phase or applied gradually during phase? 
 thank you

Micha van der Sloot · Accepted Answer

Hi Sheng-Hsien Wang, 
 For some reference on stiffness changes, please see: https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45962/reduction-of-stiffness-does-not-lead-to-a-change-in-displacements 
 To answer your questions: 
 
 Activate prestress of ground anchor: in such cases, the out-of-balance of the prestress is applied gradually during the phase with the SumMstage parameter increase. So gradually during the phase where you apply it. In general, it is recommended to define a separate phase for the prestressing, as this is in reality also a stage in which the excavation is halted until all ground anchors are pre-stressed; 
 When you activate the base slab, a plate element, the element and its stiffness are immediately activated (beginning of the phase). If the activation of the plate element leads to an out-of-balance, e.g. the self-weight of the plate, this is gradually applied during the phase. If you also fix the deformation of the plate element using prescribed displacements, this is also considered an out-of-balance and a force need to develop to reach this prescribed displacement. this is also solved gradually over the phase. Note that you prescribe an absolute vertical deformation of zero compared to the original coordinates, so not a zero displacement compared to the previous phase.