1. When I simulated the process of caisson excavation and subsidence, there would be a cycle process of excavation-sinking-excavation, which is embodied in plaxis3D as freezing the soil inside the caisson and changing the soil material at the position of the caisson to the caisson material. However, the problem of soil mesh pre-deformation will cause the newly activated caisson to continue the soil pre-deformation there. How can I solve this problem?2. In the process of tunnel excavation, there will be pre-deformation of the soil grid in front of the tunnel. When the tunnel passes through there, the lining needs to be activated, and the lining will also continue the original soil deformation. I wonder why everyone does not consider this problem, is it the reason that the soil grid pre-deformation size is very small?3. Can the displacement of each stage be reset to 0 to solve this mesh pre-deformation problem? (You need to manually overlay the results of each stage in the post-processing phase)
At the end, there is an exaggerated deformation diagram to show how large the predeformation of the underlying soil is and how large the unreal deformation of the caisson is in the next stage
Dear Xin Fang,
It's not considered a problem, because in PLAXIS Output there is an option to leave out the predeformation from the displacements. It's the option called "Sum Phase displacements". This deformation plot gives you only the sum of the phase displacements without the predeformations.
With kind regards,
Dennis Waterman
Dear Dennis，
1.In my tests， Sum Phase displacements were only useful if the grid above an existing embankment to be built was never activated, such as in the Construction of a road embankment. If tunneling was involved, In the situation that the soil grid in front of the tunnel face used to be soil but would be modified as other materials in the future, there is no difference between using Sum Phase displacements and the total placements. In short, for a grid that has never been activated before and will only be activated once later, the total phase displacement can be used, while for a grid that has been activated and subsequently modified the material, there is no difference between using total phase displacement and using total displacement. my plaxis version is V22,Has the v23 solved this problem?
2.And another question, why did we ignore this kind of pre-deformation problem in the previous version? I think this kind of problem always exists in the process of tunnel excavation. I would appreciate it if you could answer my question。
1) Yes, that is solved in PLAXIS 2023.
2) Because for structural elements it is not considered so much of a problem by our users. For embankments we received many requests to be able to discard pre-deformation and so it was implemented. For structural elements we didn't really get this request and so it was implemented only recently after some request from users that had a problem with pre-deformed retaining walls in excavation projects.
In my opinion, the pre-deformation of the grid will have a great impact on the internal force of the structure. I still take the stage activated caisson shaft wall as an example (plate element). The preset position of the shaft wall in the structural mode is "|" , but because of the continued deformation of the soil, it will form an unreal position,just like "丿".If I apply a concentrated force at the bottom or top of the shaft wall, the calculated internal force of the structure is seriously inconsistent with the reality。What do you think about this problem, teacher？
The predeformation has no influence on the internal forces. Predeformation does not give internal forces, it only determines the initial position of the nodes. Further deformation is not influenced either since deformations and/or forces are always calculated incremental relative to the undeformed mesh. Hence, it only doesn't look very nice, but apart from that it is not really a problem.With kind regards,
Dennis WatermanP.S. I'm not a teacher, please don't call me like that.
Answer Verified By: xin Fang