Hello
I am studying the seismic behavior of a soil-nailed vertical wall in a dry cohesionless medium (mainly in order to evaluate site displacement). Since I can’t have enough information about seismic movement, im considering using a pseudo-static approach. I found a lot of studies about the subject using PLAXIS and a pseudo static approach but from my understanding, this is not a suitable choice for excavation studies since the results are dependent on the chosen project size (a bigger project area will result in higher forces and displacements). I would like to have your thoughts on the subject? We have done already a site stability study using a Limit equilibrium mathod, and we have an idea about the position of the critical slip surface, could we use these information in order to fix the vertical boundaries of The PLAXIS model) (i.e. place the vertical boundaries a few meters away from the critical surface).Thank you
Best regards
Alia HATEM-QUIRIN
The first part of your question depends on whether you are considering elastic or elastoplastic behaviour. This question is similar to the question answered at the following link: RE: Size effect on modelling Plaxis 2dPLAXIS determines the most critical slip surface in the safety analysis. You can try extending the model boundaries and the geometry and material properties will dictate the shape the critical slip surface. There is also an option to exclude materials from strength reduction as this option simply instructs the cluster not to "participate" in the strength reduction. You can then create the model geometry in such a way that the soil clusters form the shape of a pre-defined slip surface and you can the exclude strength reduction in the remaining soil clusters thereby forcing the slip surface to pass through the pre-defined zone.
Dear Mr Khan
Thank you for your answer, it is quite helpeful. From what i understand: if I’m using an elasto-plastic model like HS, HSsmall, I could run my calculations using a pseudo-static approach (the geometry of the model could be defined according to the second part of your answer)?
I want to make sure since some previous posts on the site suggest that using a pesudo static approch for excavation problem is not a suitable choice (whaterver soil model have been used):
pseudo-static analysis in PLAXI - GeoStudio | PLAXIS Forum - GeoStudio | PLAXIS - Bentley Communities
As I have mentioned before, the idea of my study is to calculate the site displacements so I have to make sure that those displacements have a physical meaning (which will not be the case, if the calculations are model size dependant) and hence the idea to fix the model boundries near the position of a pre-defined critical slip surface.
Thank you again. I appreciate your help.
Alia QUIRIN
Dear Alia,
As you already mention, the displacements and also the structural forces have no physical meaning because they depend on the chosen geometry size. So if you arbitrarily choose the geometry size "a few meters larger" than the slip surface ... what makes you think that exactly that size would have a phyiscal meaning? Not to mention that "a few meters" by itself is very inaccurate ... would that be 2 meters, or 5 meters, or something else?
The problem is that with pseudo-static analysis a constant acceleration is applied in 1 specific direction for an infinite long time period. In reality in a seismic event the acceleration is not constant but continuously changing and over a limited amount of time.
Hence, pseudo-static analysis is fine to determine a failure mechanism for problems that have free boundaries, like a dam or an embankment, but not for either failure mechanisms or displacements that are in between fixed boundaries like an excavation.
With kind regards,
Dennis Waterman
Dear Dennis
Thank you. Your answer is enlightening and it confirms my understanding of the application of the pseudo static approche. The idea behind fixing vertical boundries few meters away from the critical slip surface is only a suggestion to overcome the problem of a mesh-dependant results since i imagined that resulting forces /displacement could be slighlty greater (but still acceptable for my project) than those resulted from a failure analyis. This idea is of course very discutable. Thank you again.
Kind regards