Hello everyone,
My first question here in the forum!
I'm modeling a set of foundations on weathered granite and I'm using the Hoek-Brown (HB) model. Everything in the material model works perfectly fine, however, three questions regarding the material model behavior surge:
1 - Does water/pore pressure have a relevant effect in the HB behavior?
2 - Aiming to understand the influence of pore pressure in a footing over a rock mass using HB, I did some simple models with a rectangular surface load with a pressure of 500 kPa. Without pore pressure the SF analysis runs without any problems, however, When there is pore pressure, the Safety Factor analysis has a very bizarre behavior. What should I conclude about this, that one can not rely on this analysis when there is pore pressure, or more generally that one should use this analysis with Hoek-Brown carefully?
2.1 - If instead of modeling the footing as a surface, I model it as a rigid body loaded with a point load on the top surface with a load that would develop a pressure of 500 kPa in the rock, there isn't such behavior with the pore pressure. How does the way I model the footing influence the final results?
Below I show two pictures of the safety analysis when the groundwater head is set equal to the top surface of the rock and when there is no groundwater at all. The atypical behavior of the SF analysis when there is groundwater, on the left, is quite obvious if compared when there is no groundwater head (on the right)
Fig. 1 : LEFT: SF analysis for HB model with z_groundwater = 0. RIGHT : SF analysis for HB model with NO water.
If modeling the footing as a rigid body the results are also quite different:
Fig. 2 : LEFT: SF analysis for HB model with z_groundwater = 0. RIGHT : SF analysis for HB model with NO water
Should I expect changes in the safety factor due to the water pore pressures? Does the way I model the footing really influence the final results?
Thanks in advance,
My kindest regards, Afonso
Which version are you on? I think it might be relevant to this, so if you are not at V22.02, please update and make sure that you enter manual cut-off.
https://bdem.notion.site/Hoek-Brown-Model-in-Finite-Element-Analysis-d31aa80256b143cfb61b03a8278d72c2
Answer Verified By: Afonso Anjos
Hej Berk,
Thanks for your reply and suggestion. I've dove into the tension cut-off issue within Plaxis 3D but unfortunately it didn't sort it out...
The issue is that when loading in compression a Hoek-Brown material, the model doesn't reach collapse by shear plastification, instead the collapse occurs in a bizzarre way, where the soil just pops up in the vicinities of the footing. thus reducing the overall safety factor. Like this:
If I then bury the footing the safety factor increases and this mode of collapse is reduced, but it does nevertheless continue to occur.
Cheers,
Afonso
Aaah, this happens when you don't have any plate or footing (as you mentioned in your main question) beneath the surface load. This is just a very very small bearing capacity failure, because you do not distribute the load. Always put some kind of a plate under surface loads if they are not very small.
Thanks again for the tip. Berk!But so far I have also tried with a plate below the load, but the problem still persists. It seems that these points yield in tension, but increasing the tension cut-off doesn't unfortunately solve the problem. I'm starting to doubt if using Hoek-Brown with Plaxis is really the most appropriate way to model footings on rock...
Dear Afonso,
To be able to conclude anything it would require some investigation from our side. I am sure we will manage to explain things if we are given the opportunity.
As far as I can see we are still waiting on you to send us your project.
Can you please double-check your emails?
Dear Stefanos,
Sorry for venting like this! I didn't indeed see the email asking to share the projects. I will share them immediately . thanks in advance.