Dear Suraj,

I used Drucker Prager in modeling shear slip. A slope loaded by gravity then fails, beginning with a 'hole' or something at the bottom which grows upward and on the surface this slip line may show as a sinkhole. There are two reports on my profile with essentially the same model as a basis. You can choose that with damage, where there also is a case with zero damage.

It is not so easy to obtain the state and solve numerically. There are references to Pamin and deBorst, from where I got the geometry, parts of model and load case.

(I think there are also other kinds of sinkhole, below e.g. asphalt where empty space)

Kind regards,

Lena

HI,

I developed a softening behaviour of the CYsoil model in FLACv5.0. If you are interested, I can help you to put it in your model.

Just use the cohesion, friction and tension tables (relating them with strain) with MC SS CM.  

Hi Khadka,

Your question sounds very interesting to me. I work on the mechanisms of sinkhole development over abandoned mine workings in my PhD and I have some ideas that might be useful for your research.

First of all, it is not that difficult to use the strain softening/hardening Mohr-Coulomb or the softening ubiquities joint models in FLAC/FLAC3D. Generally speaking, in all softening models, the mechanical parameters of rocks are gradually reduced as a function of plastic work/strain.

The only things that you need in FLAC/FLAC3D, to include softening effects, are a few tables to reduce the shear strength parameters of rocks as a function of plastic strain. You, of course, need some experimental tests to calibrate your model (to identify how to reduce the parameters).

If you need a good understanding of the strain softening/hardening models in geo-mechanical engineering, I would suggest reviewing the paper published by Prof. Pieter Vermeer and Prof. René De Borst that you can download it, for free, from ResearchGate:

https://www.researchgate.net/profile/Rene_Borst/publication/42621223_Non-Associated_Plasticity_for_Soils_Concrete_and_Rock/links/544e34100cf2bca5ce8f1619/Non-Associated-Plasticity-for-Soils-Concrete-and-Rock.pdf

The only question is that the mechanisms of sinkhole formation is time-dependent. The degradation of the mechanical parameters is not normally due to plastic work (e.g., like micro crack propagation due to mechanical loading, or creep). But it is due to very complex deteriorating factors such as weathering, swelling and even moisture softening.

Weathering, for example, is a very challenging coupled B-C-T-H-M prosses and involves lots of factors. Humidity and exposure to the atmosphere are among the main reasons accelerating the rate of weathering.

One, therefore, may say that it is not reasonable to use the softening models in FLAC for simulating such complex time-dependent processes that impact on the rock mass mechanical behaviour. This is because the mechanisms of softening / weakening is different. I developed a few simple but practical approaches during my PhD. One of them has been presented at:

The effect of rock mass gradual deterioration on the mechanism of post-mining subsidence over shallow abandoned coal mines

https://www.researchgate.net/publication/310592736_The_effect_of_rock_mass_gradual_deterioration_on_the_mechanism_of_post-mining_subsidence_over_shallow_abandoned_coal_mines

You may find this technique useful.

Best regards,

E. F. Salmi