When I excavate the highlighted mass in the following image, the model fails to analyse the data. Some one suggested to reduce the sigmaM stage parameter to 0.3. in that case model is working. however getting warning massage mentioned as "Prescribed ultimate state fully reached. Unfinished construction stage. Next calculation must be staged construction. [Warning code: 2]" Hence i feel its not correct to reduce sigma M stage. some one please help with the modeling approach.
Hi Sumit,
The beauty of the model and the question does not match each other. The modle looks perfect with umbrellas, curves on the edges etc. It's a work of art
But, Mstage is the first step of NATM modelling, so I assume modelling has been done by some other person and you are trying to find out how to solve this model. Basically, what we do in NATM modelling is: Excavate (with Mstage < 1.0) unsupported, add support (with Mstage = 1.0) in next stage. So, what happens is Plaxis gives part of the load to the unsupported excavation.
Let's use some example numbers. To excavate your first round:
- Apply umbrella (yellow material), because that is a pre-support, that happens before.
- Remove the material on round 1 and in project settings select Mstage = 0.3 (example number). This means that only 30% of the total unbalanced load (due to excavation) is applied. Plaxis stops at 30%. This is a workaround to simulate 3D effects of tunnelling. Because, most tunnel will not solve with Mstage=1.0 without support. But if you skip this stage and put all load on to the lining, lining will be overstressed. Anyway, let's continue.
- Activate plates and bolts. Mstage is by default 1.0. So, this means, remaining 70% is applied in this stage (assuming you didn't change any load on the model, so loading condition does not change.)
- You can also sub-divide the previous stage into couple more stages to simulate the hardening of shotcrete.
Anyway, you need to read a little bit before jumping to the model. Even the Tutorial 5 of Plaxis shows a simple NATM tunnel. communities.bentley.com/.../plaxis-2d-tutorial-05-excavation-of-an-natm-tunnel
Hi Berk,
Thanks for your kind response, now i understand this approach modeling.