Face bolting in PLAXIS 3D

Question

Hello, 
 in the PLAXIS bulletin "Preliminary 3D Modelling of Structural behaviour of Face Bolting and Umbrella Arch in Tunneling" face bolte were moddeled with embedded beams and spring elements. In this paper, no major differences between these two methods were observed. I tried to repeat this investigation, but I struggle to get the model with the embedded beams work. The spring element works perfectly, but the modell with the embedded beam needs a lot more computational time and often closes with the warning "Picos_Back: incorrect solution, slow convergence". I changed stiffnes parameters and the geometry of the face bolts, but this didn't affect the calculation as much as I hoped. So at the moment I'm not able to get reliable results for a model with very few face bolts (5 bolts). Do I have to connect the bolt to a plate element or another structural element to get to a result? Are there any experiences? I'm currently working with PLAXIS 3D V 2018. 
 Thank you!

Faseel Khan · Answer

When you get a Picos solver error, we can change the solver type and that helps. First we need to define the solver types in PLAXIS which are as follows: 
 
 Picos (multicore iterative). It stands for Plaxis Iterative Concurrent Solver library. Picos is an efficient iterative solver that solves the system of equations in parallel on multi-core processors. It is generally the fastest way to perform calculations. 
 Pardiso (multicore direct). It stands for Parallel Sparse Direct Solver. Pardiso is a direct solver that solves the system of equations in parallel on multi-core processors. It is generally the most robust way to solve equations, but it has also the highest memory consumption. 
 
 The error message 204 can be due to several reasons, please have a look at Reference manual appendix on Calculation Warning and errors in PLAXIS. 
 These are the most common causes in giving this error: 
 
 Badly shaped elements: good elements are those with the same size for width, length, and height of the element. Please inspect the mesh quality in the Output program via the menu item Mesh > Quality. 
 There might be elements with very low quality (flat elements). If the issue is related to badly shaped elements, you might want to apply some local refinements. General information about mesh improvements and identifying problematic areas can be found here: 
 https://communities.bentley.com/products/geotechnical1/w/wiki/45545/error-when-generating-3d-mesh 
 Large stiffness differences in your model (e.g. a difference ratio > 1E6). This also includes the use of structures (beams, walls). In that case, please review the stiffness values. If the error occurs in a specific phase in which you are activating structures, this gives a good hint in which materials to look for the stiffness values. 
 Or a combination of these two above. 
 
 General information about mesh improvements and identifying problem areas can be found here: https://communities.bentley.com/products/geotechnical1/w/wiki/45545/error-when-generating-3d-mesh 
 If the error keeps occurring, please follow the rest of the above mentioned hints (see also the appendix in the Reference Manual): 
 
 Missing boundary conditions. Review the boundary conditions for this phase. 
 Floating/unconnected clusters/volumes. 
 Plate only connected to embedded beams without (inactive) soil elements connected to the plate. This can be solved by adding soil volumes below the plate and deactivating them.

Sometimes, it can help to change solver to e.g. Pardiso from the Numerical control parameters by double clicking on the phase and changing the solver type to Pardiso, but this will be slower and uses more memory compared to Picos (default solver). But it does not always help. So please focus on the points mentioned above. 
 
 In order to be able to assist you with your project and identifying the reason for the problem you experience, please submit a service request. 
 Then, one of our support engineers can help you in detail: https://apps.bentley.com/srmanager/ProductSupport

Face bolting in PLAXIS 3D

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