When modeling a coal mine in RS2, is it possible to limit the slot convergence?

When modeling a thin slot in RS2, it is certainly possible that you can get convergence greater than the thickness of the slot.  There is nothing in RS2 to limit the amount of deformation based on some predefined condition. Obviously if you find that the deformation exceeds the thickness, this indicates that the excavation is unstable and would close without some sort of support.  This assumes all the properties you've defined are realistic.  The factors that influence the amount of displacement are 1) strength parameters (if you are getting a lot of failure, then most of the displacement is plastic displacement), and 2) the modulus of the material.  You should always make sure that these are correct.

With regards to limiting the slot convergence, this is not an easy thing to do in RS2 , and we do not have anything built into the program to handle this.  Boundaries are allowed to deform without restriction and there is no way to define a maximum amount of relative displacement between two points.

However, you can try to approximate this with sliding gap liner elements (Tutorial 20) and staging the stress field using the induced stress option (Tutorial 33).  

Sliding gap liner elements are liner elements that do not allow any axial or moment until a certain amount of strain has occurred.  So, if you use them as struts between the roof and floor and then define a strain of 100%, basically they’ll kick in once the floor meets the roof.  If you then set the modulus of these liners to an extremely high number ( 1e10), then no relative movement will occur between the roof and the floor once they kick in.

Now it’s not a simple matter of adding these liners and doing a quick 1 or 2 stage model.  What you have to do is relax the boundary and allow it to displace over a number of stages.  This allows the sliding gap elements to properly lock at a certain amount of strain (see tutorial 20).  To do this, the method outlined in Tutorial 33 is used.

The models are attached.  The slot in the attached model is 2m high.  An unsupported displacement of 3.5m occurs at the center of the excavation.  You’ll notice that after the sliding gap elements are added, the roof and floor displacements are around 1m at the center, for a total of 2m convergence.  Notice also how the liners pick up the load from the center outwards as the roof and floor meet from stage to stage.

 

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Comparison of support and unsupported slot excavations in RS2