I have modeled a tunnel with Plaxis program.The output shear forces in the bottom of linning( at the end section next to the ground) are extremely large and unreliable. 1-Is it a problem with plaxis? 2- Hoe can we overcome this difficulty considering the fact the in real cases the linning(shtcrete) can bear the shear forces?
If i am reading it correctly, the symptom refers to a common problem inherent to use of Mindlin plates in FE analysis. This known as "shear locking" happens not just in Plaxis but in any other fe codes attempting to model shear deformation bending action of thin beams where the shear strains become very small as compared to the bending strains. For tunnel lining analysis application, the curvature of the beams "amplifies" this situation and difficult to get round this problem.
For circular tunnels with significant overburden loads, rarely transverse shear force in the lining is of concern due to the existence of hoop thrust. If you like, you can do an approximation to the shear force by calculating Vmax=(Mmax-Mmin)/R (where R is the tunnel radius).