I want to carry out tunnel with length 100m and diameter 7m below 5m ground surface in PLAXIS programm 3d. The profile of soil consists from 11m soft clay and 39m dense sand.
Check also your initial load state. In case of shallow tunnel there might be a problem, when appied full load to a tunnel with support installed. Try to apply a small portion of load to a tunnel without support, and see what happens. If the problem persist, your probelm is innitial stress state definition.
What material model are you using for the analysis? In case you modeled the initial phase correctly, this might happen if you are using linear elastic model (with MC failure envelope), where the loading and unloading stiffness modulus is the same. Try using Hardening Material model, which takes into account different moduli for loading and unloading.