Advise to choice the value of normal and shear cohesion

Dear yade users,

I'm trying to do biaxial undrained test with cohesive contact law.

Actually I get the strange result if I choice the normal and shear cohesion that is very high (=1e10 N/m2), the confinement stress achieve the zero value for axial_strain=0.027
I trace the results: iteration VS (s_top= axial stress, s_right= confinement stress)
http://imgur.com/9N4mE

If I take the normal and shear cohesion =1e6 N/m2, the result is more reasonable.
http://imgur.com/VT6D3

Is there any reference or advise how to determine this value (either based on the "contact stiffness" or "average radius or another else")....?

Thank you very much for your reply.