[Triaxial test]:unbalanced force & axial stress

Dear all,
       Thank you for noticing my question. I'm a new Yade user and I'm trying a triaxial test with it. But I encounter some problems in the testing process. When I test the particles which young modulus are MPa, the result of the test is regular. But when I change the young modulus to GPa, there are some troubles.

       I conduct the test using triaxial tutorial script-session1.py and modify it a little:
       1) After generating the particles, I add 10KPa to the three directions to make the particles touch.
       2)When the particles have touched each other, they are compacted by decreasing the friction angle gradually with 10KPa confining pressure. After this procedure, a sample with specific porosity is got.
       3) Adding the required confining pressure 0.8MPa till the unbalanced force<0.01.
       4) Loading the deviatoric stress by controlling the axial strain, its rate is -0.02.

        When I change the particles' young modulus to GPa, there are some problems:
        1)When the confining pressure 10KPa is added, the particles in the packing decrease. Maybe the packing has exploded?
        2) I increase the stress from 0.5KPa to 10KPa gradually, decrease the time step as:

                timestepSafetyCoefficient=0.1(before, it is 0.8)

              triax.goal1=triax.goal2=triax.goal3=-500
              while 1:
                   O.run(3000, True)
                   unb=unbalancedForce()
                   print 'unbalanced force:',unb,' mean stress: ',triax.meanStress
                   if unb<stabilityThreshold and abs(-500-triax.meanStress)/500<0.1:
                       break
                                     ....... .......
               triax.goal1=triax.goal2=triax.goal3=-10000
               while 1:
                       O.run(3000, True)
                       unb=unbalancedForce()
                       print 'unbalanced force:',unb,' mean stress: ',triax.meanStress
                       if unb<stabilityThreshold and abs(-10000-triax.meanStress)/10000<0.1:
                             break
The specimen is compacted and the particles doesn't disappear. It is defective that the unbalanced force can't <0.01, increasing the damping is no use. I increase the max unbalanced force to 0.03, the script can just continue. But when I load the deviatoric stress by controlling strain rate=-0.02, the axial stress is decreasing gradually from 0.8MPa even to 0.4MPa.(The confining pressure is 0.8MPa.) And when deviatoric loading, I can see the obviously moving of the particles.

       Thanks again!

Sincerely Geo