How to solve the problem of particles penetrating through boundary in triaxial test?

Hello everybody:
I am using the following code to conduct the triaxial test. But the particles penetrate through the boundary in the triaxial test. How to fix this problem?

from yade import pack, qt

############################################
### DEFINING VARIABLES AND MATERIALS ###
############################################

# The following 5 lines will be used later for batch execution
nRead = readParamsFromTable(
 num_spheres = 1000, # number of spheres
 compFricDegree = 30, # contact friction during the confining phase
 key = '_triax_base_', # put you simulation's name here
 unknownOk = True
)

from yade.params import table

num_spheres = table.num_spheres # number of spheres
key = table.key
targetPorosity = 0.43 #the porosity we want for the packing
compFricDegree = table.compFricDegree # initial contact friction during the confining phase (will be decreased during the REFD compaction process)
finalFricDegree = 30 # contact friction during the deviatoric loading
rate = -0.02 # loading rate (strain rate)
damp = 0.2 # damping coefficient
stabilityThreshold = 0.01 # we test unbalancedForce against this value in different loops (see below)
young = 5e6 # contact stiffness
mn, mx = Vector3(0,0,0), Vector3(1,1,1) # corners of the initial packing

# create materials for spheres and plates
O.materials.append(FrictMat(young=young,poisson=0.5,frictionAngle=radians(compFricDegree),density=2600,label='spheres'))
O.materials.append(FrictMat(young=young,poisson=0.5,frictionAngle=0,density=0,label='walls'))

# create walls around the packing
walls = aabbWalls([mn,mx],thickness=0,material='walls')
wallIds = O.bodies.append(walls)

## use a SpherePack object to generate a random loose particles packing
sp = pack.SpherePack()
sp.makeCloud(mn,mx,-1,0.3333,num_spheres,False, 0.95,seed=1) #"seed" make the "random" generation always the same
O.bodies.append([sphere(center,rad,material='spheres') for center,rad in sp])
 #or alternatively (higher level function doing exactly the same):
 #sp.toSimulation(material='spheres')

############################
### DEFINING ENGINES ###
############################

triax = TriaxialStressController(
 maxMultiplier=1.+2e4/young, # spheres growing factor (fast growth)
 finalMaxMultiplier=1.+2e3/young, # spheres growing factor (slow growth)
 thickness=0,
 stressMask=7,
 internalCompaction=True, # If true the confining pressure is generated by growing particles
)

newton = NewtonIntegrator(damping=damp)

O.engines = [
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Box_Aabb()]),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom(),Ig2_Box_Sphere_ScGeom()],
  [Ip2_FrictMat_FrictMat_FrictPhys()],
  [Law2_ScGeom_FrictPhys_CundallStrack()]
 ),
 ## We will use the global stiffness of each body to determine an optimal timestep (see https://yade-dem.org/w/images/1/1b/Chareyre&Villard2005_licensed.pdf)
 GlobalStiffnessTimeStepper(active=1,timeStepUpdateInterval=100,timestepSafetyCoefficient=0.5),
 triax,
 newton
]

# Display spheres with 2 colors for seeing rotations better
Gl1_Sphere.stripes = 0
if nRead == 0: yade.qt.Controller(), yade.qt.View()

#######################################
### APPLYING CONFINING PRESSURE ###
#######################################

# the value of (isotropic) confining stress defines the target stress to be applied in all three directions
triax.goal1 = triax.goal2 = triax.goal3 = -10000

while 1:
  O.run(1000, True)
  # the global unbalanced force on dynamic bodies, thus excluding boundaries, which are not at equilibrium
  unb = unbalancedForce()
  # print(f"unbalanced force:{unb},mean stress:{triax.meanStress}")
  print 'unbalanced force:',unb,' mean stress: ',triax.meanStress
  if unb < stabilityThreshold and abs(-10000-triax.meanStress)/10000 < 0.001:
   break

O.save('confinedState'+key+'.yade.gz')
print "### Isotropic state saved ###"

###################################################
### REACHING A SPECIFIED POROSITY PRECISELY ###
###################################################

import sys #this is only for the flush() below
while triax.porosity > targetPorosity:
 ## we decrease friction value and apply it to all the bodies and contacts
 compFricDegree = 0.95*compFricDegree
 setContactFriction(radians(compFricDegree))
 print "\r Friction: ",compFricDegree," porosity:",triax.porosity,
 sys.stdout.flush()
 O.run(500,1)

O.save('compactedState'+key+'.yade.gz')
print "### Compacted state saved ###"

##############################
### DEVIATORIC LOADING ###
##############################

# We move to deviatoric loading, let us turn internal compaction off to keep particles sizes constant
triax.internalCompaction = False

# Change contact friction (remember that decreasing it would generate instantaneous instabilities)
setContactFriction(radians(finalFricDegree))

# set stress control on x and z, we will impose strain rate on y
triax.stressMask = 5
# now goal2 is the target strain rate
triax.goal2 = rate
# we define the lateral stresses during the test, here the same 10kPa as for the initial confinement.
triax.goal1=-10000
triax.goal3=-10000

# we can change damping here. What is the effect in your opinion?
newton.damping=0.1

O.engines= O.engines + [PyRunner(iterPeriod=20,command='history()',label='recorder')]

# Save temporary state in live memory. This state will be reloaded from the interface with the "reload" button.
O.saveTmp()

#####################################################
### Example of how to record and plot data ###
#####################################################

from yade import plot

# a function saving variables
def history():
 plot.addData(
  e11=-triax.strain[0],e22=-triax.strain[1],e33=-triax.strain[2],
  s11=-triax.stress(triax.wall_right_id)[0],
  s22=-triax.stress(triax.wall_top_id)[1],
  s33=-triax.stress(triax.wall_front_id)[2],
  DeviatorStress=(-triax.stress(triax.wall_top_id)[1]-(-triax.stress(triax.wall_right_id)[0]-triax.stress(triax.wall_front_id)[2])/2)*1e-6,
  i=O.iter
 )

plot.plots={'e22':('DeviatorStress')}
plot.plot()

O.run(124700)