Sphere particles pass through the walls or facets under gravity

Hello everyone,

I run the scripts periodicSandPile.py as shown in:
https://gitlab.com/yade-dev/trunk/blob/master/examples/PeriodicBoundaries/periodicSandPile.py
where the bottom plate is made by box instead of wall or facet. I am curious about the situation if wall or facet is used instead of box, so I changed the box into wall. But the spheres under gravity will passes through the wall or facet that I defined, which is confusing. Would you please tell me why this happened, or where I was wrong? Thank you! The script using wall is presented:

#------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
# import yade modules that we will use below
from yade import pack, plot, qt
O.periodic=True

lx=0.05
ly=0.1
lz=0.1

# creat horizontal 4 periodic boundries by cell
O.cell.hSize=Matrix3(lx, 0, 0,
        0, ly, 0,
        0, 0, lz)

# creat plate
O.bodies.append(utils.wall(position=(lx/2,ly/2,lz/5),axis=2))

# define material of sphere particles
idSand=O.materials.append(FrictMat(young=1e5,poisson=1,frictionAngle=radians(20),density=2650,label="Sand"))

# creat sphere particles
radium=0.0005
sp=pack.SpherePack() # create an empty cloud; SpherePack() contains only geometrical information
sp.makeCloud((0.0,0.0,0.3*lz+4*radium), (lx,ly,0.7*lz-4*radium), rMean=radium, rRelFuzz=0.0, num=500, periodic=True)
sp.makeCloud((0.0,0.0,0.3*lz+4*radium), (lx,ly,0.7*lz-4*radium), rMean=1.5*radium, rRelFuzz=0.0, num=500, periodic=True)
for s in sp:
 if s[1]==radium:
  O.bodies.append(utils.sphere(center=s[0],radius=s[1],material="Sand",color=(1.0,0.0,0.0))) #
 else:
  O.bodies.append(utils.sphere(center=s[0],radius=s[1],material="Sand",color=(0.0,1.0,0.0))) #
#---------------------------------------------------------------------------------------------

O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Box_Aabb()],allowBiggerThanPeriod=True),
 InteractionLoop(
  # handle sphere+sphere and facet+sphere collisions
  [Ig2_Sphere_Sphere_ScGeom(),Ig2_Box_Sphere_ScGeom()],
  [Ip2_FrictMat_FrictMat_FrictPhys()],
  [Law2_ScGeom_FrictPhys_CundallStrack()]
 ),
 GlobalStiffnessTimeStepper(),
 NewtonIntegrator(gravity=(0,0,-9.81),damping=0.4),
 PyRunner(command='checkUnbalanced()',realPeriod=2),
 PyRunner(command='addPlotData()',iterPeriod=100)
]
O.trackEnergy=True

def checkUnbalanced():
 if unbalancedForce()<.05:
  O.pause()
  plot.saveDataTxt('bbb.txt')
  # plot.saveGnuplot('bbb') is also possible

def addPlotData():
 # each item is given a names, by which it can be the unsed in plot.plots
 # the **O.energy converts dictionary-like O.energy to plot.addData arguments
 plot.addData(i=O.iter,unbalanced=unbalancedForce(),**O.energy)

plot.plots={'i':('unbalanced',None,O.energy.keys)}
plot.plot()