Andy suggests:

One caveat here..... the UF cable normally specified for underground
burial is supposed to
be BURIED by itself, not BURIED in conduit.

I have been told that the CODE does not permit UF to be buried in the
smaller diameters
of PVC conduit. I haven't checked it, but it sort of makes since....

HOWEVER, I ALWAYS bury my UF in PVC conduit, usually half inch,
whether code
compliant or not for one basic reason.... :

When I go to dig it up , or find it, or dig another trench over it
later, my shovel will
hit the PVC pipe first, and NOT nick the UF before I can stop
myselft........

Only cost about another 10 cents a foot, here, but makes me feel a
lot safer about
digging around a buried cable.......

I don't put joints in, just slip the pvc over the cable. This way
I know for sure I
am not dealing with a water line......
Andy