I bured a total of 600 feet of coax covering 70 feet to the base of my tower
that fed multiple antennas.  I used everything, back in those days, to the
cheep and poor radio shack coax to pretty good coax.  It last literally for
a good 15 years and I never experienced any leakage of any kind.  Putting
tubing down is ok but one problem is moisture build up inside the tubing so
I never bothered with the underground tubing.  We literally have no frost
line to speak of here in Denver so I don't know if coax freezing and thawing
would effect it or not.

Phil.
K0NX