For outdoor, flexible coax I have found that Bury-Flex (TM) from Davis is
ideal. This link takes you to the part of his site where quality coax may
be found:
http://www.davisrf.com/ham1/coax.htm

I used bury flex when I put up my 80 meter loop last year..burried about 20 ft
of it..so far so good..the stuff is very rugged..works good lasts long time..

Steve
kb8viv