[Heathkit] How do I lay coaxial cable underground?

[Howie WA3MCK]

Wireless friends;

I want to ground mount my R5 vertical.

To do so I need to run about 40 feet of coax, underground,   from the base
of the antenna to my shack.

What type of an RG-58 equivalent coaxial cable is rated for direct burial
underground?

I was thinking a slit trench several inches deep would do the trick. What
say you?

Thank you.

Howie WA3MCK