This question has come up in my system before. We have the manual groud rod pounders (ya better have ear plugs in!) and sometimes we hit BFR's (big f*&^%$#@ rocks) so I was told 8 feet is 8 feet. Instead of going 8 feet down dig a trench and go 8 feet long. I tried it once and my clamp on ammeter said 12 ohms. Not bad really. Still that advice was given as a last ditch effort. The only worry is difference of potential.
This is CABL.com posting #323881. Tiny Link: cabl.co/mbwp3