I am installing a short tower. The soil is very dry, mostly volcanic ash
and low conductivity. I had the backhoe operator drill several holes 4
feet deep for grounding. It is impossible to drive a ground rod deeper
than 4 feet due to bedrock. What should I use to fill the holes after
the ground rods are installed?

Do a web-search for "bentonite" and "ground", and you'll learn lots!

Although I've never used it (but I'm going to start!), "bentonite" is
the appropriate stuff to use. When wet, it expands inward to squeeze
the ground rod and outward to press firmly against the soil, thereby
resulting in (effectively) a very large-diameter ground well-connected
to the soil.

Clumping-type kitty-litter is supposedly bentonite (a clay material),
so it should be relatively cheap.
--
--Myron A. Calhoun.
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PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)