According to PipeDown :

Will the ground on top be subject to heavy loads like parked cars or an
occasional delivery truck or is it strictly foot traffic or garden area.

For the lighter loads I might try expanding foam mixed from a 2 part mix and
poured right on the pipes with minimal form around it to control the
expansion. May even be insulation contractors who can dispense the stuff
from a truck.

Styrofoam SM (the blue stuff) is often used in direct burial applications.

Ie: under slabs, on foundation outside faces, etc.

I would assume it could withstand the temperatures involved here.
[It's used in contact with radiant flooring.]

I suggest it because it's probably easier/cheaper than on-site
mixing/installation of foam for a DIY.

If suitable, I'd recommend putting several inches of gravel, then
two 8" wide layers of 2" foam, a slot for the pipes, and another
two layers of 2" foam. Glue the layers/"joints" together with foam adhesive
(available as a caulking tube).

A 2'x8' slab of foam is around $15 IIRC. That would do about
12 linear feet of insulation (8"x8"). What is that anyway? 4" of foam
is R20, right?

Wrap it with tape to keep it from shifting if the glue is still wet,
and you're backfilling.

Even at only a foot down, it should withstand considerable surface
traffic, but to be absolutely sure, fling on a few treated 1x6 fence
planks before backfilling. Even strips of exterior grade OSB would do.
[It'll eventually rot out, but by then the soil will be compact enough
to not crush the foam.]

A building supply house that's "advanced enough" to know and supply
the various grades of SM available (ie: under slab vs. interior wall
etc) would be able to advise you which grade is right for this, or whether
this is a dumb idea.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.