I'm not 100 percent sure this is the best group to post this to, so
apologies if I'm in a wrong place for this question.

When we bought out house, the previous owners had to renovate the
septic system to include the placement of a new 4-inch pvc pipe to run
from the tank to the drain field. The line runs down a problematically
steep slope, all the way down to the drain field, where the topography
still slopes a bit, but is much more level. We're probably talking
about a drop of two to three feet from the front of the house, where
the tank is, to the rear of the house, and maybe another three feet or
so to the start of the drain field 27 feet from the rear of the house.
We're getting a lot of erosion on that side of the yard, and are
considering ways to resolve that issue and give us more usable land.

Our favorite idea right now is to terrace the slope on the side of the
house and build a stone retaining wall about 20 feet out from the back
yard to level that part of the property. We intend to expand our deck
and build a lower-level patio in this back yard, and I think
eliminating the slopes and incorporating some drainage features into
the work will stop a lot of the moisture problem we have elsewhere in
the yard. The side yard would be planted with grass and some flower
beds, with a simple stone walkway down the center to provide access to
the air conditoning units that are there, as well as the flower beds.

The thing I'm concerned about is whether that 4-inch pvc pipe will be
happy with the weight of up to two feet of additional soil and a
portion of a stone retaining wall on top of it.

I realize that this plan will make it more difficult to service that
line should we need to, but as we fully intend to have the tank
serviced religiously and we're very careful about what goes into the
system, I'm not particularly worried about that.

Can anyone comment on this? If we were to go ahead with this plan,
should we excavate down to the pipe and build back up, and if so,
should we take any specific steps in doing so? Or can we just build up
the soil above it?

Thanks in advance for any advice!