Question about buried pipe and how much soil can go over it

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!

In my opinion, the more soil that is over the pipe the happier the pipe will be. It does not cause a weight problem, it protects the pipe from impact and surface pressure with the thickness of soil. That said, a stone retaining wall "may" cause problems depending on how close the bottom of the wall is to the pipe. You should identify exactly where the pipe is and where the wall will cross it. It may be best to provide a solid lintel of pre-cast stone over the pipe with a considerable amount of vertical space between the pipe and the lintel. The lintel stone should be supported on each side of the pipe, in a way that places no pressure on the pipe. The wall can continue on up supported by the lintel.