What dirt for septic field berm?

Hey all quick question here. I'm in Pennsylvania and I'm installing my own septic system. The system is a long at-grade system...1.5" pressure line runs from the pump tank up hill to a T in the field, one lateral goes left 90 feet and the other goes right 90 feet. They call it an at-grade system because all you do is chisel-plow the top few inches of topsoil and lay 6" of 3/4 clean stone right on grade...Then the laterals, 2" of stone to cover the distribution laterals, filter fabric, and topsoil to cover. From starting grade to surface, the whole system's only about 18" tall. It's just really long...

On the downhill side, the plans call for a 3' wide berm around the sides and downhill face of the stone. Goal here is to hold back any effluent trickling through the 6" of stone until it can percolate into the topsoil beneath. For the berm, the plans call for mineral soil less permeable than the aggregate (which is easy - the aggregate's 3/4 clean), with less than 20% of rock, of which none can be bigger than four inches. So from the side profile view, you have a 4' wide lateral, then on the downhill face there's 3' of lightly compacted dirt (the mineral soil), then the whole thing gets capped with 8" of topsoil.

Here's my question. Do you think it matters what dirt I use for the downhill berm? I have a topsoil pile that's been there for four months that's clean with no grass rocks or sticks. My thinking is if the effluent in the aggregate ever begins to rise above original grade and high enough for the downhill berm to have to hold anything back, it's probably because the system has failed and won't accept any more effluent anyway. I think the designer doesn't want water to percolate through the berm and find a way out on the downhill side, but unlike a sandmound where the sand is the filter media and sits above grade, the filter media here is actually the topsoil under the aggregate.

What am I missing? What am I not thinking about? What's the difference really between mineral soil and topsoil if there's only 4% organic matter in my topsoil anyway? Will the world stop turning if I use topsoil for the 3' berm around my septic field?

Also, will a 5 ton machine like a kx040 do the job or should I rent a 9 ton machine?

Posting to the excavator thread bc that's what I'm doing the install with, but mods feel free to move.

I think topsoil for the berm would probably be fine, especially if it has a little clay in it. I just don’t understand how the system actually works, are you installing the stone right in the existing topsoil after it’s scarified? Or are you removing the topsoil then scarifying the subsoil and putting your trenches there? I’m in Connecticut, and I’ve done a lot of septic installs here but I’ve never seen/heard of anything like you’re doing, that doesn’t mean that it doesn’t work there, I’m just asking because I find it interesting how Septic Systems vary state to state. It sounds to me like your excavator would be fine for doing it, it’s not like you’re digging deep or moving massive amounts of materials. We install a lot of the systems here with a six ton machine and it’s absolutely fine, even for digging the tank hole.

Yep exactly...Scarifying with a chisel plow and then installing stone right on top. Actually if you cant get the stone down in time and it rains and the dirt crusts over, you have to scarify all over. The system's designed for tough soils...We have 11" to the limiting layer, mostly clay, on a slight hill. The long at-grade follows a level contour and its best used on a bulge so the effluent works through ever-expanding topsoil. Wouldnt want to install it in a draw or the sewage would work through the topsoil and funnel into one ravine.

Whole system is 29LF from building sewer to 1500 gal 2 chamber tank, 220 feet of sewer run to an ecoflow 500 gal concrete coco tank, then pressure distribution uphill another 140 feet to the field, which itself is 190 feet long. I'm sitting at $22k in materials alone. Got a quote for just the field using my pipe and fabric and it was $17k. I'm doing that myself now too...

Wow,I’ve done some septic systems here in Illinois,I’m with Tags,never heard of anything like that,very interesting good luck.

I moved this thread from the Excavator forum over to General Industry Questions as it fits better there.

A 5 ton machine is a good size for septic work. A friend on mine here has 2 305's and recently purchased a 306 all for septic work. He loves the larger size of the 306, just a little bigger with more reach.

You're building a dam. In my experience with buried wood, any organic matter will decompose and settle. So your 3 foot berm with organics in it will shrink with time. The effluent will speed this up. I would think making the berm + organic % in height & width will work for your situation.

Clean fill would be fine. No large stones, no woody debris, and no trash that would facilitate the movement of water through the berm. Think of it this way, do you want that black water getting treated in the soil or surfacing? If you have a way to screen the soil, that would be idea but may be overkill.

Sounds like a "poor mans" pressure mound system.
Here, the mound is coarse sand with a little topsoil on it.

The idea behind it is to allow evaporation, not absorption.

Ed