First off, this is my first post here, so I apologize in advance if this isn't the right place to ask this. Please feel free to move it around and/or re-tag it if you think it fits better somewhere else. Also, I can't imagine that nobody has ever run into this situation before, but after spending half a day searching the internet and StackExchange and finding nothing, I guess it has never been asked before:

My question is simple: Can a septic system leach field be installed over/within clean fill? Assume that the fill is about 4 to 8 feet deep and has been approved by the applicable regulatory agency in regards to compaction, setbacks, etc. Also assume that there is enough space to install such a leach field without encroaching into any setbacks, easements, etc. Furthermore, assume that the fill material will pass a perk test and is free of trash, large boulders, organic material, etc.

Some more background:

I am asking this question first and foremost from a theoretical standpoint. However, my specific situation is as follows: I own a vacant one acre lot in northern Arizona that has approximately 12 feet of elevation change between the highest corner and the lowest corner. While I have not done any excavation on the lot or conducted a perk test I have gathered a small soil sample and observed a nearby road cut. Thus, judging by these observations, I presume that the soil is approximately 2 to 3 feet thick and is very clayey, expansive, and rocky. Beneath this layer of rocky clay soil is lava bedrock. I would not say this bedrock is impermeable, as it seems to be broken up into large chunks judging by the road cut. Furthermore, I have used the county's GIS system to figure out what type of septic permits were taken out by by my nearest neighbors. This showed that most of them required an alternative system if they had one at all. I don't plan on building on this lot for many years. However, in the interim, I plan on clearing a space on the property and grading a flat area where I can park an RV and dry camp occasionally. In the future though, I may want to build a small house on the property and live there permanently. Thus, while I am grading the flat area, I am wondering if I can do the following: 1. Strip away all of the existing soil from approximately the lower quarter of my property. (It is expansive remember.) 2. Create or bring in a new soil that is more suitable to septic system leach fields and building foundations. 3. Use this new soil to create a flat area for my camping purposes and then at some point later on, use it for a leach field and perhaps a small house.

Thanks in advance for any alternative ideas/thoughts/answers.

  • Consult with a septic engineer/designer who practices (and is licensed, if applicable) in your area so that you do this in the most efficient manner, rather than going to a lot of work that may or may not suit the purpose. Design first, then build. Not assume first, build, then try to design and find out you "shouda/wouda/coulda" at the build phase. In general, yes, suitable fill is a way that septic can and does get done in otherwise unsuitable areas. But you want to be "not so general" about getting it done in a cost effective way that will be approved by your LAHJ.
    – Ecnerwal
    May 10, 2021 at 15:07

1 Answer 1


I have built on filled areas in the past, you have asked all the right questions the first being compaction. In the area I built we had added 4-6' where the house ended up and closer to 8' closer to the back half of a drain field we filled and compacted and met the code back then but we're still required to wait 1 year and repeat the 3rd party compaction test. After that perculation testing after we had 50% of our normal annual rain fall took 2 more years because of a drought. But then we were able to get permits to build. If you are going to be waiting a few more years you may not need the additional compaction testing it was a bit expensive but turning a worthless lot into one worth 80k for just the land it was a no brainer for us. You have the soil to move around but you would be amazed how many quality loads of fill dirt can be had for a couple of cases of beer. May not be quite the same as this was many years ago at the start of mound systems and they cost up to 10x of a standard field plus the maintaince. But your plan sounds good to me.

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