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I'm grading my yard and have found a deep layer of sand (at least 2 ft) extending out about 10ft. Perhaps the sub base for a patio? I understand I need an impermeable layer to ensure rain runs away from the house rather than draining downwards. I plan to use regular, fairly dense soil from elsewhere in the yard.

Considering that no soil is completely impermeable, how thick should I make my soil layer to be effective? Do I really need to test the permeability of my soil?? That seems like overkill...

Tenz
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    Why do you think you need an impermeable layer of soil around your house? – Jurp Aug 08 '23 at 23:46
  • I have never heard of this. Infact, the opposite is true for downunder. There are regulations on maximum percentage of your property that can be impervious. – Rohit Gupta Aug 09 '23 at 00:34
  • My understanding is that you want to direct water as far away from your foundation as possible. Isn't that why grading a yard is important? If you have permeable material by your house, why bother grading? – Tenz Aug 09 '23 at 02:40
  • @Tenz Water will flow away from your home underground as well. Here's a cool diagram https://www.unomaha.edu/international-studies-and-programs/center-for-afghanistan-studies/academics/transboundary-water-research/DLM15/GW5.jpg – MackM Aug 09 '23 at 11:57
  • Thanks @MackM. The thing is I have sand going down a long way, so I don't know what the slope is below that. If the impermeable material under the sand slopes towards the house, that's bad news. That's why I'd like to grade above the sand. – Tenz Aug 09 '23 at 21:12
  • @Tenz I see what you're saying, I have an area in my yard where the top of a heavy clay hill was 'leveled' with top soil. The former low areas were muddy messes after a rain, basically subsurface puddles. That said, it did always drain eventually. I've never heard of any issues with subsurface layers channeling water back towards a structure but I don't disbelieve. – MackM Aug 10 '23 at 00:40
  • I've also never heard of adding an impermeable layer to help with runoff, just regrading. I assumed that was because the subsurface water movement would saturate the covered area anyway and regrading changes the hydraulic pressure differential underground, but I really don't know. Definitely out of my depth :) – MackM Aug 10 '23 at 00:41
  • Thanks for the input anyway @MackM. I maybe overthinking this and just ensure there's a slope and be done with it! – Tenz Aug 11 '23 at 04:09
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    @Tenz Maybe it's too basic for them, but I see the Earth Science stack exchange site has a Hydrology tag. Maybe they'll have a better answer? https://earthscience.stackexchange.com/questions/tagged/hydrology – MackM Aug 11 '23 at 13:30

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