09:19PM | 04/03/02
Member Since: 04/03/02
1 lifetime posts
Just dug a few test pits on my lot and have discovered a 1 inch to 2 inch organic layer at a depth of 3'6" Water table begins about 6 inches below that (or at this time of year seepage was happening at that depth) I am in a flood plain. The soil above the organic layer is mostly "sandy silt" "silt" and "sand" and the same below. For those engineers out there what would you reccomend. Is this a significant organic layer? It looks a bit like mulch(top soil bit of wood bits) and I suspect the developer just filled over it in 1998. Would it be best to excavate to below this - I was hoping to do just a two foot wall plus 8 inch footing but now??... The guy next to me did just that - he says he didnt find anything but it seems unlikely that the organic layer doesnt run through his lot too. If I start the foundation below the organic layer would that be a worse problem with the water table - I'd be starting just where seepage is occuring at this time of year. It's not particularly deep so pressures not so bad?

If the organic layer is consistent would differential settling be very likey if I didnt excavate below it when there could be anywhere from 1'6 plus of sandy silt above the organic that the foundation would rest on?

Inevitably it is up to the building inspector to decide but if there are any experts out there I'd appreciate your knowledge.



04:02PM | 04/04/02
Your building inspector or soils engineer will most likely make the final decision on this but I have encountered similar situations quite a few times in the past. What we did was excavate to sound material below the organic layer. Then we filled the trench with compacted stone (ABC or #57) to the desired depth and poured the footing on top of the stone. As long as the stone is confined on all sides by the trench you should have no problems. It is never a good idea to build on top of a layer of organic material.

[This message has been edited by GlennG (edited April 04, 2002).]

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