Footings for retaining wall on disturbed ground

29 Oct 2012
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United Kingdom
I have just had a new inspection chamber installed along with a new soil pipe ran to the house, which involved digging a trench about 600mm deep. The new plastic soil pipe was bedded in gravel and then backfilled with the excavated sub-soil (clay/stone),

I now need to dig some footings for a short retaining wall (about 300mm high) for a raised patio. Unfortunately where the footings will run is directly over the trench that was dug. I was planning to dig 300mm deep and put in 150mm concrete footings.

Given the ground will not be as compacted as it was before, any thoughts on how to deal with this? Was thinking either:
Go over ground with whacker before digging?
Compact base of trench after digging?
Put Type 1 and compact in base of trench before laying concrete?
Put down more than 150mm concrete?
Put rebar in the concrete footings?

Or all of the above?

It is only a small retaining wall for a patio so probably not a big deal, but still want to minimise the chance of any settling of the footing if I can.


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Your wall will not carry a lot of weight.
Strictly speaking the foundation should go down to below the level of the soil pipe. But the patio wall is only decorative, not really structural.
Don't use the whacker, and don't use hardcore.
You could put rebar across the area of the soil pipe.

You haven't said how long this wall will be. An alternative would be to have two halves of the foundation concrete, and bridge the soil pipe with a lintel(s).
But that may depend on the length of the residual foundations.
I probably wasn't clear - the retaining wall will run in line with the new soil pipe (directly over the top where the empty trench is in the photo), not perpendicular. So using a lintel won't work. The wall will run for about 3 metres.
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It will most likely settle and non of your ideas will stop that. To reduce the risk a wider foundation is required to reduce that risk. Two layers of 600mm 50mm grey slabs, bonded may help.

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