Partially collapsed extension foundation footings

17 Oct 2022
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United Kingdom

I am hoping you can help me with some independent advice regarding footings that have been dug for a single-story extension at my sons property.

Due to a neighbouring tree roots, we have been advised by the council to dig extra deep footings for the foundations, I'm afraid I don’t have the specific measurements but we are having to go a meter deeper that the original house foundations ☹.

Since the first trenches have been dug, ready for concrete on Wednesday, we have had a lot of rain which has caused some of the walls to partially collapse! Now this is where we are really worrying, one of the partially collapsed walls is against the neighbour’s patio and the other against a public footpath. From what we can some of the material under both the edge of the patio and the footpath have dropped into our trench.

I have attached a number of pictures so you can see what i am trying to explain, any advise would be most helpful.

1 - Should we pump out today and re-shutter the walls or wait until the day before we concrete?

2 - What can we do about the neighbour’s patio and the footpath? Can we “underfill” once the concrete has been poured?

Many thanks


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It happens - reshutter and add some support for the trench, it back fill the shutting to give it some rigidity, get the concrete in, build the walls to ground and backfill and make good..

I would suggest that you build shutting for the trench floor, at the moment you are trying to both retain the bank and provide a shutter for your are best to rip those boards down by a third and make shutter boxes for the pour. The banks have collapsed so there is nothing you can do bar reinstated the ground once you have poured and built the walls.
You wont be able to concrete the trench and then backfill the adjacent voids at the same time or even after.

I may be easier to fill the whole lot with concrete.
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You wont be able to concrete the trench and then backfill the adjacent voids at the same time or even after.

I may be easier to fill the whole lot with concrete.
Why...just make some formwork pour concrete build up and back as below.


Why...just make some formwork pour concrete build up and back as below.

View attachment 282873
View attachment 282870
They look lovely for a building site. However not quite the same as the OP's situation are they?

ie how is he going to backfill the land which is across the boundary under the public highway and under the neighbour's land - and ensure that the backfill is consolidated so that the public highway and the neighbours foundations don't slip in a few week's time ?
..oh no every time you dig a trench in the road you need to fill it with concrete...its a patio and a walkway...some type1 and a wacker plate job done.
Assuming the OP son is paying a builder for the job, I would assume this to be the builder's problem to solve. I imagine filling the lot with concrete will be the easiest way to resolve...albeit at a cost for the builder of a couple of extra m3 of concrete!

I just wouldn't want to be down in those trenches clearing the slippage!
For gods sake don't go down (or send anyone else) into those trenches.

You might be able to shutter the top bits but you'll need to pump, clear out the trench (edit - with the digger, as per jks below) and pour some concrete. If you get the concrete up to a level around 1m from the surface, then you look at shuttering to reduce concrete volume near the top and backfill with stone. But to sort the bottom of the trenches it's just about getting the concrete in there.
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You need to redig the bottom of the trenches to get rid of all the slop and soft material once you've pumped the water out and before you pour any concrete
The site needs tidying and all that spoil covering the oversite needs shifting. More importantly the temporary works look dire. Any disconnected or leaking drains need addressing urgently. A proper shutter needs building with props between or the whole trench mass filling. All looks poorly thought out with zero consideration for water influx (rain, drains etc).

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