Foundation next to underground drainage run

[darrenh]

Hello

I need to dig a run for an alteration to a drainage run - the underground pipe will run parallel to the new foundation.

Issue i have is once I have dug out the drainage run and laid it, I then need to dig out the foundations.

The two runs are so close that I suspect the back filled drainage run will just fall in and I need this to 'hold in the concrete' while it sets.

So dig it out all at once or one at a time?

Any thoughts?

Darren

 

[RedHerring2]

I've recently done a similar project for a kitchen/bathroom extension.
I dug the footings first, poured that and built up to dpc. Then dug to relay/replace/modify the foul drain. I was able to dig right up to footings, which became the one side of the trench.

The footings were going to cover a pipe which would become redundant in the big scheme and BCO said I didn't need to lintel over it, just let the concrete pour over/round it.
The footings should go below the level of the invert of the drain pipe.

If your existing pipe is in the way of your footings and you need to relay the pipe first, that's a different matter.

 

[darrenh]

yes - I need to relay the pipe first unfortunately I think - the existing pipe and manhole is sitting right on the corner of the proposed extension (where no lintel can cover)

 

[RedHerring2]

I think, then, there's only one way: shutter it.
I would dig out both, (don't forget the footings will be deeper than the drain) lay new pipe and then shutter it from the bottom of the footings, fill as necc' with pea gravel around pie. By sinking the shuttering slightly that should hold the bottom of the shuttering in place to restrain the pea gravel and later when you pour the footings the side of the trench will restrain the shuttering. You'll only need to restrain the top of the shuttering in both directions until the footings have been poured.

Leave any backfilling until after the footings have been poured (and brick/blockwork raised), then you should be able to remove your shuttering relatively easy. As a precaution, fix/provide some anchor points high on the shuttering to provide some leverage when you come to remove it.

You'll have plenty of room to raise the brick/blockwork off the footings.
Then you can backfill the pipe trench.

 

[darrenh]

Do youmean use some ply as shutterings? How do I remove the ply afterwards - will it not be lockedin place by the concrete?

 

[RedHerring2]

I think you have two choices:
Use sufficient ply (or OSB) to allow you to remove it afterwards, i.e. sufficient material to grab hold of, after the footings are poured.
Only the concrete between the footings and the shoulder of the trench will be holding it. (The shoulder is the difference between the footings trench and the pipe trench, which maybe as little as 150mm). Only you know at the moment the difference between your footings depth and the drain depth.
or
Use minimal ply to allow it to remain in place after the footings are poured. i.e just sufficient ply to the top of the footings. But you'll need sufficient to allow bracing until you start to pour.
Once your pour has started you can remove the bracing on the footings side.

I suppose there is a third option, run a reciprocating saw along the top of the footings to remove any ply(OSB) projecting above the footings but some projection is not a problem.

 

[darrenh]

thanks - I'll go with option 1 I think!