Foundation trench left too long, saturated and collapsed

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22nd May 2020, 10:59 AM
debunk

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Foundation trench left too long, saturated and collapsed

Hi

Im replacing a derelict lean-to at the back of my old double-brick house, and have already demolished it to replace with similar.

We dug a trench for the footings when we had an excavator here for the retaining wall, and due to a number of factors have not been able to pour the footings for a number of months. The trench has collapsed, expanded, and become saturated after many months with rain etc.

I'm still getting engineering advice on whether to go with a slab or suspended floor on piers - either way the trench is in the line of where the foundation will be. This particular trench will support the wall and also a large bifold door in a steel frame, for which I understand straightness is essential for the smooth operations of a bifold door.

The soils are Class M reactive clay, and the site is on a hill sloping 1:7, the lean-to at the rear of my house faces the hill, so cops most of the water coming downhill. Hence the retaining wall.

Is there any treatment for a saturated trench?

Is the situation salvageable? Thanks for any tips and experience with similar

ps the third photo shows the other sunroom to the right which I will also most likely demolish to makeway for a clean 9m x 3.8m rectangular box which will be the replacement lean-to. The last photo shows the sorry state of affairs atm. Please don't have a go at me for site tidiness, I know it's a mess and not usually how I run things :no:
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23rd May 2020, 12:55 PM
debunk

bumping this - hoping to get a response from a weekend owner-builder :2tsup::cry4:
23rd May 2020, 01:05 PM
Bros

All I can contribute is in my previous house which had a besser block wall to 2.4m the the house was above similar happened to me with the builder.
The trench was dug and it rained and filled the trench and all they did was bail the trench out dig out any slop and poured the foundations and the house never moved in 20 yrs. I was surprised but he said it will be OK and he was correct.
23rd May 2020, 05:55 PM
debunk

Quote:

Originally Posted by Bros

All I can contribute is in my previous house which had a besser block wall to 2.4m the the house was above similar happened to me with the builder.
The trench was dug and it rained and filled the trench and all they did was bail the trench out dig out any slop and poured the foundations and the house never moved in 20 yrs. I was surprised but he said it will be OK and he was correct.

Hi Bros thanks for the response - with your situation was the trench left soaking for a long time? In my situation its been a number of months with lots of rain recently, so I think the clay soils have got really sodden and possibly collapased
23rd May 2020, 05:57 PM
Bros

Quote:

Originally Posted by debunk

Hi Bros thanks for the response - with your situation was the trench left soaking for a long time? In my situation its been a number of months with lots of rain recently, so I think the clay soils have got really sodden and possibly collapased

Only a few days as we just get rain and at time is is heavy and then it goes away for a while.
23rd May 2020, 06:06 PM
Marc

Concreting a flooded trench is an everyday occurence for a concreter. No big deal.
What you must learn from this is that you are building in a place that will cope water all the time and therefore needs proper drainage.
23rd May 2020, 07:19 PM
Tools

Pump out the water, dig out the slop down to solid ground again, fill with blinding concrete to the underside of the footing, for up the sides of your footing if you can be bothered then continue with your footing as designed. As Marc said it is no big deal

Tools
23rd May 2020, 08:21 PM
havabeer

Yep, go buy a cheap submersible and pump the water out.

Then when getting close to the pour date, tidy up the trench
24th May 2020, 01:57 PM
johnc

Follow Tools and havabeer, the only crucial bit is the trench bottom must be solid. Clay soils don't collapse, they just get sodden, then shrink when dry, your trench problem will not make that better or worse but do try to pump it out everytime it gets wet until you pour.
24th May 2020, 09:25 PM
debunk

Quote:

Originally Posted by Tools

Pump out the water, dig out the slop down to solid ground again, fill with blinding concrete to the underside of the footing, for up the sides of your footing if you can be bothered then continue with your footing as designed. As Marc said it is no big deal

Tools

thanks Tools Havabeer JohnC andMarc - glad to hear it's not such a disaster as I've been fearing!