Wet area - concrete pour, footings - help

[James Davis]

Hello again.

My house extension is under way.

The trenches are dug and the concrete is due to be poured tomorrow for the footings. We have got down to a good clay layer that the building inspector is happy with. The only dubious part is at one of the corners. We found a mad, really deep wall, which we think was an old mechanic pit, which people used to have sometimes. The inspector wanted us to dig down to the bottom of the wall (doesn't have a footing) to the clay meaning we've had to step down to it and the ground is aroumd 1.5m deep there. The trouble is, that corner keeps slowly filling up with water, even after we've emptied it, and when we do empty it, the ground is quite soft and sloppy because it is saturated.

If we empty as best we can will it be ok to pour the concrete? I'm wondering if the trickle of water that starts to fill it up by a few inches will screw up the mix? It probably ends up 5" deep in addition to the sloppy clay. The footings will be about 850cm thick in that corner.

Also, as some rain is forecast , will it be sufficient to just put boards over the trenches whilst the concrete sets?

Thanks

 

[tony1851]

Don't worry, freshly-poured concrete loves water. It will be fine.

 

[James Davis]

Would it be necessary to throw a wheel barrow of more 'dry' concrete into that corner to begin?

 

[JohnD]

Dig out the wet mud last thing before you start laying.

Put in a few inches of drylean mix to the depth you just dug out, and beat it down well, it will absorb residual moisture. Then pour your mix on top.

 

[James Davis]

Sorry to be a pain, but I just want to make sure we are doing things properly. Once this concrete goes in and the extension is built on top of it, it won't be easy to fix if there's a problem with poor concrete at the bottom in that corner! Concrete footings are being poured tmw now.

As I say, footings will be 1.5m deep in that corner but the ground there is wet slop. See previous photos. Digging it out is just digging slop because there is a trickle of water coming in at the bottom of the ground alongside that big existing wall. So it starts to fill up again.

As the wall is outside the square of the footings, I wonder if we stick a tall shutter board down there to help block the water getting in, just before concrete arrives? So remove water before they arrive, remove top couple of inches of slop, then stick a tall shutter board down there as shown on the attached? This will hopefully reduce the amount of new water going into the footing area.

Otherwise, it seems to be a case of:

A) remove water as best we can beforehand, and then just pour concrete in up to the wall and fill the whole thing. Don't worry about the new water coming in.

B) put a couple of barrows of lean mix in first to that corner. My concern with this is that could this be a form of creating "made" ground which the whole building will bear down on?

 

[noseall]

Sorry to be a pain, but I just want to make sure we are doing things properly. Once this concrete goes in and the extension is built on top of it, it won't be easy to fix if there's a problem with poor concrete at the bottom in that corner! Concrete footings are being poured tmw now.

As I say, footings will be 1.5m deep in that corner but the ground there is wet slop. See previous photos. Digging it out is just digging slop because there is a trickle of water coming in at the bottom of the ground alongside that big existing wall. So it starts to fill up again.

As the wall is outside the square of the footings, I wonder if we stick a tall shutter board down there to help block the water getting in, just before concrete arrives? So remove water before they arrive, remove top couple of inches of slop, then stick a tall shutter board down there as shown on the attached? This will hopefully reduce the amount of new water going into the footing area.

Otherwise, it seems to be a case of:

A) remove water as best we can beforehand, and then just pour concrete in up to the wall and fill the whole thing. Don't worry about the new water coming in.

B) put a couple of barrows of lean mix in first to that corner. My concern with this is that could this be a form of creating "made" ground which the whole building will bear down on?

Stop fretting.

Bale it out with a bucket, just before the pour. Scrape the crud off and pour the conc'.

 

[James Davis]

Thanks Nose.

Concrete went in at 11am this morning. No major downpours forecast so far . What is forecast is low temperatures over night. Down to 1 centigrade in the early hours, and zero at around 6am tomorrow. Do we need to cover the footings over with boards? Currently enjoying a balmy 8C outside, but will start to cool from a couple of hours' time

 

[gemius]

I'm a retired site manager and I remember a job where, when we started to pour the concrete there was a sudden tropical down poor of rain which flooded the whole site. I watched in dismay as it was too late to do anything about it. However, concrete is heavier than water and when the flood abated it was all in place as intended. As far as I know the school is still standing.

 

[JohnD]

You should protect it from freezing for a few days. It will hold some warmth, but cover the top and ensure that cold winds cannot blow under your covering. A tarp spaced off the concrete to give an air gap, and weighted down at the edges, is fine.

If warm and sunny, your cover will also prevent the top from drying out too fast.

 

[James Davis]

Better than nout...?

 

[JohnD]

You can also shovel sand on the poly, once the concrete is firm, to hold it down, and sweep it up later.

 

[noseall]

Thanks Nose.

Concrete went in at 11am this morning. No major downpours forecast so far . What is forecast is low temperatures over night. Down to 1 centigrade in the early hours, and zero at around 6am tomorrow. Do we need to cover the footings over with boards? Currently enjoying a balmy 8C outside, but will start to cool from a couple of hours' time

I doub't the temp's will be low enough to have any effect whatsoever, other than help prevent the concrete setting too quickly.