Footings with a high water table

[laviscus]

Hi.

I'm digging some footings for a two-storey extension. BCO, is happy with an 800 mim depth as we are on to sand, but the water table is only 600 mm down so it leaves 200 mm of water on top of the sand.

This will obviously get pumped out before we pour the concrete footings, but with all the rain we've had some of the wall has slipped in, I've now shuttered it but it'll be next week before i can get the footings poured and I'm worried about more slop getting in before then.

Given I've sand at the base of the trench, i was thinking about doing a lean mix (30-40mm) by mixing in some cement so the base of the trench is hard and easy to clean up before the fittings are poured.

If there s downside to this? Thanks.

 

[ree]

OP,
For me, the downside is that it wont work.
Rent a pump, & then pump & pour in almost one go.

 

[laviscus]

Thanks ree. We're having a nightmare with these footings. As soon as i hit the water table, the trench is filling with water and in only minutes it turns the hard sand into slurry (i originally thought it was where some of the wall has fallen in but i did a small test hole and it's definitely not, it's the firm sand from the bottom being liquefied). The sand under the slurry is still quite firm, but as soon as i scrape the slurry off, the hard sand then turns to slurry.

My BCOs advice was 'just dug it and pour it quickly' (and has now gone on holiday for 2 weeks), this doesn't really seem ideal given the slurry is quite deep and comes back as soon as i remove it. I'm assuming i shouldn't really get the concrete poured onto this slurry (even though it's really just sand and water).

Would the simultaneous pump and pour work given how quick it is?

One blog i saw from an engineer suggested compacting 80-100mm cobbles into it until it's a firm base and then pour the concrete in top, is this really viable??

I talked to an architect friend who suggested i could give up on the trench and go with a raft instead.

Any other suggestions?