Sides of foundation walls collapsing

[macadonald]

I finished digging 1m by 0.5m trenches yesterday. Woke up this morning and there is significant collapse on the inside walls. By inside I mean the square we have left against the house for this single storey rear extension. Is this usual? The soil is heavy clay, so seemed stable. There is a lot of water, which prob isn't helping, so off to get a water pump today. Think the concrete will only go in on Wednesday Thursday, so very worried.

Am I,likely to just be able to pour more concrete to fill these areas or building control want them to be regular, in which case I could only imagine propping lots of boards against the side and I filling. Will be costly though.

 

[macadonald]

In addition to e above question, are supporting boards and cross timber supports ever left on the side or foundations when they are poured?

 

[noseall]

It is not unusual to have the sides collapse in, we always dread arriving on a Monday to see how much spoil we have to shovel out! Especially bad when you have already pegged the trench and have to shovel around the pegs.

The trench sides do not need to be regular. In fact they will have a greater resistance if they are not.

Don't bother pumping the trench until you are ready for the pour. Because you have clay it will only fill up again. You may have to remove the mush in the bottom and be asked to say pour a few barrows of dry gear in before the wet stuff goes on top.

We have used shutter boards and props in a trench but this more when the banks are very unstable, like when there is a drain trench adjacent.

 

[macadonald]

Thanks for the response noseal. Feel better having read it, but I did go the whole hog today and spent the best part of 10 hrs removing the collapsed spoil and shuttering and pumping. At leat I know what I have already removed, won't return over the coming days. The shuttering should hold the walls nicely. Couldn't believe how well the dirty water pump worked.

What I did do and am concerned it may back fire, is back filled the collapsed part of the walls after placing the shuttering. Saved a lot of removal as well. I compacted it pretty well. A little concerned that when I remove the shuttering, minutes or hours before the concrete is poured that e back fill will collapse back in. Out of interest can you leave shuttering in when pouring concrete.

 

[noseall]

Leave the shuttering and props in place.

As long as you have only wedged-in the props you can hoof them out as the concrete level rises to them.

 

[macadonald]

Great idea. Out of interest when you mentioned the pegging and having to dig around them, what do you mean. Assume you're not talking about the pegs and string fro marking out the foundations to be dug?

 

[noseall]

No.

I always peg the trench for concrete depth. I set the first peg about 525mm (7 metric courses) down from dpc. this first peg is set next to the existing building.

I then laser a series of pegs around the trench level with the first peg, right around till it meets the existing building again. It is then interesting to see how level the existing building is by measuring 525mm at the other end.

We set the pegs about 1.5m apart. Always have a peg where the trench changes direction.

 

[macadonald]

Very useful to read that, as this was my task for tomorrow. Big relief this afternoon when the inspector passed my trenches. Also had the 400 bags of clay removed phew..

I did ask him what depth below ground level he would recomend the top of the foundations be. He suggested not going more than 225 (1 block) below ground level. Just measured approx depth below DPC and it is just short of 500, about 450. Was a little surprised that after having to dig 1 meter depth, that concrete is only required to 750ish. Guess it's about the tickness of concrete, but rather what it sits on..

I suppose every house is different. Is it always advisable to ensure that the number of bricks + mortar beds will tie in with DPC from top of foundation or can the difference always be made up in mortar thicknesses?

 

[noseall]

I say 525mm below dpc for my own reasons but i would go at least 450mm (2 block courses) below, so you can install bottle gullies etc without intrusion from the concrete.

Mass filling with concrete is simpler and cheaper than laying masonry below ground.