3 Final questions about trenches before pouring concrete

[macadonald]

Hi guys,

Had great advice already from this forum about my trenches, but last few before the concrete gets poured on Friday.

1. The building inspector passed the trenches, but advised I remove all the wet slush before pouring. Did this for a few hrs today, real clay muck that ended up being easier to remove by hand. Is it worth putting in a bit of small (or big) hardcore in the base of the trench to stop any concrete mixing with this mud?

2. After my post about the sides having collapsed last weekend I ended up placing lots of shuttering on one side. The garden side of the trench is fine and stable, but the inside isn't and I back filled some of the clay. Worried this will fall in if I remove the boards just before pouring. In fact the clay has stuck to the sheets. Can I just leave the 12mm mdf on the one side. Surely this can't have any negative impact?

3. Lastly, noseal advised that I measure down from the dpc to mark the top level of the concrete to be poured. I decided 450mm down from the dpc would be fine (6 coarses of brick). I attempted to use a water level to do this today. Nearly impossible by myself, so will buy a laser level tomorrow. 450 from dpc, leaves about 600mm of depth in the trenches for concrete. The trenches were dug 1m. Is this okay. I would have thought that after digging 1m, en the concrete would not be far of this depth. Guess I'm wrong?

Many thanks guys.

 

[noseall]

No hardcore.

Leave the shuttering, no problem at all.

You can build a two storey extension on concrete that is only 225mm thick. Yours will be more than adequate!

 

[r896neo]

Hire a good self leveling laser. Buying one will set you back 3-400 even for a crap one.

 

[macadonald]

Thanks no seal.

R896neo. I see your point, but was just going to buy a 30 pound job, which is simply a traditional level with a laser pointing out. No good? Would my 2 metre level not work by marking each end and moving it along?

Trying to save where i can. 180 squid to hire one from hss. I know that's a week rate, but looking expensive for a few hrs needed.

 

[nickjb]

Can't really help with your technical questions as I'm going through the same thing but I can offer my 2 pence on the laser. I picked up a Bosch Quigo laser from B&Q for £42, probably cheaper elsewhere, but I wanted it that day. It works really well. I'm sure it's not as good as a £400 one but it's level and bright. Puts out a self levelled cross so should be handy for various bit of my build and it's cheap enough. You can even use the bracket for self timer photos

Also, is it my maths but it sounds like you DPC is only 50mm above the ground level (600+450-1000), but I thought it needed to be 150 minimum

 

[ldt]

£180 will buy you a decent professional self leveling cross line laser suitable for your job, red glasses will help to see the line outside. See my thoughts here:

//www.diynot.com/forums/tools-materials/laser-level.337192/

Before I poured my foundations I bought a couple of packs of Tesco value colouring pencils and using the laser pushed them into the sides of the trenches to mark the top of the concrete fill. I ran some string between them too but this was overkill tbh. Might be unconventional but worked a treat for me and the bright colours made them easy to see against the clay.

 

[r896neo]

Your spirit level and a straight edge is more than enough. A rotary laser is what nose and i were referring to as you use it with a staff and can very quickly and easily set a series of pegs at exactly the same level.

Just use what you've got and even if your 10mm out by the end this is easy to take up in a few courses of block.

 

[macadonald]

Thanks for all the advise guys, I was reading some reviews about the Bosch Quigo self leveling laser. Def a diy piece of kit and not intended for the professional, but I think it would be a useful affordable piece of equipment in the tool box and is likely to do the what I need. Can get one for £40 today, if not will use my straight edge and level.

 

[JohnD]

I've seen specs for Utilities cabins that mention if the bottom is wet after pumping out, lay 100mm of drylean mix.

I interpret this to mean ballast and cement with no water, shovelled in and consolidated to absorb the moisture, and did it myself after repairing a broken drain at home.

Have never watched it being done onsite, though.

Would it be useful here? How long would you leave it before pouring the founds?

 

[macadonald]

Thanks all. Quick update:

After reading a few reviews I bought the Bosch quigo laser level. Hate to sound like the people on the review sites, who prob work for Bosch, but it was perfect for the job. I can really recommend it for any similar job. Comes with a handy bracket that can be manipulated into any position. I simply placed stakes of timber into the base of the trenches and clamped it at the right level. The laser line extended down both sides of the trench on both sides. Used cheap colouring pencils as recommended and the moved the stakes and worked my way around.

One word of advise which caught me out. Don't place your first peg closest to the laser, work from the far end closer, otherwise the laser is obscured by the first pencil.

Regarding the bottom of the trenches, its too late now as the concrete arrives tomorrow lunch time. Going to go with noseals advice and leave. There are little puddles of water and slushy mud a couple of cm thick, but unless you are digging in dry soil, this must be the case in most trenches. Certainly in London clay.

Thanks all.

 

[nickjb]

Sounds very similar to mine. I got rid of the slushy mud and puddles with a stiff yard broom. Just pushed it to a corner and scooped it up (by hand and shovel). No idea if this was needed but it was pretty easy. I found another way to remove loose mud was to walk in the house after walking the trench, mrs is not happy

I poured mine today. Seemed to go OK. I used the Quigo again the check the finished levels. Set it up 100mm or so from the surface then went around the concrete projecting the laser onto a tape measure making sure it was 100mm all round.

 

[^woody^]

Surprisingly, footings were poured level before the invention of the laser level

Throw the concrete in wet, and level it via the puddle of water on the surface

 

[macadonald]

Who would have thought new concrete could look so good. Very satisfied, and relieved!

Thanks all for your advice, particularly noseal...

 

[noseall]

Excellent!

We are always mighty relieved watching the concrete pouring into the trenches as this is a rather nervous time. You never know when the sides might collapse in or a human may fall in!