Concrete blinding base of ring beam trench?!

[Delinquent]

Question is in the title really - why?!

Just had a quote for some assistance with the ring beam, in which the quote mentions putting 2" blinding in the base of the ring beam trench - the trench is all shuttered and ready, with the base being of level sand around 25mm below the bottom of where the ring beam should be.

What exactly is the difference between just pouring the 425mm of concrete (400mm ring beam plus 25mm), and digging out another inch of sand, blinding with 50mm of concrete, then getting bloke back out a few days later to pour another 400mm?!

OK so there is the obvious additional 25mm of concrete - but that's not called for in the structural engineers reports so in my mind is a waste of money - unless I'm missing something...

 

[Shytalkz]

It's not a structural requirement per se, but is specified so as to give a firm and level base off which to support the rebar cages. With heavily reinforced beams with, say, H32s in, the cover blocks would soon sink into the sand blinding or subsoil, if it's soft.

Edited to add: makes it easier to clean out the bottom before pouring as well. But, if you can find another way of supporting the rebar, such as hanging it off the shuttering, then it can, of course, be omitted.

 

[Delinquent]

well the rebars are all going to be sitting on the steel piles that we've cut to height of the base of the rebar (50mm above base of beam, so in reality 75mm above base of trenches)

The spans are not that great (largest span between piles is 3m) so I was going to simply use a large piece of timber (or some of the floor beams!) across the trenches at the mid-span point to suspend the rebar on - which will give the longest span of rebar unsupported of 1.5m - sound OK?

 

[JohnD]

there is also the point that rain or groundwater may turn the bottom of the excavation into mud, and anyone walking in it will stir it up.

 

[Shytalkz]

Sounds perfect! Don't forget the side cover blocks as well, otherwise when you poker it, the cage could move over against the shuttering. Clean out the bottom too of big lumps of crud too.

 

[Delinquent]

there is also the point that rain or groundwater may turn the bottom of the excavation into mud, and anyone walking in it will stir it up.

LOL - well yes that is a slight issue at the moment... in that the trench is actually 300mm deep in water! oh well, I suppose at least the heavy rain held off long enough for me to get it all dug out.

 

[Shytalkz]

Clay site, perchance?!

 

[Delinquent]

lovely job, cheers.

Hadn't decided what to do about lateral movement yet - though the chaps that did the piling left a large bag of round things that went around the rebar in the top of the piles... I reckon a half dozen of those cliped along the bars should prevent it. (very handily they appear to have a diameter of 100mm so should be perfect, just hope there are enough... )

Strangely enough, No clay! In the original reports we were told to expect masses of London Clay, and would need to use clayboard all round "to suit the conditions" - when the piling team turned up and tried to use the auger, they got down to 9m and found nothing but gravel and sand. Which promptly collapsed the pile - so they went home and got a driving rig to replace the auger

When I then dug the trenches out, I came across something that looked like a cross between clay and sand, but was assured that it's just a sand. When I quizzed them on what clayboard would be appropriate "to suit the conditions" they suggested the one I left in the shop

 

[Shytalkz]

When I quizzed them on what clayboard would be appropriate "to suit the conditions" they suggested the one I left in the shop

Lol, sounds about right too: if it's sandy clay then the heave potential will be on the low end of things. Was there an SI carried out on the site, as it's interesting that the conditions are vastly different from what was expected?

 

[Delinquent]

No site inspection was carried out, it was to be based on other projects worked on in the area - however the piling company informed me that they've done dozens of properties stretching miles either side of ours, and there experience has been that the majority is indeed London Clay, however there is a seam about 1/2 mile wide where they've yet to come across any.... our house being right in the middle of that seam!

Bonus really, as I didn't want to dig any deeper - I'd run out of places to hide the soil. Pockets, turn ups....was even considering getting a gym horse

 

[Shytalkz]

Makes you wonder if it even needed piling...still, you did save about £2k by not having an SI!

 

[Shytalkz]

Bonus really, as I didn't want to dig any deeper - I'd run out of places to hide the soil. Pockets, turn ups....was even considering getting a gym horse

Nah, you just need to dig a hole, put the spoil in it, then dig another hole, put the spoil from the first one in it...erm, is there a flaw in this theory?!

 

[Delinquent]

well yes that did cross my mind, though as its on a very steeply banked site I think it was probably worth it - plus the cost of the piling work was IMO really quite reasonable - by my current calcs I'll be up to and including floor level (including the Jetfloor system itself ) for c. £6.5k - as it's getting on for 30m2 floor area I'm pleased enough!

 

[Delinquent]

Bonus really, as I didn't want to dig any deeper - I'd run out of places to hide the soil. Pockets, turn ups....was even considering getting a gym horse

Nah, you just need to dig a hole, put the spoil in it, then dig another hole, put the spoil from the first one in it...erm, is there a flaw in this theory?!

LOL yeah - I'm doing all the digging with a spade only, couldn't cope with all the new holes