Hardcore fill in excess of 900mm between foundation and FFL

alan86

Hi all,

We have dugout and poured foundations and we have a nice bit of block work to do to bring it back up to FFL. As far as I'm aware once I pass the need to fill beyond 900m of hardcore I need to look at other options such as suspended concrete floor etc. I have reached out to my engineer and waiting for a reply but wondering has anyone else here ran into this issue in past.

hydrus21

Did a beam & block in UK 5yrs ago. So much easier all round.

Needed weed suppression and a 400mm void so ask about that.

PaddyCar

You can make up the ground below the 900mm with T0 struct material and then SR21 material for the remaining 900mm to DPM level.

alan86

i have taken a look at that. Just a bit concerned around leaving a void under house with the likes of vermin etc.

alan86

https://www.boards.ie/discussion/comment/121780467#Comment_121780467

Might sound like a silly question. What's T0 material?

hydrus21

I'm guessing it's ' take out '.

Hard-core saved and set aside for infill.

Could be wrong.

PaddyCar

It's basically 5 inch clean stone. Have a look at technical guidance document part c which has good info. https://roadstone.ie/sites/default/files/2022-05/SR-21-Brochure-NOV-17.pdf also has useful info.

hydrus21

https://www.boards.ie/discussion/comment/121780810#Comment_121780810

No different to a suspended wooden floor.

Panjandrums

What height have you from top of foundation to finished floor level?

alan86

https://www.boards.ie/discussion/comment/121781265#Comment_121781265

Approx 1250mm from foundation to ffl. But there's a bit of give and take with that depending on how blocks make it up. We have 125mm screed,150mm indulation and 50mm sand blinding so we are probably talking between 900mm to 1000mm hardcore fill to make up.

monseiur

https://www.boards.ie/discussion/comment/121781544#Comment_121781544

Fill with clause 804 - get stuff with plenty of dust, compact well in layers. Forget leaving a void your're regret it later and it's almost impossible to rectify.

Panjandrums

That depth isn't a problem for a ground bearing slab. Concrete slab, insulation,50mm of T3 stone, 200mm of T2 stone and the rest T1.

Once it's compacted in layers, there won't be any problems.

Might be worth exploring the use of XPS insulation, it will reduce the depth of fill.

hydrus21

OP Has not mentiond m2 of job and no mention of ease of access. Big factors in choice of build.

alan86

https://www.boards.ie/discussion/comment/121781814#Comment_121781814

Good point should of mentioned at start. The house is a 200m2 bungalow. We have stepped the foundation so this is only an issue with approx 32m2 of the base.

Access to house is very straight forward only about 20m from road

alan86

https://www.boards.ie/discussion/comment/121781770#Comment_121781770

Might sound like a silly question what's the difference between T1 amd T2?

Panjandrums

T1 is 0-31.5mm so it has fines and up to 31.5mm stone size

T2 is 4-40mm, it's a gas permable layer, the radon sump is located within this layer.

alan86

https://www.boards.ie/discussion/comment/121782079#Comment_121782079

I get ya. Just did a bit of more reading online to try and understand it all a bit better. Looking at what road stone have up for annex e that governs all this what you saying is spot on. Need to make sure we leave 200mm T2 for radon and the rest can be T1 as you said.

SodiumCooled

Maybe I’m missing something but why wasn’t the level raised with certified stone before pouring the foundations? After we dug out it was filled with certified stone in layers (T0, T1 etc) until it was the correct height that the top of the raft sat at the correct height for finished floor (+ insulation under screed) to be at the correct level.

I would have thought that’s the normal way of doing it?

muffler

https://www.boards.ie/discussion/comment/121787955#Comment_121787955

He probably has a strip foundation as opposed to your raft. 2 different animals completely.

alan86

https://www.boards.ie/discussion/comment/121787955#Comment_121787955

As muffler says above we have gone with a strip and not a raft. The site was a bit low so we know we would need to bring up the blocks abit but then had to dig down further than planned to hit solid ground so it's all added up.