Sue B

What is this Passive Slab I hear you talk of

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I have done searches on here and also outside of BH looking at Passive Slab foundations but want to know more.

 

Our ground has a high water table - in the winter the water table is regularly 2 inches below the surface.  In summer it drops to 3.5 ft.

 

The ground is made up, down to a level of around 2.5 - 3 ft.  Under that is a sand / gravel

mix.

 

These are our findings from digging out the koi pond.

 

When the previous owner originally got planning permission his discussions with engineers were steering him to piled foundations.  However, by the time he decided to sell, he had received advice that ordinary trench foundations could be used on the site - we have not seen any documentation to back up either position and we are obviously going to seek structural advice before proceeding.

 

We started with the assumption that we would probably need piled foundations but then I started to hear about the passive slab.  With the conditions that we have found so far, does that rule out a passive slab completely or is it still worth investigating if it is possible?

 

From what I have read, the passive slab solution helps alleviate the issues that our ground will give - do we just start searching for structural engineers with experience in passive slab?  Not sure where to start.

 

 

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This sounds very much like my own site.

 

For strip foundations you would dig through the made up soil down to 3ft or so until you are into original firm sand / gravel.

 

You might have to remove all / some of the made soil under the house as well.

 

The passive slab can probably best be described as an insulated raft foundation.  You would have to strip most if not all the made ground and build up in layers of compacted stone before laying the passive slab system.  It usually incorporates a perimeter French drain.

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Which way did you go for your foundations?

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8 hours ago, Sue B said:

Which way did you go for your foundations?

Strip foundations and insulated suspended timber floor.

 

The main issue for me is the slop of the site and I wanted to raise, not lower, the ground level. At the back, the house floor is 1 metre above ground level (and that is after the ground level was raised using all the spare soil)

 

To do that with a passive slab would have been a lot of material to import, and would have meant building up a raised platform that extended beyond the perimiter of the house and would not have worked so well with the landscaping plans.

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We have a passive slab on dug out ground.  Dead easy and quick to install, if done by a team who know what they are doing.  Not great for a team who have little experience of laying them.  There are photos and a description of ours being laid here:  http://www.mayfly.eu/2013/10/part-sixteen-fun-and-games-in-the-mud/

 

Start to finish the laying of the blinding, insulation, DPM, reinforcement fabric, UFH pipes and pouring and power floating the concrete slab and ring beam took four days.  The slab is laid directly on a free-draining 200mm thick layer of MOT Type 3, a bit like clean railway ballast, laid on to geofabric.  We have drains around the edges to ensure water flows away.  The photo below shows the dug out ground and the layer of compacted Type 1 stone:

 

5741978318605_Housebase-Copy.thumb.JPG.182ed4223419def89122bf1c53810cdb.JPG

 

 

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9 hours ago, Sue B said:

From what I have read, the passive slab solution helps alleviate the issues that our ground will give - do we just start searching for structural engineers with experience in passive slab?  Not sure where to start.

 

 

Ground that has a very high water table or that is prone to flooding would definitely need the input of an engineer before designing the foundations.

Take a look at the power of floating polystyrene on a flooded site in this link of a flooded car park in London:

https://www.telegraph.co.uk/news/2016/10/09/cars-crushed-against-the-ceiling-in-underground-car-park-as-floo/

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32 minutes ago, JSHarris said:

... free-draining 200mm thick layer of MOT Type 1, a bit like clean railway ballast, laid on to geofabric.  We have drains around the edges to ensure water flows away.  The photo below shows the dug out ground and the layer of compacted Type 1 stone

 

Just for clarity, MOT type 1 includes fines, and isn't free-draining. I think the stuff specified by MBC's engineer is something like "18-35mm, no fines". 

 

We used recycled railway ballast. If you choose to do this, make sure you're getting stuff that's been steam-cleaned. Ours was filthy with god knows what, but it was delivered and laid while I was at work so there was nothing I could do about it.

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19 minutes ago, Ian said:

Take a look at the power of floating polystyrene on a flooded site in this link of a flooded car park in London:

https://www.telegraph.co.uk/news/2016/10/09/cars-crushed-against-the-ceiling-in-underground-car-park-as-floo/

 

 

Perhaps passiv slab house owners should fit marine navigation lights to cover all possibilities.

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Just now, jack said:

 

Just for clarity, MOT type 1 includes fines, and isn't free-draining. I think the stuff specified by MBC's engineer is something like "18-35mm, no fines". 

 

We used recycled railway ballast. If you choose to do this, make sure you're getting stuff that's been steam-cleaned. Ours was filthy with god knows what, but it was delivered and laid while I was at work so there was nothing I could do about it.

 

 

Sorry, that was a typo, should have read MOT Type 3 - I've corrected it now, thanks for highlighting it.

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1 minute ago, JSHarris said:

Sorry, that was a typo

 

Thought it might have been!

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3 minutes ago, epsilonGreedy said:

 

Perhaps passiv slab house owners should fit marine navigation lights to cover all possibilities.

 

Some of ours (EPS300)  took flight in a gale: hence aviation anti-collision lights  too

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Just to put the floatation aspect into perspective, we have an 85m² slab, which is EPS that's 300mm deep.  If the water level rose to the top of the slab (DPM level) then the upward acting buoyancy of the EPS would be around 242 kN. 

 

The mass of concrete and steel in the slab is around 14.5 tonnes, so that exerts a downwards force of around 142 kN.

 

The mass of the house built on top of the slab is around 30 to 40 tonnes, so that exerts a downwards force of between 294 to 392 kN.

 

Summing these forces, using the minimum house mass, we end up with:

 

Downwards forces = 142 kN + 294 kN = 436 kN

 

Upwards force = 242 kN

 

So, even if flooded up to the top edge of the slab, the point where the house would start to get flooded, there is zero risk of the house being displaced upwards by the buoyancy of the EPS, all that would happen is that the force acting on the ground beneath would reduce from around 436 kN to around 194 kN.

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10 hours ago, Sue B said:

From what I have read, the passive slab solution helps alleviate the issues that our ground will give - do we just start searching for structural engineers with experience in passive slab?  Not sure where to start.

 

We have a water table that varies from road level down to 2.3m below. We had an Isoquick insulated raft foundation installed in 2010 which was the first in the UK. It consisted of 200mm compacted type1 sub base with 50mm granite fines on top. The insulation was 300mm Peripor EPS with 200mm thick upstand and 200mm concrete raft. We had soil tests carried out and it is essential that a structural engineer who has a good knowledge of that type of foundation is employed.

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1 hour ago, Ian said:

Ground that has a very high water table or that is prone to flooding would definitely need the input of an engineer before designing the foundations.

Take a look at the power of floating polystyrene on a flooded site in this link of a flooded car park in London:

https://www.telegraph.co.uk/news/2016/10/09/cars-crushed-against-the-ceiling-in-underground-car-park-as-floo/

Bloody hell!

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Posted (edited)

We have the same slab as @JSHarris all done by MBC.  We are on clay and before the slab was laid the area was basically a lake as the water table is high. We were advised by the MBC engineer to lay a land drain 1m out around the area of the slab which we did in advance of laying the slab as you can see from the attached picture.  

IMG_20150926_121024458_HDR.thumb.jpg.24fd8c7584fd2e877464b7ddae9cd191.jpg This has done the trick as we have no problem at all now.

Edited by JanetE
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We adapted the passive slab design for our basement. Even more concrete and steel sitting on top of the 300mm EPS :) 

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11 hours ago, Sue B said:

I have done searches on here and also outside of BH looking at Passive Slab foundations but want to know more.

 

If you want to talk to an engineer who knows about raft foundations, MBC used (and may still use) Hilliard Tanner. Website is http://www.tsd.ie/

 

Hilliard REALLY knows this stuff inside and out, and I found him very approachable when we had a few questions during our build.

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Before I talk to an engineer I have one more question.  I know that some, if not all of you had your UFH pipes laid in the concrete pour and not above the slab in the screed.  On our last build we had a piled raft, then we put insulation down, put the UFH pipes down and laid the screed on top.  This appeared to be normal procedure at the time (2008).

 

The house that we visited locally, who are using Durisol, have a passive slab but they have not put their UFH pipes in that concrete.  They are going put the UFH pipes in the screed.  I can’t work out from the emails that we have exchanged if they will be putting insulation down under the UFH but thinking about where they said the floor height would be I would guess that they must be.

 

I prefer the idea of UFH pipes going in once the shell is up.  I also prefer the idea of screed after the shell as An ICF build will of course have spills to clear up during pours.

 

Our raft was laid when we were at work last time and it was discovered shortly after that it was 50mm out diagonally.  We have learnt our lesson and will be on site during this slab but it did allow us to fudge a solution (the groundworkers picked up the bill for the majority of the corrections). By increasing insulation to level the floor.

 

So, getting round to the question,........ for an ICF build is UFH in screed more sensible than trying to get everything done in the first slab pour?

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The main reason people have built the way they have in the past is responsiveness. If you don't have much insulation, you lose a lot of heat, so you can treat a thin screed almost like radiators, in that it will heat up fast, but also cool down fast.


Once you have a lot of insulation, like those of us with passive slabs, the slab becomes more like a storage heater. For that reason, having it thicker works better, plus why pay to lay two loads of concrete when one will do?

 

We did actually effectively end up with a screed on our slab, but that's because we went for polished concrete (late decision). There's no insulation between the slab and the screed though.

 

There'd be nothing stopping you from topping a slab with a separate screed, but why would you if you plan to cover with flooring anyway? If you have any bits of concrete spills left sticking up, you can just grind them flat with a scrabbler or a floor grinder. Lot cheaper than a complete screed!

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I could see the cost saving but was worried about the more precise laying requirement.  Still, at least I know what is possible when I talk to the engineers.  Thank you .

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You do need skilled people to lay the slab, for sure. A couple of people on here have had bad experiences, but that's possible with screed too.

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On 10/01/2019 at 20:29, Sue B said:

On our last build we had a piled raft....

I am very interested in this as I am looking inot doing this with my current project-

could you give me some more info on this? Please just PM me if you can.

Edited by Patrick

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Raft vs piles, etc. You really need your SE to advise you on this.  As @JanetE  said we used and MBC passive slab and they / Hilliard Tanner required that we did a Geophys survey which involved coring down in the 4 corners of the slab and taking regular core samples at a range of depths.   The result of this was a clean bill of health for a passive slab.  The type and depth of sub-base depends critically on this survey and in some case it will lead to the conclusion that this type of slab is just not suitable for the site -- e.g. if there is any differentials across the site.

 

As to doing the slab, UFH, etc. in one go I would endorse this approach to anyone.  Yes, you have to do careful prep and take create care over getting your levels right, but this was still a case of the slab crew turning up on day 1 and leaving on day 8 with the slab all finished and power-floated ready for tiling and the UFH loops all installed.  Job done.

Edited by TerryE

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A piled foundation is not one that I would choose to do @Patrick the cost was enormous but I no longer have the spreadsheet where our costs were stored to see remind me just how expensive it was.  The company we used (I can’t remember their name) I would never recommend due to the issue of the slab being out by such a large amount.  However, the SE said that we had no choice and we would have struggled to insure the finished house if we had not gone this route.  This was in 2008.  Things have moved on in those 10 years - I had never heard of passive slab at that point and would have investigated then if I had known about it.  Our SE was very much old school, when I wanted to build in ICF he told me that we would never get a mortgage on it.  I believed him because naively I thought he was the expert.  You live and learn.

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FWIW, we have a passive slab AND piled foundations.  Although we don't have a high water table, we are on highly shrinkable dessicated clay down to 12m and beyond.  We could have managed without piles but then would have had to reduce dig to about 3m and backfill - based on my now known cost of muckaway, this would have cost in the order of £40k +.  Instead, we had a reduced dig to 800mm, then mini piles.  The mini piles were put in in one week and the muckaway was far more modest as well as being considerable cheaper on the groundworker labour and plant hire.

 

We also had the UFH pipes buried in the slab at the time it was done and the heating zones were planned in advance to allow this to be done.

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