What is this Passive Slab I hear you talk of

[Sue B]

That is good to know @vivienz - it means I know we can still have the passive slab, even if piles are required.

[Dreadnaught]

@Sue B, another one here. This time mine will be (not started yet) a passive slab combined with screw piles (because of unstable soils, clay beneath that can heave, and lots of tree roots that need to live a long and happy life). I have collected a small sheath of cross-sectional diagrams of piles and rafts. Any of the raft of pile companies will have one that they can send you.

[Patrick]

14 hours ago, Dreadnaught said:

@Sue B, another one here. This time mine will be (not started yet) a passive slab combined with screw piles (because of unstable soils, clay beneath that can heave, and lots of tree roots that need to live a long and happy life). I have collected a small sheath of cross-sectional diagrams of piles and rafts. Any of the raft of pile companies will have one that they can send you.

@DreadnaughtPlease could you foward these to me , I am looking for something like this as well.

Edited January 14, 2019 by Patrick

[laurenco]

On 08/01/2019 at 23:08, Sue B said:

I have done searches on here and also outside of BH looking at Passive Slab foundations but want to know more. The ground is made up, down to a level of around 2.5 - 3 ft.  Under that is a sand / gravel mix. 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.

 

 

 

@Sue B We went with an MBC passive slab on 1m-1.8m of Made Ground on top of sand, and we learnt the hard way... if you have Made Ground you must either remove it or pile around it, even with a passive slab going on top. Terrified of the expense of mini-piles, we opted to remove circa 700 TONNES of Made Ground, which we were lucky we could dump into our [lower] garden. We then replaced the land with MOT Hardcore Type 1. In all it cost around £9k in hardcore plus say £2k labour. When the Building Inspector came, he delayed the foundations install by a week whilst we commissioned a Plate Test (£300+vat) to determine the absolute force of the Hardcore, which thankfully was fine. But I'd get this done asap if I was doing it again, and I wouldn't wait to be told by MBC as their engineer (Hilliard Tanner) waited until the run up to Christmas to advise us to get this done for a January install.

 

I'm sorry this doesn't help with the water table question - I expect a drainage engineer could advise better on this..?

 

@vivienz Can I ask how much the mini-piles ended up being? Ballpark figures. I still don't know if we made the right decision removing the Made Ground in favour of mini piles.

[scottishjohn]

On 08/01/2019 at 23:08, Sue B said:

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.

 

 

your engineer will tell you 

but wet ground will be twice the heat loss of dry ground 

so maybe you lay foundations (strip type) some block work  then use a block and beam floor above the water table with suitable insulation ?

i read a very good article on insulated raft system ,which strongly suggested that if it was in water all the time it would eventually absorb some water ,which would then drop its insulation value by up to half .

so maybe better  to just not get your insulated floor wet to start with.

you have to remember this insulated raft idea came from countires that have permo frost  or very cold winters ,which we do not  

It strikes me a its all about selling us something we don,t really need,

I know all the guys that have done will get defensive --but just think about it logically .

yes insualet founds --but 300mm !! 

a bit OTT for cost verus actual savings possibly

 quite a fair bit of controvesy in Ireland as they have been pushing htis for a while now 

[Jeremy Harris]

10 minutes ago, scottishjohn said:

your engineer will tell you 

but wet ground will be twice the heat loss of dry ground 

so maybe you lay foundations (strip type) some block work  then use a block and beam floor above the water table with suitable insulation ?

i read a very good article on insulated raft system ,which strongly suggested that if it was in water all the time it would eventually absorb some water ,which would then drop its insulation value by up to half .

so maybe better  to just not get your insulated floor wet to start with.

you have to remember this insulated raft idea came from countires that have permo frost  or very cold winters ,which we do not  

It strikes me a its all about selling us something we don,t really need,

I know all the guys that have done will get defensive --but just think about it logically .

yes insualet founds --but 300mm !! 

a bit OTT for cost verus actual savings possibly

 quite a fair bit of controvesy in Ireland as they have been pushing htis for a while now 

 

 

I looked into this for a long time before opting for a passive slab, and frankly most of what you've written is complete tosh, with no evidence to support it.  In fact the opposite is true; there is a lot of evidence that EPS in wet ground doesn't absorb any water after several decades.  Apart from this, a passive slab has a drainage layer underneath, so that the EPS doesn't even sit in water at all, even on wet ground (bear in mind that the base of the EPS is close to ground level anyway).

 

I don't know where you get the idea that countries like Germany (which have been using EPS raft systems for basement foundations for over 40 years now) have permafrost, or that even the populated areas of Scandinavian countries have permafrost (they don't).

 

I'm wholly unaware of any controversy in Ireland either, and I have been following the use of passive slab foundation systems for around 7 or 8 years now.

 

Finally, a passive slab isn't that costly when compared with a traditional foundation.  The excavation requirement is a lot less (we excavated 200mm deep over 85m² for the sub-base) and the labour is less (two blokes laid ours from start to finish in four days).  Add in that the UFH can be simply tied to the steel fabric before the pour (included in the four days it took to lay our complete foundation slab) and there can be significant overall cost savings. 

 

Like any foundation system, it does depend very much on the underlying ground as to whether it is cost-effective and efficient, but for us it was actually the cheapest foundation option, when everything was accounted for.

 

[Onoff]

Pretty sure I read somewhere that EPS, even though it's closed cell can potentially absorb 6% water by volume IF SUBMERGED FOR A YEAR. To do with micro cracks in the surface. 

 

I'd take those odds under my slab all day long. The fact it's generally laid anyway on a draining aggregate makes the argument against pretty null and void.  

 

Interesting further reading in how some worms and beetle larvae can happily live on EPS and that their droppings are safe as a fertiliser. Could be the Passive house version of the termite!

 

 

[vivienz]

3 hours ago, laurenco said:

 

@Sue B We went with an MBC passive slab on 1m-1.8m of Made Ground on top of sand, and we learnt the hard way... if you have Made Ground you must either remove it or pile around it, even with a passive slab going on top. Terrified of the expense of mini-piles, we opted to remove circa 700 TONNES of Made Ground, which we were lucky we could dump into our [lower] garden. We then replaced the land with MOT Hardcore Type 1. In all it cost around £9k in hardcore plus say £2k labour. When the Building Inspector came, he delayed the foundations install by a week whilst we commissioned a Plate Test (£300+vat) to determine the absolute force of the Hardcore, which thankfully was fine. But I'd get this done asap if I was doing it again, and I wouldn't wait to be told by MBC as their engineer (Hilliard Tanner) waited until the run up to Christmas to advise us to get this done for a January install.

 

I'm sorry this doesn't help with the water table question - I expect a drainage engineer could advise better on this..?

 

@vivienz Can I ask how much the mini-piles ended up being? Ballpark figures. I still don't know if we made the right decision removing the Made Ground in favour of mini piles.

 

No problem - about £14.5k and I only needed a reduced dig of 800mm to get the desired finished floor level.  The piles were over an area of about 200 sq m, including an attached garage and were done in a week.  It was the right choice for me as a deeper reduced dig would have been crippling in muckaway costs - no one wants free clay and there's no way we could have lost that much on the site.

[scottishjohn]

I will not reply directly to JS reply to my post  as he is only looking at half of what I wrote ,

as that suits his view point to try to convince all that passiv slab is the only way  for all situations .

which it is not .

As for his saying that germany don,t have permofrost --who mentioned germany ?

plenty of permofrost in nordic countries  and lots of europe that has a frost line down to 1m -1.4m 

which we never get here -thats why you bury water pipe at 500mm

that is why it was developed --so there would be no foundation heave as well as ther insulation value --but note all systemsrequire drainage so it don,t get  WET.

now if as JS says foam is water proff why would they require drainge ?--my case proved I think 

study the spec --other systems are avaialble --i,m not pushing any --just first one i found 

https://econekt.co.uk/wp-content/uploads/2016/03/ECONEKT.-Passivhaus-Raft-Foundation-System..pdf

the case we are discussing has a water table of 2" --we are told 

so why would you want to build into the ground to have soggy foundations --unoless you going to tank them like a basement ..

this idea that eps or foam is water proof is TOSH,as JS says --thats why you have to tank basements even if built from ICF-- and have positive edriange layers as well 

my case proved again  I think 

the SE+ground specialsit will advise --but it would seem that building out of the ground is the simplest and cheaperst way -

use blocks to get out of ground  then  beam and insulated block floor +use as much eps as you like --but above the ground

no sort of french drain around your passiv slab is going to work if water table is at ground level --wheres it going to drian too?

Edited January 30, 2019 by scottishjohn

[Alexphd1]

Sorry late to the party SueB. 

 

I have done a EPS insulated raft foundation with UFH pipes in the raft with Nudara ICF above on a DIY basis (apart from pouring the 60m3 concrete which a professional squad done in a day). 

 

Any questions I can answer fire away.

[Jeremy Harris]

2 minutes ago, scottishjohn said:

I will not reply directly to JS reply to my post  as he is only looking at half of what I wrote ,

as that suits his view point to try to convince all that passiv slab is the only way  for all situations .

which it is not .

 

 

You may have missed this sentence from my post:

18 hours ago, JSHarris said:

Like any foundation system, it does depend very much on the underlying ground as to whether it is cost-effective and efficient, but for us it was actually the cheapest foundation option, when everything was accounted for.

 

Does that read to you as if I'm doing as you say and advocating that a passive slab is the "only way for all solutions"?

 

I mentioned Germany because it is in Germany that the concept of the passive house first gained prominence, and that is where the Passivhaus Institut is based.

 

As for your observations on permafrost, I suggest you have a look at this:

 

 

As you can see, you are completely wrong about permafrost existing in any of the major populated areas of Europe, it is confined the the high, northern areas of Norway, Finland and Sweden, not the areas where the majority of the population of northern Scandinavia live at all.

 

[scottishjohn]

so sweden don,t count then ?? thats not a nordic country 
carry on knit picking if you like 

[Jeremy Harris]

4 minutes ago, scottishjohn said:

so sweden don,t count then ?? thats not a nordic country 
carry on knit picking if you like 

 

Look at that map, @scottishjohn, and tell me what percentage of the population of Sweden live inside the permafrost area. 

[Declan52]

34 minutes ago, scottishjohn said:

I will not reply directly to JS reply to my post  as he is only looking at half of what I wrote ,

as that suits his view point to try to convince all that passiv slab is the only way  for all situations .

which it is not .

As for his saying that germany don,t have permofrost --who mentioned germany ?

plenty of permofrost in nordic countries  and lots of europe that has a frost line down to 1m -1.4m 

which we never get here -thats why you bury water pipe at 500mm

that is why it was developed --so there would be no foundation heave as well as ther insulation value --but note all systemsrequire drainage so it don,t get  WET.

now if as JS says foam is water proff why would they require drainge ?--my case proved I think 

study the spec --other systems are avaialble --i,m not pushing any --just first one i found 

https://econekt.co.uk/wp-content/uploads/2016/03/ECONEKT.-Passivhaus-Raft-Foundation-System..pdf

the case we are discussing has a water table of 2" --we are told 

so why would you want to build into the ground to have soggy foundations --unoless you going to tank them like a basement ..

this idea that eps or foam is water proof is TOSH,as JS says --thats why you have to tank basements even if built from ICF-- and have positive edriange layers as well 

my case proved again  I think 

the SE+ground specialsit will advise --but it would seem that building out of the ground is the simplest and cheaperst way -

use blocks to get out of ground  then  beam and insulated block floor +use as much eps as you like --but above the ground

no sort of french drain around your passiv slab is going to work if water table is at ground level --wheres it going to drian too?

The cheapest method is build the founds with blocks then fill with clean stone,  put your insulation in and then cast your slab,  with UFH pipes if your going that way.  

Beam and block floor shouldn't be used if you don't have do.  

[scottishjohn]

not a clue what percentage --but you can bet the areas close will have a very deep frost penetration level requiring these sort of founds .

And again sweden has summers and winters not like here 

[scottishjohn]

1 minute ago, Declan52 said:

The cheapest method is build the founds with blocks then fill with clean stone,  put your insulation in and then cast your slab,  with UFH pipes if your going that way.  

Beam and block floor shouldn't be used if you don't have do.  

but all  well above the water table 

that is the way VELOX do it in poland  and suggest in build manual --strip found ,then 500mm panels  to make perimeter --fill with concrete ,then clean stone inside +pour slab

Edited January 30, 2019 by scottishjohn

[Jeremy Harris]

8 minutes ago, scottishjohn said:

not a clue what percentage --but you can bet the areas close will have a very deep frost penetration level requiring these sort of founds .

And again sweden has summers and winters not like here 

 

 

I can tell you.  Less than one half of one percent.  The majority of the populated areas in Sweden are hundreds of miles away from the high northern permafrost zone.