Passive foundations on clay?

[Gus Potter]

On 27/08/2024 at 08:28, IanR said:

 

While the EPS300 typically used under the integral ring-beams of an insulated raft is rated at 300 kPa @ 10% compression, it's not going to see those loads in a typical foundation for a timberframe structure. For mine, worst case is less than a 10th of those loads, on the top layer of EPS, that obviously spreads across a greater area as it goes through the the EPS layers. We're talking 1 - 2mm compression.

Hi Ian and all.

 

I can see where you're coming from Ian but there are a couple (well a lot of steps actually) of steps missing. Generally Ian your right about  typical slab edge TF line loading only being about a tenth of the load that a 300kPa insulation can carry at 10% compression.

 

On 27/08/2024 at 08:28, IanR said:

Now say the insulation is 300mm thick. You load that up and it needs to compress by 30mm to achieve its design strength.  Now 30mm compression will play havoc with the concrete slab.! It won't work!

Say Ian's load is 10%.. that results in a settlement of 3.0mm at the edge of the slab say. Now it gets a bit more complicated as that amount of movement at the slab edge is on the boundary of crack / over stress the slab if it is not reinforced.

 

For all G and J sum it up the concept.. sometimes we need to go back to basics!

On 26/08/2024 at 10:41, G and J said:

As I understand it, a passive slab foundation is basically a big lump of concrete cast inside a polystyrene tray.

Simplistically we make a concrete slab that generally spreads the load over a large area and that all works fine. Take a 300 kPa insulation at 10% compression at 300mm thick. That's 30 tonnes per square metre on the face of it but it will; compress like fury, crack your walls, make your floors off level, burst cladding fixings, maybe cause the roof to leak and stop your doors/  windows from opening and shutting.. !

 

When we design foundations and raft slabs we as an absolute limit the settlement to 25mm for a domestic dwelling over a 50 year life span.

 

Now it's not just the compressibility of the insulation it's all the stuff under that... the soil and so on which adds to the amount of movement / settlement.

 

Take the outside walls for example.. these load the slab edge and thus at the slab edges we get an overstress / too much compression of the insulation.  Simplistically we calculate what the insulation can carry at the slab edge. The bit it can't carry we throw back into the slab by reinforcing the slab with steel bars until we get it all to work.

 

The technical term for this is what we call a semi flexible edge thickened raft.. The semi flexible bit is important as what we do is balance the movement / flexibility of the layers with the reinforcement and usually the insulation behavoir so the slab does not fail and crack too much.

 

Now that all sounds complicated but all we are doing is to look at how "squashy" the layers of stuff are under the concrete and design the concrete raft to cope with that.

 

In summary when it comes to EPS or anything like that think about how much it needs to compres by before it achieves it's declared strength.

 

 

 

 

[zzPaulzz]

7 hours ago, Gus Potter said:

Hi Ian and all.

 

I can see where you're coming from Ian but there are a couple (well a lot of steps actually) of steps missing. Generally Ian your right about  typical slab edge TF line loading only being about a tenth of the load that a 300kPa insulation can carry at 10% compression.

 

Say Ian's load is 10%.. that results in a settlement of 3.0mm at the edge of the slab say. Now it gets a bit more complicated as that amount of movement at the slab edge is on the boundary of crack / over stress the slab if it is not reinforced.

 

For all G and J sum it up the concept.. sometimes we need to go back to basics!

Simplistically we make a concrete slab that generally spreads the load over a large area and that all works fine. Take a 300 kPa insulation at 10% compression at 300mm thick. That's 30 tonnes per square metre on the face of it but it will; compress like fury, crack your walls, make your floors off level, burst cladding fixings, maybe cause the roof to leak and stop your doors/  windows from opening and shutting.. !

 

When we design foundations and raft slabs we as an absolute limit the settlement to 25mm for a domestic dwelling over a 50 year life span.

 

Now it's not just the compressibility of the insulation it's all the stuff under that... the soil and so on which adds to the amount of movement / settlement.

 

Take the outside walls for example.. these load the slab edge and thus at the slab edges we get an overstress / too much compression of the insulation.  Simplistically we calculate what the insulation can carry at the slab edge. The bit it can't carry we throw back into the slab by reinforcing the slab with steel bars until we get it all to work.

 

The technical term for this is what we call a semi flexible edge thickened raft.. The semi flexible bit is important as what we do is balance the movement / flexibility of the layers with the reinforcement and usually the insulation behavoir so the slab does not fail and crack too much.

 

Now that all sounds complicated but all we are doing is to look at how "squashy" the layers of stuff are under the concrete and design the concrete raft to cope with that.

 

In summary when it comes to EPS or anything like that think about how much it needs to compres by before it achieves it's declared strength.

 

 

 

 

That’s a helpful explanation, thanks.  In my case, the presence of clay and tree roots must mean the ground itself is quite ‘squashy’ - and its degree of squashiness will change through the seasons. I fear the solution for me is going to be expensive! 

[G and J]

10 hours ago, Gus Potter said:

Hi Ian and all.

 

I can see where you're coming from Ian but there are a couple (well a lot of steps actually) of steps missing. Generally Ian your right about  typical slab edge TF line loading only being about a tenth of the load that a 300kPa insulation can carry at 10% compression.

 

Say Ian's load is 10%.. that results in a settlement of 3.0mm at the edge of the slab say. Now it gets a bit more complicated as that amount of movement at the slab edge is on the boundary of crack / over stress the slab if it is not reinforced.

 

For all G and J sum it up the concept.. sometimes we need to go back to basics!

Simplistically we make a concrete slab that generally spreads the load over a large area and that all works fine. Take a 300 kPa insulation at 10% compression at 300mm thick. That's 30 tonnes per square metre on the face of it but it will; compress like fury, crack your walls, make your floors off level, burst cladding fixings, maybe cause the roof to leak and stop your doors/  windows from opening and shutting.. !

 

When we design foundations and raft slabs we as an absolute limit the settlement to 25mm for a domestic dwelling over a 50 year life span.

 

Now it's not just the compressibility of the insulation it's all the stuff under that... the soil and so on which adds to the amount of movement / settlement.

 

Take the outside walls for example.. these load the slab edge and thus at the slab edges we get an overstress / too much compression of the insulation.  Simplistically we calculate what the insulation can carry at the slab edge. The bit it can't carry we throw back into the slab by reinforcing the slab with steel bars until we get it all to work.

 

The technical term for this is what we call a semi flexible edge thickened raft.. The semi flexible bit is important as what we do is balance the movement / flexibility of the layers with the reinforcement and usually the insulation behavoir so the slab does not fail and crack too much.

 

Now that all sounds complicated but all we are doing is to look at how "squashy" the layers of stuff are under the concrete and design the concrete raft to cope with that.

 

In summary when it comes to EPS or anything like that think about how much it needs to compres by before it achieves it's declared strength.

 

 

 

 

Crumbs, funny how different things can seem when a more interesting and illuminating viewpoint is highlighted.  You should consider doing this for a living @Gus Potter  🙂 

 

I think of concrete as totally inflexible and solid, but nothing really is.  If I think about two people standing on a thin sheet of ply on a lawn, no one would be surprised at it bending if they stood right on the edge.  But if they moved in a bit it would bend less.  Shock!

 

The more I think about this the more I like strip foundations.  

[saveasteading]

2 hours ago, G and J said:

The more I think about this the more I like strip foundations.  

I consider all options but 99% I will choose strip footings or pads, except on loose fill.

The building sits on solid foundations. The slab and insulation only support room loadings....an upright piano being the heaviest it will ever support, and a theoretical, slight compression of insulation won't matter.

 

No need to get more complicated and use specialist workers either.

[Simon R]

On 26/08/2024 at 08:23, MikeSharp01 said:

Passive slabs are more expensive than strip foundations but your soil conditions are the same as ours, although we have no large trees close, so no reason not to. You need to get a proper structural design based around a few cores taken by a soil mechanic as we did. The rest was just hard work - but worth it.

I agree completely. Our build is on clay over gravel (Bracklesham beds). The structural engineers we used were Tanners who used input from our soil samples. Yes, it's move expensive that strip but the results in terms of insulation are worth it. Regarding strength, these raft are recommended for earthquake areas.

[saveasteading]

59 minutes ago, Simon R said:

the results in terms of insulation are worth it.

Are there figures? I would hazard a payback in heat loss of £50 / year for the small area of lower resistance  at the sole plate. For an extra foundation cost of £10k on an average house?

There are other reasons for the decision but that isn't a conclusive one for me.

 

59 minutes ago, Simon R said:

these raft are recommended for earthquake areas.

Which supports my point too. It does not need to be that strong.

[Nick Laslett]

As a complete novice, what I liked about the passive slab, was how simple it was to execute and understand. The EPS tub created a continuous insulation layer you could see. No cold bridges, and no extra steps. The power floated concrete gave you a finished floor, which needs no additional work, no screed no additional moisture being pumped into your build. I went with fermacell for the same reason, no wet skim adding moisture to the build. The simplicity of putting the UFH in a large piece of concrete with a lot of mass and the ability to hold a lot of heat. This suits the ASHP for heating and cooling. I had ICF on top of my slab, so this was even more simple, the continuous insulation layer around the whole house could be inspected and easily understood. I realise these options aren't for everyone and not applicable in a lot of builds. But don't underestimate the value of simplicity, even if it costs more.

[Gus Potter]

On 31/08/2024 at 06:23, zzPaulzz said:

In my case, the presence of clay and tree roots must mean the ground itself is quite ‘squashy’ - and its degree of squashiness will change through the seasons.

Yes that sums it up but also the ground swells up.. some grund can swell / shring a lot.. several inches / cm! Things like trees and the desication of the ground are essential to know about.

 

On 31/08/2024 at 18:03, Nick Laslett said:

As a complete novice, what I liked about the passive slab, was how simple it was to execute and understand. The EPS tub created a continuous insulation layer you could see. No cold bridges, and no extra steps. The power floated concrete gave you a finished floor, which needs no additional work, no screed no additional moisture being pumped into your build. I went with fermacell for the same reason, no wet skim adding moisture to the build. The simplicity of putting the UFH in a large piece of concrete with a lot of mass and the ability to hold a lot of heat. This suits the ASHP for heating and cooling. I had ICF on top of my slab, so this was even more simple, the continuous insulation layer around the whole house could be inspected and easily understood. I realise these options aren't for everyone and not applicable in a lot of builds. But don't underestimate the value of simplicity, even if it costs more.

Nick makes a good point here. If your motivation is to achieve something close to passive then a raft is a good simple way of doing it. Yes the raft may cost a little more than a strip found but it can be much easier to build in some circumstances.. it's simple if you take care and lay the insulation and rebar correctly.

 

All these things come with a lot of complexity. Say you are in a Radon area then a raft makes the Radon Barrier easier to detail out / buildability for example. Trees for example in clay soils can make things much more complex.

 

But just say you have good ground and just want a raft as Nick favours and because YOU CAN and WANT IT.. no harm in that.. it's your house and your design decision.

 

If you have good ground then this can be easy to achieve without chucking loads of rebar at the slab and making it massivly thick. You need some kind of floor anyway! It usually needs a bit of edge thickening in some form or another as folk want to put in big glass doors these days  which cause point loads at the slab edge.

 

One biggy that BC / NHBC ask about is cover for frost and height to DPC. Generally height to DPC from ground level for sensitive wall cladding is 150mm which is about 50mm less than the thickness of an ideal raft slab on good ground. Now add 300 mm of EPS to that takes you down to 350mm below finished ground level. Add say 150mm of type one and now all our materials (which are not suceptible to frost) extend to the min of 450mm required for frost cover. Box ticked.

 

For all.. Raft slabs.. and a bit of info that may help.

 

There are argueably three at least kinds of generic rafts. Within each type there are permutations. There are others but let's run with this for now.

 

The main types are:

 

1/ A rigid raft. This tends to be a bit of a beast. We may use this in a domestic context where we have past mining that can cause the ground to move / crack horizontally at the surface. This used to be associated with long wall mining where you get a rolling wave of horizontal movement in the ground. We don't often design this way now in a domestic setting.. as they shut all the mines long ago.

 

2/ A semi flexible raft. Here we thicken the edge a bit but if you have heavy walls the EPS say at the edge compresses too much which causes the edge of the raft to rotate. To stop the rotation we reinforce the slab so it carries some of the vertical loads and stops the rotation of the edge.

 

3/ An edge thickened slab. Here the thickening at the slab edge deals with the frost cover and the differing line and point loads around the slab edge. The slab it's self is just designed so it does not crack thus has a light reinforcing mesh.

 

Now within all these generic types there are permutations and the design is often driven by how good and consistent the soil is under the whole thing. If we have ground that has local soft spots then the edge of the slab and internally often needs to be thicker so it can span over the soft spots and thus needs more reinforcement.

 

In summary it's a pretty complex undertaking in term of the structural / soil things you need to know about . but the solution is often simple once you draw it out. The main thing is to spend a bit of time and money understanding and investigating your ground as this reduces your risk and helps you design the right way. With a fair wind a raft slab does not always cost a lot more when you take everything in context.

 

 

 

 

 

 

 

 

 

[Simon R]

On 31/08/2024 at 17:43, saveasteading said:

Are there figures? I would hazard a payback in heat loss of £50 / year for the small area of lower resistance  at the sole plate. For an extra foundation cost of £10k on an average house?

Good point, our raft cost 5,248 just for the eps. Quite a bit of money, plus another 20k for the ICF blocks, with a total construction cost around 150K for the completed structure.  I can't equate the cost to a conventional build. The result was considerably reduced risk and a very quick construction time. It's so well insulated that we didn't require UFH, the slab is the same temperature as the house even in winter, no need for slippers etc. and an annual heating bill around the 300 mark.

I don't doubt the cost is higher than a conventional build but the comfort is worth every penny.

[saveasteading]

6 minutes ago, Simon R said:

we didn't require UFH, 

That's worth a lot.

Of course any slab or wall can have lots of insulation.

Is the slab the finished surface? Thinking about weather affecting the pour.

[Simon R]

7 hours ago, saveasteading said:

Is the slab the finished surface? Thinking about weather affecting the pour.

We initially wanted a polished concrete finish, fortunately our builder talked us out of it and we ended up putting down self levelling screed topped with vinyl planking.

 

[zzPaulzz]

7 hours ago, Simon R said:

... It's so well insulated that we didn't require UFH, the slab is the same temperature as the house even in winter, no need for slippers etc. and an annual heating bill around the 300 mark...

Wow.. did you install UFH and just not turn it on, or decide early on you didn't need it. 

[saveasteading]

38 minutes ago, Simon R said:

self levelling screed

Good. I wouldn't dream of pouring a finished floor out in the open, even for industrial use.

[Simon R]

14 hours ago, zzPaulzz said:

Wow.. did you install UFH and just not turn it on, or decide early on you didn't need it. 

The figures we got from the SAP calculations suggested we only needed a couple of kW to maintain a 20c temperature difference. As we had gas available we decided on a 3kW combi boiler to provide hot water and heat two towel rails and a single radiator in our living room. It has worked out well both from a comfort and budget point of view. We did have a problem with too much solar gain which we solved with a a small A2A unit which has ended up being used for heating in preference to the gas boiler.

Installing an ASHP and UFH made no sense given the 2kWh input requirement and saved a lot of complexity and budget.

[zzPaulzz]

On 04/09/2024 at 09:00, Simon R said:

The figures we got from the SAP calculations suggested we only needed a couple of kW to maintain a 20c temperature difference. As we had gas available we decided on a 3kW combi boiler to provide hot water and heat two towel rails and a single radiator in our living room. It has worked out well both from a comfort and budget point of view. We did have a problem with too much solar gain which we solved with a a small A2A unit which has ended up being used for heating in preference to the gas boiler.

Installing an ASHP and UFH made no sense given the 2kWh input requirement and saved a lot of complexity and budget.

That makes sense. My modelling (amateur trying the PHPP model) says I’ll have a similar heat demand so I’m going with a small ASHP with UFH in slab. That way I can power it off my PV and battery, offsetting in winter to keep the cost down to gas equivalent or less. No gas to the house. 

[G and J]

2 hours ago, zzPaulzz said:

That makes sense. My modelling (amateur trying the PHPP model) says I’ll have a similar heat demand so I’m going with a small ASHP with UFH in slab. That way I can power it off my PV and battery, offsetting in winter to keep the cost down to gas equivalent or less. No gas to the house. 

Snap.  Only I’m too tight to buy PHPP so I used Jeremy’s spreadsheet.

[zzPaulzz]

I resisted for a long time but found I was struggling to assess the value vs performance.  PHPP 10 is a beast of a spreadsheet and some the terminology is unfamiliar, but I feel I’m through the worst of the learning curve and starting to get sensible answers out of it.  

[zzPaulzz]

Finally got a soil report for my site.  I’ve got medium plasticity clay, so that plus the various trees on site means my SE says I’ll need foundations 1.8m deep in places and a suspended raft. Hopefully he can find a good solution. The guidance from everyone here has been a great help, thank you.

[saveasteading]

3 hours ago, zzPaulzz said:

Hopefully he can find a good solution.

Good news. Standard `construction. Strip footings and beam & block, I would think.