Our raft idea

[ianfish]

Who have people used for a raft foundation? I am struggling to get any info to compare systems. Isoquick - called but no reply yet 72 hours Passiveframe - they say they are too far away 75 miles Kore - quoting but giving us heads up they dont tend to supply indivdual builds Insulhub - want 700 to deliver about 80 miles... Anyothers worth contacting ?

[Russell griffiths]

Buy your own materials and do it yourself. 

What is the reason for wanting this for an extension. 

My thoughts are that you will create a lot more agro than you need, the difference in materials from the old to the new will be a pain all the time. 

[dpmiller]

Polarwall will supply XPS...

[ianfish]

On 10/08/2020 at 15:04, Russell griffiths said:

Buy your own materials and do it yourself. 

What is the reason for wanting this for an extension. 

My thoughts are that you will create a lot more agro than you need, the difference in materials from the old to the new will be a pain all the time. 

There is always gkin

[ianfish]

Finally we have planning!

 

We are looking at engaging a SE but to get a grasp of what and why what they will advise.

 

The house as it exists was built mid 30's and is on a relatively shallow raft foundation from what I have seen so far.

 

There is also a leanto side end extension which has trench footings built circa 2000.

 

Can the EPS raft be just butted up to the current building?

 

My first question what have others done?

[Gus Potter]

On 10/08/2020 at 11:50, ianfish said:

Who have people used for a raft foundation? I am struggling to get any info to compare systems. Isoquick - called but no reply yet 72 hours Passiveframe - they say they are too far away 75 miles Kore - quoting but giving us heads up they dont tend to supply indivdual builds Insulhub - want 700 to deliver about 80 miles... Anyothers worth contacting ?

A few words of encouragement.

 

We have been designing rafts for many years (decades). The method is tried and tested, proven to work. Now we want to have rafts / basements with insulation (ICF) underneath.. the insulation is essentially  just treated as another layer of soil... The structural engineering principles are the same, the harder part is the detail so you can avoid thermal bridging and so on, stop the building lifting up when the wind blows say.

 

An old building may well have shallow founds and move about up and down between the seasons as the ground shrinks and swells. A ICF raft founded at the same depth as the original founds is more likely to move up and down at the same rate as the original house. It's common sense.. but you may  need to spend a bit of money exposing the existing founds etc so the SE can justify the design.

 

Find an SE that understands these principles and you are up and running.

 

 

[syne]

On 01/02/2021 at 01:02, Gus Potter said:

We have been designing rafts for many years

  -snip--

An old building may well have shallow founds and move about up and down between the seasons as the ground shrinks and swells. A ICF raft founded at the same depth as the original founds is more likely to move up and down at the same rate as the original house. 

_snip-

Find an SE that understands these principles and you are up and running.

 

 

 

So am I correct in thinking just like 'a rising tide raises all boats' - 'a ground movement, moves all foundations'?

 

If a sips extension on an icf was attached to a brick house on a strip found -

 would the disparity in building weights create a different raise/movement amount between the two building types?

ie the heavier brick house creates more resistance/moves less

Or do they both pose an insignificance resistance  and the ground moves them both by the amount the ground decides to move? 

 

When you say 

"A ICF raft founded at the same depth as the original found"

I presume you mean the top, So the bottom of the new raft would sit level with the top of the old? 

 

 

[Gus Potter]

Hello Syne.

 

Your nautical approach will appeal to some of the members on BH. It's a good analogy and good questions.

 

I simplify, but if you think about the ground as layers of a cake. Each layer (if clay say, not rock ) moves - expands and contracts by different amounts depending on the moisture content. Each layer of soil also compresses by different amounts depending on how much load you put on it. You can be technical here and try and account for the different layers of soil and found the ICF at a different depth. You could maybe analyse this using an advanced soil model.. academic research..  But in reality the layers of soil are not generally level as this is nature. Any model would be just that. By different depths I mean say plus 300 mm (not a few mm) as this is the point where you could start to excavate too much near the existing founds?

 

A way of simplying this is to found things at roughly the same depth. A clay soil as it shinks and swells will easily lift a house up and down. Syne .. yes your right about the ground. If the clay (say) swells and the house stayed at the same level then this would mean that the founds are sinking into the soil? If you are on rock or chalk then it's different, I have assumed clay here.

 

Now, if you found the ICF raft at the same level as the old house each will move roughly up and down at the same rate at this level (datum). What you are doing here is making educated assumptions. This is the first part.

 

Next (second part)  is to look at the ICF material. Often you see a spec for IFC as being able to carry say 150 kPa at 10% compression. That's about 15 tonnes per sq metre. Sounds like plenty, but the insulation is elastic. If you have a thickness of ICF of say 300 mm then to carry this amount of load then the ICF would need to compress by 10% of 300mm = 30 mm..that causes an issues for say a floor slab, the windows and doors won't open, you'll get cracking that will overstress the other parts of the structure. One reason being is that some parts of the ICF will be more heavily loaded than others and move differentially.

 

What we do here is to manage the loads on the ICF down to about say 40 Kpa as a starting point. Now the 30mm compression at 150 kPa is (40/ 150) * 0.1 * 300 = 8.0mm.. this is more on the ball park.. a timber kit can shrink this amount easily for example. The 8.0mm is very rough as there are other factors that are too lengthy to explore for now.

 

Put the two parts back together. If you load up the floors on the old house, say get a heavy fall of snow, the walls are solid, go all the way to the founds, thus the old house does not move (compress) down that much over the eaves to foundation level. It's a solid old house. The extension walls subject to the same load won't compress that much but the IFC will compress.. by say the 8.0mm under full loading.

 

In summary with a bit of good detailing you can account for the ICF movement between the old and the new which is doable, but founding at different depths (bottom of ICF ~ = to bottom of old found depth)  can introduce other issues. A good way of looking at this is to say.. well the old house shows no signs of distress, let's not found something at a different depth, more, let's found something at the same depth, try and avoid heavy point loads on the soil, especially near the old founds and see if we can develop a design along these lines that will account for the different behavoir of the ICF insulation and the old walls of the house.

 

Hope this helps.