Help me understand Foundations.... (please)

[Mulberry View]

Here is the output from our Structural Engineer...

 

 

I'm not aware of Clay on the site. Our site investigation was attended by the Structural Engineer, with myself operating a 3T digger. 3-4 trial pits were dug, observed by the SE, who also took a soil sample away. He seemed fairly happy with ground conditions, once we got down to around 750mm-1000mm below, we were well into the hard stuff, which I assume to be chalk? By the time our 1.2m percolation test pit was dug, the bottom 400-500mm was hard, solid and did not percolate well at all.

 

The plot has had trees removed (mostly over 12 months ago), some are remaining and have been designed into the scheme. Our foundation is pure concrete it would appear, no steel required. 12mm Starter Bars will be chem-fixed in for the ICF.

 

I'm curious as to the purpose of the Claymaster in the design on the inside edge of the trench?

 

Also, if we wish to implement a deeper void, is it a case of setting the 'top of concrete' height in accordance with the sum of the parts above and with the concrete lowered, we will need to use more blocks to construct the walls?

 

My plan is to construct a dwarf-wall inside the ICF structure on which to bear the beam & block floor, leaving the ICF fully intact and butting the blockwork right up to it, does this sound OK?

[ToughButterCup]

5 hours ago, Mulberry View said:

...

My plan is to construct a dwarf-wall inside the ICF structure on which to bear the beam & block floor, leaving the ICF fully intact and butting the blockwork right up to it, does this sound OK?

 

I had loads' a similar questions about mine - our ICF company retained an SE who was just brilliant. Answered everything by phone, and saved us a couple of grand at least.

 

Give your ICF company a ring and ask to speak to their SE...

[George]

Where are you in the country?

 

Why don't you think there is clay? 

That detail is definitely assuming there is clay! 

 

Quote

 

I'm curious as to the purpose of the Claymaster in the design on the inside edge of the trench?

 

Purpose of the claymaster (and the 150mm void) is to prevent shrink/swell behaviour (aka heave) - governed by the clay content and plasticity - from pushing the foundations and floor around.

 

Quote

Also, if we wish to implement a deeper void, is it a case of setting the 'top of concrete' height in accordance with the sum of the parts above and with the concrete lowered, we will need to use more blocks to construct the walls?

Yes. There is a minimum thickness of concrete you need. Usually for mass concrete this is simply the projection from the edge of the walls or, say 300mm (it is a bit more complicated but that would be OK). However in your case you will need to consider the minimum thickness for the resin anchors for the ICF reinforcement. 

 

Quote

My plan is to construct a dwarf-wall inside the ICF structure on which to bear the beam & block floor, leaving the ICF fully intact and butting the blockwork right up to it, does this sound OK?

Sounds like a detail the ICF supplier should be able to readily give to you. 

 

[Mulberry View]

11 hours ago, George said:

Where are you in the country?

 

I'm in Norfolk.

 

11 hours ago, George said:

 

Why don't you think there is clay? 

That detail is definitely assuming there is clay! 

 

Purpose of the claymaster (and the 150mm void) is to prevent shrink/swell behaviour (aka heave) - governed by the clay content and plasticity - from pushing the foundations and floor around.

 

Yes. There is a minimum thickness of concrete you need. Usually for mass concrete this is simply the projection from the edge of the walls or, say 300mm (it is a bit more complicated but that would be OK). However in your case you will need to consider the minimum thickness for the resin anchors for the ICF reinforcement. 

 

Sounds like a detail the ICF supplier should be able to readily give to you. 

 

 

There's been no mention of Clay and I've not seen it. Apparently the site investigation work was straight-forward and the outcome of the physical examination removed the need for plasticity testing etc. The specification is to found onto the hard chalk bedrock and with a minimum concrete depth of 300mm.

 

I'm struggling  to understand who sets the top of concrete height. It's not been specified by Architect or SE. If it's me, as the Principal Contractor, that's fine, I just need to somehow ensure I'm correctly understanding the process. That is, I know the floor levels, and the floor make-up, but is that enough?

 

Our building has a ground floor level with steps into the kitchen (up 600mm) and steps into the living room (up 400mm). The design shows 2 steps in the foundation, but our Architect doesn't have a good understanding of ICF and so perhaps doesn't realise that we don't really have to consider coursability.

 

At the Southern end of the building, for example, the current design appears to put the top of concrete height at 13.050, with a surrounding average topography of 13.200. To me that's not deep enough to give frost protection, so I made the decision to remove the 2nd step from the foundations, adding another course of blocks underground, this dropping the foundation down by a further 457mm at that end, it felt like a simple decision, but now I'm doubting myself and wonder if I should be doing something else to justify this decision.

Edited July 19, 2022 by Mulberry View

[Mr Punter]

3 hours ago, Mulberry View said:

To me that's not deep enough to give frost protection

 

That applies to the base of the foundation concrete, not the top.

 

If you are on chalk you would notice by the colour.

[JohnMo]

Ask the structural engineering or architect why, way easier than loads of second guessing.  There may be a simple explanation, or they have just and pasted from another job and not removed some details.

[Mulberry View]

14 minutes ago, Mr Punter said:

 

That applies to the base of the foundation concrete, not the top.

 

If you are on chalk you would notice by the colour.

 

I took the basic opinion in the end that pushing the top of concrete lower in the ground means we're less likely to encounter it later. 150mm below just feels like it's barely below ground in places. Adding a course of Nudura at that end of the building, according to my calcs, reduces foundation concrete by 4.5 cube, increases wall concrete by only just over 1 cube and we have enough blocks, but is it that simple?

 

Here is the pit we dug to do the percolation test. Not sure if you can see from this what's in there? It's the second pit that was dug to about 800mm depth from memory after the deeper pit was too slow.

 

 

 

[PeterW]

27 minutes ago, Mulberry View said:

 

I took the basic opinion in the end that pushing the top of concrete lower in the ground means we're less likely to encounter it later. 150mm below just feels like it's barely below ground in places. Adding a course of Nudura at that end of the building, according to my calcs, reduces foundation concrete by 4.5 cube, increases wall concrete by only just over 1 cube and we have enough blocks, but is it that simple?

 

Here is the pit we dug to do the percolation test. Not sure if you can see from this what's in there? It's the second pit that was dug to about 800mm depth from memory after the deeper pit was too slow.

 

 

 

so have you cleared the top surface off all of the site as you need to get the top soil off and then start your levels. It doesn’t look like clay, and I think your SEng has copy pasted but that would also make me wonder what else they have not done properly…. 

[George]

7 hours ago, Mulberry View said:

 

I'm in Norfolk.

 

 

There's been no mention of Clay and I've not seen it. Apparently the site investigation work was straight-forward and the outcome of the physical examination removed the need for plasticity testing etc. The specification is to found onto the hard chalk bedrock and with a minimum concrete depth of 300mm.

 

I'm struggling  to understand who sets the top of concrete height. It's not been specified by Architect or SE. If it's me, as the Principal Contractor, that's fine, I just need to somehow ensure I'm correctly understanding the process. That is, I know the floor levels, and the floor make-up, but is that enough?

 

Our building has a ground floor level with steps into the kitchen (up 600mm) and steps into the living room (up 400mm). The design shows 2 steps in the foundation, but our Architect doesn't have a good understanding of ICF and so perhaps doesn't realise that we don't really have to consider coursability.

 

At the Southern end of the building, for example, the current design appears to put the top of concrete height at 13.050, with a surrounding average topography of 13.200. To me that's not deep enough to give frost protection, so I made the decision to remove the 2nd step from the foundations, adding another course of blocks underground, this dropping the foundation down by a further 457mm at that end, it felt like a simple decision, but now I'm doubting myself and wonder if I should be doing something else to justify this decision.

 

Chalk doesn't necessarily rule out clay (broadly speaking there are four types of chalk, going from A to D, which D effectively being silt, i.e. clay).  Would look different to Marl / mudstone clay. Can't remember the susceptibility to shrink/swell, though. Have a look here if you want to find what soil type you are likely to have : https://mapapps2.bgs.ac.uk/geoindex/home.html?layers=BGSBedEngGeol,BGSSupEngGeol,BGSEGFSReports,BGSUSAReports

 

However - as others have said, could well be a copy and paste detail. Not the worst crime but your SE should be able to clarify for you.

 

You can change the TOC level as required, so long as the thickness that remains and the dimensions of the step meets the general building regulations requirements for steps and the depth needed for the anchors. If there's a significant cost saving then I would certainly do it - just run it past the SE. The architect doesn't need input on the foundations. 

 

As already said, the frost depth refers to the formation (bottom) of the foundation. If in non-shrinkable soils this is typically 450mm in the UK. (the 900mm minimum for shrinkable soils is not related to frost but to water content of the soil)

 

Mass concrete foundations are wonderful simple concepts - you are just replacing the higher level soil which might move around due to water content and frost with concrete which won't. You just need to make sure they are wide enough and thick enough to distribute the load on the lower level soils. 

 

 

Edited July 19, 2022 by George

[Temp]

On 18/07/2022 at 11:07, Mulberry View said:

The plot has had trees removed (mostly over 12 months ago),

 

On 18/07/2022 at 11:07, Mulberry View said:

By the time our 1.2m percolation test pit was dug, the bottom 400-500mm was hard, solid and did not percolate well at all.

 

I suspect the recent tree removal is the main reason for the caution. Normally an issue on clay but possibly other types.  

 

Good database of soil types and more here..

https://mapapps2.bgs.ac.uk/ukso/home.html

 

Click on the + next to "Add Map layers" and go down to "soil types". Expand that and select "Soilscapes for England and Wales".