patp

Foundations on brick and block build

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We have the foundations out of the ground and have infilled with crushed concrete. The next layer, to protect the dpc, is sand. We have laid sand over the whole of the foundations to a depth of approx 10mm it might be deeper in some places and shallower in others. The Building Inspector has just visited and told us that he thinks it is too deep. He did not say it in a way that meant we had to change it. What do others think? Is he saying it for something to say or do we really have to tamp it all down? What will happen if we leave it at that depth? I would have thought that the concrete pour over the dpc would do the job of flattening it down?

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Im guessing 10mm is a typo and should read 100mm.

Blinding layer should be well compacted to prevent voids and settlement later on,

regardless of that, is the B.I saying its too deep because there isnt enough space left above to FFL? or that he (or she) feels you could have saved a few pennies by using more crushed infill to save on sand?

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Probably a case of trying to be more clear with communication. I saw our approved inspector for the first time since lockdown yesterday. There was a couple of things she was a bit wooly about so I asked her clearly 'So what do you want us to do about x'. I followed up and confirmed this by email later which all gets logged into their system.

 

Honestly I think your simplest and best course of action is to call the inspector and ask them to clarify exactly what they want you to do. Hopefully the answer will be nothing and then just confirm this in an email so its on record. 

 

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What is this for, a floor slab?

if so it should have stone of some sort heavily compacted, then a sand blinding compacted and screeded flat, don’t be thinking concrete will flatten it, it won’t it will follow what ever shape you have left. 

 

Now is not the time time to be skimping on a couple of hours work. 

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10mm is fine but as above did you actually put 4" down? If not and your bro literally thinks 10mm is too much blinding i'd be more concerned with his knowledge

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The hardcore/infill should be compressed with something like a walker plate. I also compresses the sand.

 

Are you sure the BCO wasn't referring to the overall height? Eg he thinks there isn't enough height left for whats going on top for the planed FFL?

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15 hours ago, patp said:

I would have thought that the concrete pour over the dpc would do the job of flattening it down?

 

No it won't.

 

Aside: concrete will need levelling even if what's underneath is level. Make sure you know how to do that and have people to help when concrete delivered.

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Thanks all.

 

BCO just seemed to think we should tamp it all down more. To be honest he did not seem too bothered but just commented that where he was walking was a bit loose and deep. I have seen his report and it is not mentioned in it.

 

Our plans have asked for internal trenches (115mm deep) across the bed of concrete that are deeper than the main slab. They coincide with the internal walls. So each internal wall has a deeper foundation than the main slab. This is making it a nightmare to tamp down! We had to level the whole area of crushed concrete and put a whacker plate over it and then dig out the  these 115mm wall trenches and tamp them down separately. The BCO did not know what they were for! We explained that we were following the plans So if we use the whacker plate on the sand we will not be able to use it on these internal trenches. I suppose that we will be able to do those by hand?

 

All being held anyway as our expert groundworker is a farmer now too and is harvesting :(

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Run over the stuff with a wacker until it stops compacting.  For future reference Type 1 properly wacked makes better blinding than sand.

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On 12/08/2020 at 18:17, Mr Punter said:

Run over the stuff with a wacker until it stops compacting.  For future reference Type 1 properly wacked makes better blinding than sand.

We would if it was not for these Bl**dy internal trenches! They are just depressions in the type 1 that coincide with internal walls, 115 mm deep (I think) designed, we assume, to give the internal walls a bit more support. Our, experienced, brickie thinks they are a joke. They would cave in if we went near them with a whacker. As I said earlier the BCO did not know what they were so we could have done away with them as he, obviously, had not consulted the plans before he came out.

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4 hours ago, patp said:

these Bl**dy internal trenches!


not heard of those before, our internal walls had footings as deep as outside walls but narrower 

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21 hours ago, patp said:

we assume, to give the internal walls a bit more support. Our, experienced, brickie thinks they are a joke. They would cave in if we went near them with a whacker. 

sounds like they should have been made same as main wall founds at perimeter 

now is not the time to skimp  on foundations 

 from your description i think i would be digging out and filling them with concrete

 if your brickie is worried --then you should be 

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It sounds like you are trying to support the internal walls from a thickened raft.

 

Who designed this?

 

What do building control say about it?

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Our architect designed them. Everyone that has seen them has been puzzled by them. The Brickie is not worried just incredulous that they are on the plans. We showed them to the BCO and he had never seen them.

 

It is just a bungalow so the the internal, non load bearing, walls should not, really, need this extra support? 

 

I wonder if it is worth asking the architect for his reasons?

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If the internal walls are not load bearing, why not make them of timber and plasterboard?  easier, cheaper, easier to run services etc?

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3 minutes ago, ProDave said:

If the internal walls are not load bearing, why not make them of timber and plasterboard?  easier, cheaper, easier to run services etc?

We like the solidity of the block walls and the sound proofing etc.

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So do we. All our internal walls are block but they sit on full foundations like the exterior walls but narrower. 

 

Do you have a chimney and what does the foundation for that look like?

 

 

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If the internals are non load bearing then they are usually built on top of the slab, with no foundation below, I can see your problem. I'd go back to the architect and ask for clarification. 

 

You could fill the trenches with concrete I guess.

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When I have trenches in any sub base (for instance when I know a drain run is going through and I don't want to re dig the  compacted stone) I usually chuck a couple of planks in them and run the wacker over. Save the problem you have of being too delicate with the wacker around where these runs are. Take the planks out after, trim out with a shovel then do what you've gotta do.

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On 15/08/2020 at 11:06, Temp said:

So do we. All our internal walls are block but they sit on full foundations like the exterior walls but narrower. 

 

Do you have a chimney and what does the foundation for that look like?

 

 

Yes we do have a chimney and the foundations are the same as/deep as the rest of the foundations. 

 

Well, all the sand has blown away now!!! Still can't find any groundworkers that are free to help lay the slab :( We will hold off on laying the sand again until we are sure they are coming. Had a couple of visits but only one quote of £250 but he is not free for a month :( Mind you the brickie is busy on another, luckily small, job. Having days where those thoughts of "wish we had never started" creep into my mind......

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