Kore on Rock

[Mr Blobby]

So now that the demolition is done (and from having issues getting earth rods in) its pretty clear that our site sits on a layer of rock about 400 below the surface. 

Before the groundworks tidy up the site and excavate down for the KORE slab, however, the amount of rock and soil condition is hidden from view and largely guesswork.  And there's my dilemma.  I think.

 

Do I need to get a ground survey done now to see what I'm dealing with, or press on with the ground works and excavation for the KORE slab, breaking the rock out to go down the depth required for the 500 hardcore?

 

Generally speaking, is it a problem to have a KORE slab on (hardcore then) rock?

 

My architect insists I don't need any sort of ground survey and neither does the SE need to come visit the site until after ground works is under way. 

Am I worrying about this too much?

 

Edited July 6, 2022 by Mr Blobby

[jack]

I doubt you need to break solid rock down to the full depth, but there's likely to be a minimum depth of hardcode to ensure load distribution.

Only the engineer responsible for the design can confirm. 

[Mr Blobby]

3 hours ago, jack said:

I doubt you need to break solid rock down to the full depth, but there's likely to be a minimum depth of hardcode to ensure load distribution.

Only the engineer responsible for the design can confirm. 

 

Should have got Hilliard Tanner to sort this out...  sleep walked into the architect directing his SE to do the work without me knowing.  Oh well.

 

[Mr Blobby]

So what's the choreography here?

 

Can't place order with KORE until SE has been on site. (theres about 450 drop at one corner of build)

SE won't come out until excavation has started

Can't start excavating foundation until the KORE slab is ordered

 

I'm stuck in a loop...

Edited July 6, 2022 by Mr Blobby

[Mr Punter]

Why not just excavate down to solid rock or 500mm, whichever is less?

[Iceverge]

Gut feeling on this (not an SE) is that you need to have the raft on a consistent base underneath.

 

One corner on rock and one on infill is a recipe for differential settlement. 

 

If it's only 400mm to the rock I would get out with a spade and dig a selection of trial holes for the engineer to look into. 

 

 

 

 

[Conor]

3 hours ago, Mr Blobby said:

So what's the choreography here?

 

Can't place order with KORE until SE has been on site. (theres about 450 drop at one corner of build)

SE won't come out until excavation has started

Can't start excavating foundation until the KORE slab is ordered

 

I'm stuck in a loop...

Get the digger in and strip it back.  won't take long to dig down 400-500mm over the footprint. Only then will you know what is needed. You'll only need 100mm or so of compacted stone. The whole beauty of a floating raft is that it is much more tolerant of varying ground conditions across its area.

[Mr Blobby]

1 hour ago, Mr Punter said:

Why not just excavate down to solid rock or 500mm, whichever is less?

 

Which is what I would like to do.

 

2 hours ago, Iceverge said:

Gut feeling on this (not an SE) is that you need to have the raft on a consistent base underneath.

 

One corner on rock and one on infill is a recipe for differential settlement. 

 

 

but this is my concern too, if some of the hardcore base is on rock with some of the hardcore base on clay soil then would there be catastrophic soil heave on part of the foundation. 

Not an engineer so may be worryaing about nothing

 

2 hours ago, Iceverge said:

If it's only 400mm to the rock I would get out with a spade and dig a selection of trial holes for the engineer to look into. 

 

 

I fear this is probably the only way to be sure but the ground is mixture of hard clay and stones, not easy to dig at all. May need to get a digger in.

 

 

To get an idea of the layout, here it is with some topo levels.  The site is pretty flat, dropping away at the back of the single-storey garage, where it may be less likely to be on rock but also far less load on the foundation. If that makes any difference 🤔

 

 

[Mr Blobby]

6 minutes ago, Conor said:

Get the digger in and strip it back.  won't take long to dig down 400-500mm over the footprint. Only then will you know what is needed. You'll only need 100mm or so of compacted stone. The whole beauty of a floating raft is that it is much more tolerant of varying ground conditions across its area.

 

I think you're right.  A local grave digger dug the NIE trench for me.  I'll try and get him back on site.  Hopefully nobody's died this week and he'll have some free time.

Edited July 6, 2022 by Mr Blobby

[Conor]

Just now, Mr Blobby said:

 

I think you're right.  I got a local grave digger in to dig the NIE trench.  I'll try and get him back on site.

It's the 12th week coming up... You'll get good deals through the hire companies. Get digging

[Mr Blobby]

1 minute ago, Conor said:

It's the 12th week coming up... You'll get good deals through the hire companies. Get digging

 

Are you suggesting I drive the digger myself?  That would be entertaining 🤪

[Nickfromwales]

12 minutes ago, Mr Blobby said:

 

Are you suggesting I drive the digger myself?  That would be entertaining 🤪

If it’s just you, the plot, and a machine…… get stuck in  Just keep doing a head-count of wife and kids.

There would be very little excuse needed for me to hire a BFO machine and get cracking, and it’s got to be therapeutic too. 

Crack on, it’s not as iffy as it looks, just find out where the services are and go slow there.

 

 

[Conor]

29 minutes ago, Mr Blobby said:

 

Are you suggesting I drive the digger myself?  That would be entertaining 🤪

It's EASY. I only hit my house three times.

 

Just checked balloo. 6 days hire for the price of two. So that's a 5 tonne digger for a guts of a week for £300.

[Ferdinand]

44 minutes ago, Mr Blobby said:

 

Are you suggesting I drive the digger myself?  That would be entertaining 🤪

 

Yes. 'Tis fun.

[Jenki]

My site has bed rock at shallow levels (300- 450mm). For my house ( insulated raft) I did as previous suggestions and dig out the house plot.

I made my excavation 3 meters wider and longer to allow for scaffold and room around the build and stripped off all the soil until I hit rock.

There is a slope on this, but it's all rock ( Caithness flagstone). The intention is to raise the lower corner with free draining hardcore, compacted with vibrating roller to end up with a level plot for the EPS.

SE and BC have no issues, no requirement for load testing etc.

 

The rock is going no where and neither is the compacted rock.

[Mr Blobby]

15 hours ago, Conor said:

It's EASY. I only hit my house three times.

 

Just checked balloo. 6 days hire for the price of two. So that's a 5 tonne digger for a guts of a week for £300.

 

Just checked Balloo.  There's a 3t limit on the holiday offer so that's £308 for the week.  Which would normally be £531.

update... Balloo can't deliver tomorow.  boo 😒

 

That's a good price but this is not a 6 day job is it?  Darling wife wants a trip away.

 

Edited July 7, 2022 by Mr Blobby
3t limit

[saveasteading]

20 hours ago, Mr Punter said:

excavate down to solid rock or 500mm, whichever is less?

Sounds right to me. No point in breaking out solid rock to put back looser rock.

You are right to ask the question. Keep doing so. Pictures too.

[Mr Blobby]

So things have moved sideways a little.

 

Builder on site has suggested excluding the garage from the KORE slab to build the house first and then add the attached garage on a strip foundation afterwards. 

 

His reason is that this would leave room for a telehandler to get round the back of the build and remove the need for a crane. That side of the site also slopes away a little (about 500mm) and appears not to be on such a firm rock base as rest of site.  Image in earlier post above shows garage attached to right hand side of house.

 

On the face of it, building the garage on a strip foundation seems like a good idea.  Is it? 

Has anyone else built a block structure with part strip foundation, part KORE foundation? 

 

Of course I will raise this with my SE but very interested in feedback here as I have no idea if this is a stupid thing to do.

[Thorfun]

1 minute ago, Mr Blobby said:

Of course I will raise this with my SE 

this is the important step! definitely run it by the SE.

 

we have 'arms' attached to our main house that has a basement underneath and those arms are also insulated slabs but are sitting on strip foundations. so there's no reason why you can't have the garage on an insulated slab and 'attached' to the main house later if you want to. obviously if it's ok with the SE! 😉 

 

some might say that there's no need to insulate the ground of the garage but for the cost of the insulation I'd do it as you never know if you want to convert it at a later date to a habitable room or not. will be easier if it's insulated now.

[Thorfun]

here's a photo of our 'arms' on insulated slabs which sit on 2m deep trench fill foundations.

 

 

here are the trench fill foundations for the 'arm' closest to the camera in the photo above.

 

 

and with the insulation and rebar on top ready to pour the slab.

 

 

hope this helps! 🙂 

[Mr Blobby]

So, if I understand this, you put in a traditional trench foundation and then an insulated slab on top of that?  So no hard core under the insulated foundation for drainage, just around the outside?

 

 

9 minutes ago, Thorfun said:

...some might say that there's no need to insulate the ground of the garage but for the cost of the insulation I'd do it as you never know if you want to convert it at a later date to a habitable room or not. will be easier if it's insulated now.

 

... I agree on insulating the garage floor, but had thought the only option was to go the traditional route of concrete subfloor and then insulation and then screed on top. 

[Thorfun]

1 hour ago, Mr Blobby said:

So, if I understand this, you put in a traditional trench foundation and then an insulated slab on top of that?  So no hard core under the insulated foundation for drainage, just around the outside?

we have Cordex under the insulation laid on a bed of sand laid on hardcore. here are our SE drawings for that detail.

 

 

and here it is in-situ 🙂. you can see the three stages in one photo! the far bit has hardcore, the next has the layer of sand blinding and then the cortex in the bit by the basement wall.

 

 

we used TSD for our foundations who are excellent with insulated slabs. you definitely don't have to do it the old school way. obviously all depending on the correct SE calculations!

[Mr Blobby]

17 hours ago, Thorfun said:

we used TSD for our foundations who are excellent with insulated slabs.

 

This is where I went wrong.  I contacted TSD a few months ago only to discover my architect had already instructed his SE to do the work without telling me or providing me with any of the drawings.

 

Should have gone to TSD to get this done months ago.  Oh well.

[Thorfun]

1 hour ago, Mr Blobby said:

only to discover my architect had already instructed his SE to do the work without telling me or providing me with any of the drawings.

I presume that the architect is also paying for the SE as well? I can't see how it's right to tell someone to do some work without getting authorisation from you who will be paying for it! 🤦‍♂️

 

anyway, it's done now. hopefully your SE will be able to give you the details to build the garage at a later stage whilst keeping the insulated slab.

 

best of luck.