Over site prep after foundation masonry. Why?

[flanagaj]

When we had the footings dug, the JCB guy told me to pour the concrete footings and then dig the over site down, before commencing the foundation masonry.  That made complete sense, but unfortunately, the brick layer was committed to starting on a specific date and it didn't get done.  Now I'm thinking how blooming tricky it's going to be to manoeuvre the mini excavator around and not catch any of the block work.  So unless I am missing something 'basic' here, my advice would be to dig it down first and then commence your foundation masonry.

[dpmiller]

why do you need to dig down?

[flanagaj]

6 hours ago, dpmiller said:

why do you need to dig down?

Ground bearing slab requires 525mm to be dug out below FFL.  It's currently only dug down 200mm

[MikeGrahamT21]

You need 150mm from the finished level of your oversite to the bottom of your floor (whether that be joists or B&B).

 

so as long as you’ve got at least that amount there’s no need to dig down. Obviously it must be lower than your DPC, but that will come naturally with the 150mm measurement 

 

Edit: just seen you’re going full concrete slab, so you’ll need plenty of space for insulation below your finished slab. I’m not sure what the minimum recommended thickness for concrete slab is but I’m sure someone will know on here

Edited 20 hours ago by MikeGrahamT21

[Mr Punter]

Have you got a section detail showing the foundation, slab, insulation, DPM, DPC, screed and FFL?

[JohnMo]

56 minutes ago, flanagaj said:

Ground bearing slab requires 525mm to be dug out below FFL.  It's currently only dug down 200mm

We did similar, two of us did it by spade with a barrow. Gets you fit is one way to look at it, kills you is the other. We had a 192m² to do.

 

This is us near the end

 

Have fun

[Owain1602]

10 hours ago, flanagaj said:

my advice would be to dig it down first and then commence your foundation masonry.

With all due respect, I don’t think you should be giving any advice. Just because you’re not comfortable to be operating an excavator and dumper around some block work.

If you don’t trust yourself, get an experienced operator, or get the bulk of the material well away from the blocks out with machinery, then pick away at the rest by hand. 

 

It sounds to me like you should be spending more time doing things, rather than getting every opinion under the sun about how something should be done. 
 

Why was the material not excavated before the blocks started going in? No idea the footprint of what you’re doing, but I can’t imagine it would be more than a day of work.

Did you ask the guy doing the blocks if you could delay him by a day? 

[Oz07]

I get the machine to scrape out before he digs the last wall. You just go to whatever depth you need to bottom of stone for slab or clearance for suspended floor. Are you sure its only 200mm below ffl at present? Did you not even scrape the topsoil off prior to trenches? Presumably your ffl is around 150mm above the existing ground before you started?

[Oz07]

Say 200mm stone, 200mm insulation, 100mm slab. 500mm. Less full being 150mm above ground gets you to 350 reduced dig off ground height. Can reduce insulation with pir and prob go down to 150mm with stone

[flanagaj]

13 hours ago, Owain1602 said:

With all due respect, I don’t think you should be giving any advice. Just because you’re not comfortable to be operating an excavator and dumper around some block work.

I'm getting tired of the comments!

 

My post like many are simply about challenging the 'wisdom' in which construction activities are performed.  Digging out the oversize after block work is up to DPC, is the dumbest approach ever.  So yes, my advice to anyone embarking on a ground bearing slab, is to dig out once you've poured the footings.

[dpmiller]

20 minutes ago, flanagaj said:

dig out once you've poured the footings

why not before? There's topsoil to strip anyway...

[Oz07]

26 minutes ago, flanagaj said:

I'm getting tired of the comments!

 

My post like many are simply about challenging the 'wisdom' in which construction activities are performed.  Digging out the oversize after block work is up to DPC, is the dumbest approach ever.  So yes, my advice to anyone embarking on a ground bearing slab, is to dig out once you've poured the footings.

Before you pour is better, while the digger is still there.

[flanagaj]

5 minutes ago, dpmiller said:

why not before? There's topsoil to strip anyway...

The whole site was dug down by 200mm before footings were pulled.  Even the estimators work schedule states digging down the oversite after you've built up to DPC. I'll stop whining as it's done now, but next time I'll dictate the order of works.