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 yesterday at 15:50 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.

[Mr Punter]

Learning by doing.  You will ace this next time!

[Nickfromwales]

7 hours ago, flanagaj said:

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.

Make a ramp, and get the micro digger in there. 
 

Or just get a wet saw and remove a 1200mm section of brickwork and accept that as collateral damage aka “it is what it is”. 
 

Digging it out by hand is unnecessary, there’s always a solution. A good machine operator will tell you how he’s going to get a machine in there. My guy walked up a vertical 900mm brick wall without so much as a scratch.

[saveasteading]

8 hours ago, flanagaj said:

challenging the 'wisdom' in which construction activities are performed.

Quite right too, but you didn't take the advice do doing it before, and the majority of activities and processes are well proven.

The oversight of the over-site!

 

Edit I see @Nickfromwales has said what I had drafted below.

 

1. Why can't you knock a few blocks out and get the digger in? or a ramp?

2.Can you or your advisor redesign the slab, and will a redesign cost more than you will save by not reducing the over-site.

I don't favour a thick concrete slab over insulation anyway, and can't see much point of having strong stone under weak insulation, so I'd be redesigning completely. But you presumably are working to a professional's design so should speak to them about changing it, or live with the problem and, as above , learn to manage the project more thoroughly/ forcefully.

We all make mistakes. I discovered a biggish practical one this week, but we are qualified to adapt the design to overcome it. 

 

The sketch requested above would be useful.

 

[Nickfromwales]

1 minute ago, saveasteading said:

1. Why can't you knock a few blocks out and get the digger in? or a ramp?

By the looks of the block-work they may just fall out with a swift left boot.

[saveasteading]

1 minute ago, Nickfromwales said:

they may just fall out

Sad but true,  but take them out by whatever means, A big hammer with a block of timber should do it. Then you can clean the mortar off and relay them when ready.

I've just read that post And this was the bricklayer (sorry to abuse that term) who said he was 'committed' and has stopped you getting access.  You are entitled to knock some of his wages off for putting the work right.  Expect some aggro, but you can confidently state that you have taken advice from independent specialists and it is unacceptable to British Standards.

[Oz07]

13 minutes ago, Nickfromwales said:

By the looks of the block-work they may just fall out with a swift left boot.

Is there a photo?

[Redbeard]

8 minutes ago, Oz07 said:

Is there a photo?

 

 

[ETC]

Raise the FFL.