Setting out, MBC passive slab

[Russdl]

Is this simple or complicated, can someone put me straight?

 

We have a relatively level site, dropping 1.2m over 51m from north to south. Our southern boundary is a neighbours fence and our eastern boundary is an easily defined straight line. I know the elevation of the inspection chamber cover in the south west corner of the plot, 61.31m, and the top of slab, 61.74m

 

As we are having an MBC foundation (which we expect to be of fairly standard depth) I'm going to need a hole 600mm below the top of the slab, and covering the footprint of the building (T shaped) plus 1m all around.

 

To my mind, and in my particular circumstances, it looks like it's going to be fairly straight forward to do that. (15m and a bit from the souther boundary, 1m and a bit from the eastern boundary and dug out to 170mm below the level of the inspection chamber cover.

 

Is there more to it than that?

 

 

Edited February 20, 2019 by Russdl
Had my maths homework marked and got 2/10

[Mr Punter]

I make it 170mm dig down from the IC cover (but 15mm wont make much difference).

[Russdl]

  On 20/02/2019 at 16:47, Mr Punter said:

I make it 170mm dig down from the IC cover (but 15mm wont make much difference).

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? It clearly is complicated then if I can't even do some basic maths! I'll edit that, thanks.

[TerryE]

IMO, a key point is to dig / concrete in some datum posts near the perimeter of the site and well outside the area where your groundworks team is going to be using plant; also preferably where you have clear line of sight to the entire slab area.  If you have 3 of these to form a reference baseline then you can easily triangulate the corners (and retriangulate as needed) and recover accurate levels if you have a Dumpy or equiv. 

Edited February 20, 2019 by TerryE

[Jeremy Harris]

  On 20/02/2019 at 18:32, TerryE said:

IMO, a key point is to dig / concrete in some datum posts near the perimeter of the site and well outside the area where your groundworks team is going to be using plant; also preferably where you have clear line of sight to the entire slab area.  If you have 3 of these to form a reference baseline then you can easily triangulate the corners (and retriangulate as needed) and recover accurate levels if you have a Dumpy or equiv. 

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Excellent point. 

 

I established two fixed datum points, one was a handy OS spot height nail in the lane next to our drive entrance, the other was a fixed T post in the North East corner, outside any excavations.  These proved invaluable when doing the cross checks to ensure that the base for the slab was in the right place and that the ducts, pipes etc were all coming up in exactly the right places.  Our ground works chap though I was mad in insisting on sub-20mm accuracy, but it turned out that this was only just good enough; we ended up with a soil pipe that was right up against the edge of the frame and another 10mm of clearance would have saved me half a day of frigging around to get things to line up.

 

Well worth hiring a Total Station, if you don't have access to one, too.  Our ground works chap had one, and without it cross checking the exact positions of things would have been a pain.

[Russdl]

  On 20/02/2019 at 19:12, JSHarris said:

I established two fixed datum points, one was a handy OS spot height nail

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I remember reading about that in your blog Jeremy, my survey is referenced to a spot heigh at the nearby church. The IC cover I have on the plot is permanent fixture for the time being and so I was considering that my 'nail'. 

 

  On 20/02/2019 at 18:32, TerryE said:

IMO, a key point is to dig / concrete in some datum posts near the perimeter of the site and well outside the area where your groundworks team is going to be using plant; also preferably where you have clear line of sight to the entire slab area.

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I have space to do that, great idea.

 

  On 20/02/2019 at 19:12, JSHarris said:

These proved invaluable when doing the cross checks to ensure that the base for the slab was in the right place and that the ducts, pipes etc were all coming up in exactly the right places.

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I presume that means you had all the drains etc in position prior to MBC arriving and starting the slab? For me MBC will:

 

 "install all pipes and ducts to a distance of 1 metre from the edge of the foundation slab – leaving you and your groundwork contractor to connect onwards."

 

Am I right in thinking that's different to what happened in your build?

[Jeremy Harris]

  On 20/02/2019 at 22:00, Russdl said:

I presume that means you had all the drains etc in position prior to MBC arriving and starting the slab? For me MBC will:

 

 "install all pipes and ducts to a distance of 1 metre from the edge of the foundation slab – leaving you and your groundwork contractor to connect onwards."

 

Am I right in thinking that's different to what happened in your build?

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Yes, that's different to us.  I agreed to lay the sub-base and put in drains, ducts etc, MBC then blinded that. levelled and compacted everything and installed the slab.  If they do the drains, ducts, sub-base etc then that removes the risk of them being in the wrong place, so it's a good move, IMHO, and takes even more risk out of things.

[Russdl]

  On 20/02/2019 at 22:07, JSHarris said:

and takes even more risk out of things.

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That was what I was thinking. It seems that for me it's going to be really straight forward but just the fact that I'm thinking that makes me nervous that I've missed something really obvious that will bite me in the backside on 'D' day.

 

I'm planning to just have the hole dug and nothing else so that MBC can put the slab in it and then, after the slab is in, and MBC have left for a couple of weeks start connecting drains and services to the "pipes and ducts" they've left behind. 

[Jeremy Harris]

They will probably come out and have a look at the site a few weeks before they start, so with luck you can make sure everything is sorted out OK.  The MBC part of our build was far and away the least hassle, it was really things that I didn't think for a moment would cause problems that ended up being the ones that bit me in the bum, hard, like the bloody borehole...

[Russdl]

  On 20/02/2019 at 22:43, JSHarris said:

like the bloody borehole

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I do recall your trials and tribulations with that. Water will be a problem for me as well, not sure how hard it will bite yet. 

[Pete]

Whatever you decide to do check and re check your drain and utility points in relation to your slab. I had a discussion about the location of a hidden rainwater gully that came up the middle of our frame, outside the insulation layer, telling the timber frame guy it was positioned wrong. He was adamant it was right and consequently put it in place in some concrete. I re measured when they had gone and removed it. We discussed again next day and he agreed to move it. When frame erected it was bang on where it should have been. We all make mistakes, it is just sometimes some are worse than others. 

[Russell griffiths]

From what I have seen I would check all their measurements, twice 

mbc working in the dark and late on Saturdays would not fill me with confidence that a mistake had not been made. 

 

if you are confident re checking then do it, if not pay your surveyor to come and check all points prior to concrete placement 

[Russdl]

@Pete was that MBC? Did they ‘instal all pipes and ducts’ for you?

 

@Russell griffiths some very sound advice there. Thanks. 

[TerryE]

@Russdl make sure that you have a very clear agreement with your TF supplier who is responsible for what when it comes to drainage etc.  Like Jeremy our agreement with Trish @ MBC was that I was responsible for the exact placement of the drains, not MBC -- though they did provide the labour.  The process that we agreed was that they laid the sub base layers and binding sand layer and the EPS perimeter -- which I then surveyed to validate levels, trueness, etc.  MBC had previously sent me the AUTOCAD files, and I used a free autocad viewer to take distances and X,Y positions of the foulwater risers, etc. so that we could position these to the nearest cm or so.  As with Jeremy this saved a lot of grief after the frame was up because the risers were correctly positioned relative to the frame.  We cut the drain runs in, checked the flow rates with a bucket test and then made good and completed the EPS former.

 

You might consider another trick that @Stones did which was to terminate all of his risers with a female fitting about 1-2cm below the slab finished level.  (See his blog.)   This allowed the slab crew to power-float over the top of them.  It was then a simple matter to go around a day later when the slab was still green and chip off this 2cm layer, exposing the female coupler.

 

We used a few 110mm runs as access pipe to thread water etc. into the service cupboard.  This is also a good idea, but if you do this then consider using slow bends or even slow 45° bends to bring these up into the slab -- it's a damn sight easier pulling 25mm MDPE round a slow bend!!  

Edited February 21, 2019 by TerryE

[Russdl]

@TerryE thank you for all that Terry, I'll definitely pin down exactly what is and isn't going to be done by MBC. 

 

I recall reading @Stones's comments re the female fitting on top of the risers, a good idea.

 

  On 21/02/2019 at 09:32, TerryE said:

 

We used a few 110mm runs as access pipe to thread water etc. into the service cupboard.  This is also a good idea, but if you do this then consider using slow bends or even slow 45° bends to bring these up into the slab -- it's a damn sight easier pulling 25mm MDPE round a slow bend!!

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That is definitely on the 'to-do' list, and maybe 3 of them - 1 for water (in and out), 1 for electric (in and out, bt etc), and 1 for last minute changes of plan! As it stands we don't have an ASHP in the scheme - it was going to be Sunamps all the way but my confidence in that plan is being dented in light of recent developments.

[TerryE]

  On 21/02/2019 at 19:07, Russdl said:

1 for last minute changes of plan!

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Jan and I had real fun threading the insulated double run for the external ASHP down one -- I only wish that we'd used a 160mm run with a low bend for this!!  In the end, thanks to a suggestion by @JSHarris we ended up using flexible connectors to make the bend.

[Pete]

  On 21/02/2019 at 19:07, Russdl said:

@TerryE thank you for all that Terry, I'll definitely pin down exactly what is and isn't going to be done by MBC. 

 

I recall reading @Stones's comments re the female fitting on top of the risers, a good idea.

 

 

That is definitely on the 'to-do' list, and maybe 3 of them - 1 for water (in and out), 1 for electric (in and out, bt etc), and 1 for last minute changes of plan! As it stands we don't have an ASHP in the scheme - it was going to be Sunamps all the way but my confidence in that plan is being dented in light of recent developments.

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May be an idea to put a large duct in in case you decide you need an ASHP at a later date if the Sunamp does not work out

[TerryE]

  On 22/02/2019 at 17:33, Pete said:

it was going to be Sunamps all the way

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At the end of the day, the SunAmps are a heat store and you need to supply that heat.  If you use electric heating elements then to CoP must be 1 hence the C rating.  Pre-heat + SunAmp allows you run your ASHP at a single temp, but still get some CoP gearing.. 

 

BTW, based on this last year's historic data, using an ASHP rather than Willis for UFH will save us about £450 p.a. in electricity bills; and maybe £50-100 more if we use the buffer tank for DHW preheat.  If I am looking for a 5-year payback on any ASHP + buffer tank investment, then this sets a pretty hard limit for me.

Edited February 22, 2019 by TerryE

[Jeremy Harris]

Worth noting that the main issues with the Sunamp UniQ seem to be isolated to the electrically heated versions; those heated by hot water seem to be OK, I believe.

[Russell griffiths]

Why not use 100mm ducting instead of underground pipe and bends, 

cheaper and it’s what it’s meant for. ?‍♂️

I found a heavily insulated 32 mm pipe fitted nicely in a 65mm duct, even cheaper than 100mm. 

 

[Jeremy Harris]

I used 100mm underground ducting, worked fine, even when I dragged a length of 25mm three core SWA through a few metres of it, up through our slab.  Cheap as well, IIRC, as I had to buy loads of it when putting in cables etc around the plot.  The DNO insisted (in writing) that their cable be run in black 100mm duct, then changed their mind when they turned up, and decided they wanted the duct taken out and their cable directly buried (damned good job we hadn't filled the trench when they turned up...).