Ground Beams — Shambles

[marcus]

Hello,

This is a question for any builders on the forum, opinions gratefully received.

I need to have a chat tomorrow with the person who produced this shambles in my garden (sigh) which is supposed to be the foundation for an extension (it is heavy clay soil with trees hence the pile and ground beam construction). It has rained very heavily in last few days which hasn't helped, but still, they have had a LOT of time to get it done before the rain started....

Before our 'chat' it would really help to have an idea about the following:

1. How long would you normally expect two blokes working together to take install the steelwork in the picture (I mean not to do the digging, but just to put in the armature once the digging is done (it is 4m2, give or take).
2. Is it normal to assemble the armature above the hole and then lower it in, as they have been doing?
3. It is normal/sensible to install the formwork after the armature is in, as they are planning to do?
4. When it is finally ready, would it be reasonable for them to mix the concrete for this in a cement mixer on site? That is what is being planned, and to me it seems risky (poor quality control, what if the cement mixer breaks down before it is finished, risk of cold joints, and so on.).

Building control have seen and are insisting on approving it before any concrete is poured, but it would be useful to go into the 'chat' tomorrow with the above information.

Thank you

Marcus.

 

[ChaiLatte]

1. How long would you normally expect two blokes working together to take install the steelwork in the picture (I mean not to do the digging, but just to put in the armature once the digging is done (it is 4m2, give or take).
2. Is it normal to assemble the armature above the hole and then lower it in, as they have been doing?
3. It is normal/sensible to install the formwork after the armature is in, as they are planning to do?
4. When it is finally ready, would it be reasonable for them to mix the concrete for this in a cement mixer on site? That is what is being planned, and to me it seems risky (poor quality control, what if the cement mixer breaks down before it is finished, risk of cold joints, and so on.).

1. Two men should make that in a day, easy. I would call it a 'cage' rather than an armature.
2. Yes. If it is going in a trench, or as your own is, sitting on piles, prefabricating it and dropping into place is far more practical. You would start with the top bars of each beam on two scaffold tubes supported by four total trestles. Thread the links onto the top bars, space them out and tie them. Then insert the bottom bars and tie them. Insert the corner L-bars loosely. Remove scaffold tubes and drop down (making sure proper spacers are tied to bottom so cage is not resting on the plywood forming the base shuttering). Once all four beams are down, pull out the corner L-bars and tie them into their correct place.
3. Can be done either way. Often for ground beams, they use the reinforcement to steady and space the formwork (so the cage should have spacers on the side as well). The through ties for the formwork snug it up against the spacers. When the pressure from the concrete comes on the ties take the pressure. If they do not use through ties, they have to brace the formwork externally. The inside of the square is reasonably easy as you can backfill agianst the shutter or punch one side off the other (but you have to fill it evenly). The outside is more challenging.
4. No. Readymix and a pump will be way faster.

Think carefully about any services (water, data, electricity, drainage) that have to pass from house to garden. The ground beam is a solid barrier to them and a sleeve or two through it for future service provision will pay dividends.

 

[Garden Shed Projects]

Hello,

This is a question for any builders on the forum, opinions gratefully received.

I need to have a chat tomorrow with the person who produced this shambles in my garden (sigh) which is supposed to be the foundation for an extension (it is heavy clay soil with trees hence the pile and ground beam construction). It has rained very heavily in last few days which hasn't helped, but still, they have had a LOT of time to get it done before the rain started....

Before our 'chat' it would really help to have an idea about the following:

1. How long would you normally expect two blokes working together to take install the steelwork in the picture (I mean not to do the digging, but just to put in the armature once the digging is done (it is 4m2, give or take).
2. Is it normal to assemble the armature above the hole and then lower it in, as they have been doing?
3. It is normal/sensible to install the formwork after the armature is in, as they are planning to do?
4. When it is finally ready, would it be reasonable for them to mix the concrete for this in a cement mixer on site? That is what is being planned, and to me it seems risky (poor quality control, what if the cement mixer breaks down before it is finished, risk of cold joints, and so on.).

Building control have seen and are insisting on approving it before any concrete is poured, but it would be useful to go into the 'chat' tomorrow with the above information.

Thank you

Marcus.

View attachment 143280

View attachment 143281

1. I would estimate 3-4 days being a reasonable time frame.
2. Yes. The cages are normally built on flat ground and then lowered into place.
3. I think this is unusual. It would be easier and quicker to make the form and then lower the cages into it and tie in situ at corners and pile heads.
4. I am not sure of dimensions but if I assume the width and depth to be 500x500 and the base 4m x 4m meaning a quarter cube per linear metre x 16m you would only have a part load from a pre mix truck. I would still have expected them to go this route assuming you have decent access to the foundation.

 

[Garden Shed Projects]

Hi Chailatte. If I may comment on your point 3.

It appears from Marcuses original post and the photos that the beans ar none ground bearing, I would assume to combat ground heave because of the trees. In this instance you would not be able to use back fill to form the shutter and would need to form the ground beam as a fully independent self supporting beam.

I may have over estimated the time frame for the works as i didn’t appreciate the hole was already dug.

 

[marcus]

Thanks for your replies, Garden Shed Projects—they are using a deformable polystyrene product on the inside edges of the beams (which can be seen in the picture, much of it now broken) to allow for ground heave (nothing is specified for the outside edge) ...

 

[MARK.B.]

If you have access to get ready mix in then that imho would be the way to go, a single pour all mixed the same would be done quickly and the cost of ready mix should easily be offset by the time saved in labour charges if it were mixed on site in a small mixer.

 

[ChaiLatte]

3. I think this is unusual. It would be easier and quicker to make the form and then lower the cages into it and tie in situ at corners and pile heads.
4. ...you would only have a part load from a pre mix truck.

The difficulty with putting the shutter in place before the cage is it has to be self-supporting both forwards and backwards. It is extremely difficuly and somewhat hazardous to try to tie up the corners with both sides of the shutter in place. At the least, you need one side open (there's a lot less rebar on the sides so you can find a space to poke your arms).

It is an ideal job for a mix-in-place truck (8-wheelers with an auger at the back). Most of the companies round here withthe trucks either own a pump truck themselves or have a siter company that does or partnership with someone who does.

 

[johna.clements]

3. I think this is unusual. It would be easier and quicker to make the form and then lower the cages into it and tie in situ at corners and pile heads.
4. I am not sure of dimensions but if I assume the width and depth to be 500x500 and the base 4m x 4m meaning a quarter cube per linear metre x 16m you would only have a part load from a pre mix truck. I would still have expected them to go this route assuming you have decent access to the foundation.

If the shutter is pre made and you try to lower the cage into it, the cage will not fit or you will have to spend a lot of time getting it in.
The shutter should be able to quickly erected. I assume none of it will be seen so it only needs to be F1 quality (rough)

I doubt that a mixer would be much cheaper by the time you have got the mixer, collected the cement and ballast etc.

 

[Spectric]

First impressions are those builders are not organised, the work area is to cluttered and looks a mess which does question whether they really know what they are doing, in my books a good worker is a clean and tidy worker.

When it is finally ready, would it be reasonable for them to mix the concrete for this in a cement mixer on site? That is what is being planned, and to me it seems risky (poor quality control, what if the cement mixer breaks down before it is finished, risk of cold joints, and so on.).

Mixing it in a mixer is not a problem if the guys are fit and able, but it looks like a small working area with poor access, so getting the ballast to the mixer might be more difficult. You could mix elsewhere near the ballast but then you have the extra weight of wet concrete to move but having several people on the barrows will help but it can be done.

No. Readymix and a pump will be way faster.

You need good access for readymix as the drivers are not known for wanting to hang around whilst you barrow it away, a pump for that small amount may just not be cost effective.

 

[AJB Temple]

If they are proposing to mix on site with a small machine, then they are fleecing you, unless you cant get a mixer close enough. Labour costs will greatly exceed cost of readymix. About an hour to get the concrete in and you need it the correct specified mix.

 

[marcus]

Thanks everyone, here's one more picture of the bottom of the trench. How does it look? The idea I think is that the pink heave protection (Jablite Claymaster) is intended to act as the formwork for the bottom (Jabliite's product page says this is OK and part of the point of the product). I'm not sure what the OSB is doing on top, presumably it has to stay there now as I don't see how they can get it out with the cage in place. The jablite is crrently floating some inches off the bottom due to rain, they say they will be getting a pump....

Does this plan look normal? Is there anything additional I should be aware of about this?

Thank you!

 

[johna.clements]

It certainly does look a bit dubious.

Too much cover off the cage running to the right.
The corner rebar is buried in the mud. No cover should be 40mm or so.
Looks like no spacers under the cages. Should have been fixed before they lifted them in.
On the previous picture one of the piles was
The rebar and shutter needs cleaning before concrete is poured.

I wonder if the rebar was bent for those ground beam or was something that they just had lying around. Does it give the area of steel required by the design. why are the links at strange spacing. The links take shear but are missing over the piles where the biggest shear will be.

 

[Garden Shed Projects]

Unless they are planning to remove this material after the beam has gone off, likely to be weeks. The ground have is powerful when it occurs but will not compress the jablite and will likely crack the beam. The beam needs to totally independent of the ground to allow it to move freely.

I have used this stuff in the past which will collapse when the ground heave occurs. You shouldn’t need much but will incur a cost. Be mindful that we are at the end of a dry summer and heading into the wet weather.

https://cordek.com/market-sectors/ground-heave-solutions

 

[marcus]

I really appreciate people taking time to comment on this, thank you.

I do know that the rebar was ordered and bent specifically for this job.

The Jalite is product called Claymaster, which, according to their website, is designed to deform under heave, and was specified in engineers drawings (150mm under, 50mm to inside face of beams), to be left in place ....

 

[Garden Shed Projects]

I should mention I have done this a few times in the past but on much larger buildings so my opinions may not be relevant in this instance.

I assume that there is an engineers design in place for this which has taken ground heave into account and called for the piles. While the design won’t tell you how to build it it should advice on the materials. Is the clay master shown on the drawing? Can’t imagine the osb is.

 

[marcus]

Thanks Garden Shed Projects, I was just thinking that I will drop the engineer a line today ...

 

[MARK.B.]

No expert in this work by any means but it looks like they have used the osb as a quick and easy way to get things level, i don't think i have ever seen osb or any other timber left under cages like that before(when it rots away you would be left with voids). Can't tell from the pictures but you say there is a fair bit of water underneath , make sure they do pump out as much as possible( a small amount should not cause any major issues) or when the concrete goes in it will dilute the mix and any excess will be forced to the surface where it will unless it can run off, sit on the top and further weaken the mix and prolong drying times.

 

[johna.clements]

No expert in this work by any means but it looks like they have used the osb as a quick and easy way to get things level, i don't think i have ever seen osb or any other timber left under cages like that before(when it rots away you would be left with voids). Can't tell from the pictures but you say there is a fair bit of water underneath , make sure they do pump out as much as possible( a small amount should not cause any major issues) or when the concrete goes in it will dilute the mix and any excess will be forced to the surface where it will unless it can run off, sit on the top and further weaken the mix and prolong drying times.

The OSB is not a big concern. The polystyrene Claymaster product is designed to to compress when the clay expands. When the clay contracts again it is unlikely to fully expand so a void will be left. It is quite common to to bury shutters.

 

[MARK.B.]

Aye i dare say you are right johna ,just not something i have seen in my somewhat limited exposure to building sites

 

[Jameshow]

I thought I was messy!

I'd want the caged lifted off with proper lifting blocks wired in. You don't want water getting into the reinforcing steel.

I'd give them a couple if days to finish and get it inspected ready for the pour.

Most concrete suppliers is mix on the drive rigs round here and barrow it in with a couple of labourers. What you want is a certificate of mix which the driver will give you. Nor do you want a full over a couple if mixes it all needs to be in one pour.