The saga continues - FOOTINGS & BI

2 Oct 2007
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United Kingdom


Quick recap - BI who is new to my area told me to dig 1 metre deep x .5 metre wide footings.

The ground (reclaimed beach) is totally unsuitable, which has caused no end of problems,
and resulted in a very large and deep whole.

Two days ago, following BI instructions, we were in the middle of joining together
18mm 2400 x 1200 ply shuttering in long runs.

(we were going to use the additional 200mm as a purchase and knock this into the clay)

Out of the blue, and not requested, the building inspector turned up and was
horrified at what he saw.

At this point, I'll just point out, we have spoken to 4 local builders who said the
old BI who has just retired was great, and this new bloke hasn't got a clue regarding
footings and the local land.

The builders and local farmers said every property around here is built on "Rafter" footings.

Once the BI had got over the shock he turned around and said you should have built
on rafter footings, totally contradicting what he had said around 6 weeks ago.

We were dumfounded.

He went on to say that we need a structural engineer to visit the site immediately;

1) Because we had removed around 6" deep of clay from under the property's original
footings - extending by 24" (on a corner). Not 24" UNDER the property!

2) Because you need plans for a raft.

It would seem from other local builders that the BI has been on the job a couple of months,
and has actually now seen other local footings, and now realises he gave us
completely the wrong advice.

He turned around and said, if you use a structural engineer that I know (Mr XXX), I will
agree to anything he puts forward, anyone else, and I will scrutinise the plans to
the nth degree (or words to that effect).

So I phoned Mr XXX who came yesterday.

We asked him two questions;

1) Do you know of any other residential buildings in this area that have 1 metre
deep footings - answer NO.

2) If the original 6 metres of footings had been left in place - and a RSJ had been placed
on piers, would this have been structurally correct - taking into account the original footings
were tied into the rest of the footings - answer YES.

Mr XXX will have details in writing back to us by next Tuesday.

Meanwhile, he has said we can do what he has recommended before we receive it in writing.

With all that was going on, we still had to drag out of him what we actually needed to be
done BEFORE we receive his plan.

So that we get this correct, can I pick your brains?


The original footings are raft - they are only 7" deep.

The original property was a bungalow (quite long)- but around 20 years ago it was converted
to a chalet bungalow, the upstairs was converted to full living space
(living room/bedroom/shower & toilet) - it seems
all of this additional weight was still on 7" raft footings.

A whole new roof structure was fitted.

The depth of the hole where the new footings are to be laid is 1 metre and
around 2 metres wide.

The structural engineer has said to dig the 1 metre out a bit more (didn't say how much)
and flood the whole area with a lean mix of concrete (he said one in ten).

He said that will deal with the water table.

On top of the lean mix he wants shuttering - 1200mm wide x 228mm (9 inches) high - to accept
concrete with reinforced mesh (rebar?).

He wants the concrete/mesh to sit slightly "under" the original footings that we have exposed.

From that point we are to build up with brick & block.

The brick/block work will sit in the middle of the 1200mm.


1) When he says ONE IN TEN lean mix - is that one of concrete and ten of sand?

2) Should we add aggregate to the mix?

3) Why so lean - does this prevent cracking/

4) When we place the reinforced mesh in - should we suspended/position it about 114mm - so that
concrete flows all around.

5) Do we still use the mix originally specified ST1 - OPC / 20MM aggregate - for the mesh?

6) When he says the raft "just under the original footings" does he mean so the concrete/mesh is
actually forced against the old footings to support them OR in case the old footings start to sag, and
the raft is to stop the "sag" any further?


Sorry this has been so long winded - but we need to get it right while we have the weather on our side.

Thanks in advance.

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All the above should be answered by your SE.. if he hasn't done details for what he wants ask for them..
He said we could continue because of all the mess ups with the initial lean mix before he gets the plans to us - which is next Tuesday..

Any pointers?
We use 20/n as a lean mix in mud mats and 35/n for reinforcing.
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Ok well to help with your further questions:
1/ Lean mix go for an ST1
2/ Just get it ready mix and dont site mix it if you are unsure what you are doing
3/ Nope, it cracks to hell.. its a cheap backfill material that will be stable and will gain strength over time. Used to replace pour quality soil that may move.
4/ Mesh should go at least 40mm down from the top surface. Use chairs to support it. If you have any possible soft spots under the slab then also put mesh in the bottom (75mm cover).
5/ Definitely not! Use a RC28/35 concrete if you are putting reinforcement in it.
6/ Again this really should be answered by your designer, but at a guess he means to underpin your existing footings which is more to stop differential settlement between the two structures and make them act together.. both go up or down at the same rate..
Could sit looking at this screen and answer all your questions, how ever without seeing the existing structure, size and design of new structure, and with respect the state of the site after you have been playing around in there, soil classification and plasticity, and being able to rub a sample of soil through our hands then our answers could be completely wrong.
The only questions we can answer are, from your description, BI is a d*ck head and MrXXX is not far behind. Would suggest, if you are paying MrXXX, do nothing until you receive his drawings, specification and notes. As said if you are paying, that is the bundle Mrxxx should provide in return for his fee. You should be in a position, that you do not have to ask anybody any further questions.
Regards lean mix. It is a low strength concrete to make up levels or soft spots that have been excavated. It can be 1 to 10, but to achieve 7N after 28 days it should be 1-3-6 or 1 to 9 all in ballast. ST1, Gen 0 or C7. Disagree with Static, never known lean mix to crack. Agree with static always use a concrete with a RC preface when working with rebar.
Tip Zippy. When we go into a new area to work and do not know BC, we always have a little note wrote up ready for inspection. Example. Date, foundations inspected and approved. Then ask BI to sign it for your personal records. If he gets stropy and says that he does all that back at his office. Politely, explain that you consider you have made a reasonable and legitimate request, and how can he confidently state that he may not have a heart attack on way back to office. We even do it with a verbal instruction issued on site by BI. If you have every thing in writing, then you can not go wrong.
Curious to know what way MrXXX goes on this one.
oldun :D

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