My conservatory build starts today, and as we live in a boggy area, I
am having a "raft" built.

I know NOTHING about this kind of construction, so wonder if anyone
can tell me the key points of what I should be looking for, to be sure
that its done properly.

The builder mentioned something about the fact that it should be 750mm
deep, but as its only to support a dwarf wall and conservatory, he
only goes down to 500mm.

I have always been nervious about this, as, when I was house hunting,
I looked at another house on my estate where the conservatory was
clearly sinking (There was a 2" gap at the top of the dwarf wall). It
turns out that it was only built on standard 'footings' which wern't
up to the job considering the type of land we live on.

After seeing this, I am paranoid that the same thing could happen to
me. What should I be inspecting, just to make sure that its up to the
job?

Any info would be apprecaited

Jon

a raft is generally a concrete slab of say 150mm thick maybe more maybe
less with thickened edges where the wall goes of say an extra 300 - 500mm.
It is all cast as one and could have some reinforcing in it. It is the
foundations and oversite all cast at the same time.

Give you local BCO a call, cost pratically nothing to speak to him, local
call, and could save a packet if you have to underpin further down the line.

Cheers
Jonathan

From the chaotic regions of the Cryptosphere,
(Jon Weaver) wrote on 7 Jul 2003 07:50:08
-0700:

My conservatory build starts today, and as we live in a boggy area, I
am having a "raft" built.

The builder mentioned something about the fact that it should be 750mm
deep, but as its only to support a dwarf wall and conservatory, he
only goes down to 500mm.

After seeing this, I am paranoid that the same thing could happen to
me. What should I be inspecting, just to make sure that its up to the
job?

A true raft foundation will look something like:

| | | |
| | | |-------------Cavity wall
| | | |
| | |__|______________
___|__|_| --Raft
| ___________
| /
|_____________/

-with the toe and the slab linked together by substantial amounts of
reinforcement (I would expect at least two layers of mesh of about
8-10mm diameter bars at 200mm centres, linked to additional mesh in
the toe), designed by a structural engineer. This is designed to
spread the load of the outer walls across the whole area. The normal
depth of a raft would be about 300mm deep at the toe and 150mm in the
centre, with at least a 150mm thick base of well compacted hardcore
below. Check with the builders that they have these details on site,
and be very suspicious if they don't.

A false raft is simply the footings and the slab poured in one go.
It's no better than normal strip footings, and in all likelihood
worse, as the foundation depth is usually not enough, and there is not
usually the opportunity to properly compact the edges of the hardcore
under the slab. Don't accept this if the ground is anything less than
a firm clay, sand, gravel, etc., and even if it is, get the builders
to construct separate foundations and slab.
--
Hugo Nebula
"You know, I'd rather see this on TV,
Tones it down" - Laurie Anderson

a raft is generally a concrete slab of say 150mm thick maybe more maybe
less with thickened edges where the wall goes of say an extra 300 - 500mm.
It is all cast as one and could have some reinforcing in it. It is the
foundations and oversite all cast at the same time.

That certainly fits the description of what he is doing. He said that
he is doing 500mm at the edges (under the brickwork) and a thinner
slab (Almost certainly more than 150mm) all over.

Also, he is putting in 4 layers of steel re-enforcement too, so it
certainly seems that he is doing roughly the right thing.

Thanks for that.

Jon

Give you local BCO a call, cost pratically nothing to speak to him, local
call, and could save a packet if you have to underpin further down the line.

Cheers
Jonathan

Excuse my ignorance, but whats a BCO? My best guess is "Buildings
Control Officer". If I am right, where would I find a BCO? Are they
linked to the council?

Jon Weaver wrote:
Give you local BCO a call, cost pratically nothing to speak to him,
local call, and could save a packet if you have to underpin further
down the line.

Cheers
Jonathan

Excuse my ignorance, but whats a BCO? My best guess is "Buildings
Control Officer". If I am right, where would I find a BCO? Are they
linked to the council?


Yep, when you build a house or an exstension he is the bloke that comes and
make everyone nervouse. He will usaully visit first to check the
foundations, then to check the damp course and he will also check that the
drains are set right and don't leak. It is his job to make sure that
Building Regulations are adhered to. He works for the council, mine is
actually quite a nice bloke and responds to my emails when I send them.

Cheers
Jonathan

A true raft foundation will look something like:

| | | |
| | | |-------------Cavity wall
| | | |
| | |__|______________
___|__|_| --Raft
| ___________
| /
|_____________/

-with the toe and the slab linked together by substantial amounts of
reinforcement (I would expect at least two layers of mesh of about
8-10mm diameter bars at 200mm centres, linked to additional mesh in
the toe), designed by a structural engineer. This is designed to
spread the load of the outer walls across the whole area. The normal
depth of a raft would be about 300mm deep at the toe and 150mm in the
centre, with at least a 150mm thick base of well compacted hardcore
below. Check with the builders that they have these details on site,
and be very suspicious if they don't.

A false raft is simply the footings and the slab poured in one go.
It's no better than normal strip footings, and in all likelihood
worse, as the foundation depth is usually not enough, and there is not
usually the opportunity to properly compact the edges of the hardcore
under the slab. Don't accept this if the ground is anything less than
a firm clay, sand, gravel, etc., and even if it is, get the builders
to construct separate foundations and slab.

Can I just ask about the 'hardcore'..

Would I expect hardcore under the toe as well as the slab?

Currently the trech has been dug out and they have started
constructing the metal work in the toe. The dimensions of the metal
work is 400mm(h)x400mm(d) and the current trench approx 450mm deep.
However, the metal work is placed right on the soil below as no
hardcore has been put in.

| _______ _______ |
|| | __________________________ | ||
|| Steel | / \ | Steel ||
|| Cage | / \ | Cage ||
||_______|/ \|_______||



I spoke to a friend who is a "quantity surveyer" (And therefore not
really qualified) and he didn't really feel that this was a problem,
as he believed that the hardcore would normally go in under the slab
rather than the toe

There are 5 tonnes of hardcore on my drive and 6 sheets of steel-grid,
so its clear that they have plenty more to do and there is no evidence
that they are doing this wrong... Should I be insisting that they pack
put hardcore under the toe or will hard-core under the slab be good
enough?

Also, does 450mm seem deep enough for this kind of application?

On 10 Jul 2003 15:09:22 -0700, (Jon Weaver)
wrote:

Currently the trech has been dug out and they have started
constructing the metal work in the toe. The dimensions of the metal
work is 400mm(h)x400mm(d) and the current trench approx 450mm deep.
However, the metal work is placed right on the soil below as no
hardcore has been put in.

When you say right on the soil, do you mean that, or are there
spacers? There should be spacers so when the concrete is poured there
is 50mm of concrete under the reinforcement.
Have never seen hardcore under the foundations. (An a QS though, so
not really qualified :-) )

--
John

BigWallop wrote:

"John Armstrong" wrote in message
...

On 10 Jul 2003 15:09:22 -0700, (Jon Weaver)
wrote:


Currently the trech has been dug out and they have started
constructing the metal work in the toe. The dimensions of the metal
work is 400mm(h)x400mm(d) and the current trench approx 450mm deep.
However, the metal work is placed right on the soil below as no
hardcore has been put in.


When you say right on the soil, do you mean that, or are there
spacers? There should be spacers so when the concrete is poured there
is 50mm of concrete under the reinforcement.
Have never seen hardcore under the foundations. (An a QS though, so
not really qualified :-) )

--
John


Hard core will be used if the ground under the raft is boggy or susceptible
to flooding, and allows water to wash through under the unit without washing
away any of the soil. The raft will then sit on top of the hard core and
not be bonded to it and this will allow the raft to drain surface water
through to the hard core and back in to surrounding ground. A sort of
sub-terrainian colonial drainage system you might say.


---
BigWallop

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Yes. Hardcore isn't just any old muck. It has a definite mechanical
property - two in fact - derving from the trapped air - ability to
drain, and ability to crush ligthly under stress. Limestone is better here.

This menas it bothg stops ptential subsidence from water, and allows a
certain amount of heave to take place...

The oher property is of load spreading. Here the deeper the better.

"Jon Weaver" wrote in message
om...
John Armstrong wrote in message
. ..
On 10 Jul 2003 15:09:22 -0700, (Jon Weaver)
wrote:

Currently the trech has been dug out and they have started
constructing the metal work in the toe. The dimensions of the metal
work is 400mm(h)x400mm(d) and the current trench approx 450mm deep.
However, the metal work is placed right on the soil below as no
hardcore has been put in.

When you say right on the soil, do you mean that, or are there
spacers? There should be spacers so when the concrete is poured there
is 50mm of concrete under the reinforcement.
Have never seen hardcore under the foundations. (An a QS though, so
not really qualified :-) )

THe other thing that I ment to say is that no 'hardcore' as such has
been delivered.. What i have is 4 bags (tons) of a material which
ranges from 'dust' to approx 1" stones.. If this is what is going to
be 'packed' into the base, is it the right stuff?



I'd say your builder knows what he's doing. The hard-core is there to stop
water building up against the underside of the raft, that's all, and allow
any water from surrounding areas to flush under the new raft. If you used
larger stones, then the raft would move about on the top of them. If you
used dust, then it would wash away with the rest of the soil. So you have
to use small stones to keep them from away, but to allow the soil to vent
water through them and down and away from the raft.

It works like a sand filter (do a web search to find one).


---
BigWallop

http://basecuritysystems.no-ip.com

Outgoing mail is certified Virus Free.
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"Jon Weaver" wrote in message
om...
John Armstrong wrote in message
. ..
On 10 Jul 2003 15:09:22 -0700, (Jon Weaver)
wrote:

Currently the trech has been dug out and they have started
constructing the metal work in the toe. The dimensions of the metal
work is 400mm(h)x400mm(d) and the current trench approx 450mm deep.
However, the metal work is placed right on the soil below as no
hardcore has been put in.

When you say right on the soil, do you mean that, or are there
spacers? There should be spacers so when the concrete is poured there
is 50mm of concrete under the reinforcement.
Have never seen hardcore under the foundations. (An a QS though, so
not really qualified :-) )

THe other thing that I ment to say is that no 'hardcore' as such has
been delivered.. What i have is 4 bags (tons) of a material which
ranges from 'dust' to approx 1" stones.. If this is what is going to
be 'packed' into the base, is it the right stuff?



http://fbe.uwe.ac.uk/public/geocal/f...s/Fountype.htm


---
BigWallop

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(Jon Weaver) wrote in message . com...

Since you obviously know absolutely nothing about what you have
contracted for you had best steer clear until it is all over. In case
you do not know it: if you wind people up, they are liable to become
violent.

For your next project you might care to bear in mind the following:

1. It takes from 3 to 9 hours for a usenet post to appear. Another
three to nine for a reply and the time up to reading the post.

2. The contractor has set aside both his time and his money to get
your job done. You might at least have had the courtesy to ask around
and see if he is any good.

3. Our replies are not chapter and verse. Take careful note that the
poster who wants all that hardcore has set aside a lot of time to draw
pictures of a raft. He has no knowledge of your site. You could have
found out that much from your local library or looked up previously
posted threads. As far as I know he is correct though no-one has
mentioned the need to drop to 18 inches for the foundation.

4. Thomas Telford ran a rail line over boggy land using carpets of
willow faggots. (Bundes of withies overlapping each other.) You could
have done the same. (Just thought I'd rub salt in. There are a
suprisingly large number of people still around, who take pride in
their work and their ability. Let us don't hinder!)

me. What should I be inspecting, just to make sure that its up
to the
job?

Nothing.

A badly DESIGNED but wonderfully constructed raft will slowly
sink or drift off down the garden.

A well designed one, which will incorporate a certain margin of
error in the calcs, will probably stand some slightly off
construction under a conservatory which is a lightweight thing.

You aren't in a position to check the cals, so... Close the
curtains. Pour yourself a drink. Relax. Stay away until it is
done.

W.