raft foundation on clay ground spongy!!.. please help!

[tiredofdigging]

Hi everybody...

I am in a bit of a pickle, i am building an 8m x 3m extension on clay ground and have reached sponge! The kids love it but im pulling my hair out to find an answer.

I have dug through 1ft of topsoil, 1ft of rubble (newbuild house!!!) and about a ft of clay. The problem i have is this:

My architect placed a raft foundation on the building regs which got approved, his depth was approx. 750 below ground level and footing width is 700. I have dug down to this (am cream crackered!) and have mostly clay. 50 percent of the foundation base is pretty solid but the other 50% is spongy in areas.Im a little concerned that if i continue to put in steels and concrete the thing i will have severe problems later.
My current footing level that i dug to is exactly the same as the existing house foundations.

The internal foundation base is fine (architect put in an internal wall foundation with steelwork even though it carries no load bearing walls), Theres also a 50mm flexcell going in the internal foundation wall

The eternal front base is solid

The rear is like a sponge cake

the outer length of the house is spongy in certain areas.

We have greenbelt land to the side of us (within 10metres) which gets very waterlogged (im told theres a brook running through it, never found it though!!!) and no major trees near the proposed extension (as i ripped all the conifers out that posed any threats as instructed by the BCO)

Can someone tell me where i go from here?.. do i add a lean mix in the base to help combat the sponge?


Any advice is welcome. am pretty desperate!

 

[Static]

ok,

Firstly, you say a raft foundation? then go to discribe a traditional strip footing foundation, which do you have. A raft footing will be a concrete floor with edge thickening to support external walls. A strip footing will be strips of concrete to support the walls and then floor is built separately.

Secondly, you mention steels and concrete. Are you refering to reinforcement or RSJ's etc? If so what size are these and where have they been specified?

Thirdly, internal walls still carry a fair load assuming the wall is blockwork (a 3m long block wall weighs just under 2 tonnes)

Fourthly, did the architect get an engineer to deisgn these footings?

 

[tiredofdigging]

Ok, thanks for the rapid response!

I took a couple of pictures which may help....

Foundation details as per building regs plan.

Foundations dug ready for steelwork R10 Cage @ 300 centres with T16 through.

There are no load bearing walls inside the extension
Both floors are both open plan. The mass of soil in the centre still needs taking down a little.

all the plans / regs are approved and the footings at this level were checked and approved by the BCO but until we had a serious amount of rainfall the footings were pretty solid and i am now a little concerned. As my BCO is on Holiday i am pretty desperate for any advice.....

 

[Static]

One last question, you say the worst soil is crap, how bad is it. ie when you pick it up does it run through your fingers like slime, or can you mould it like putty, or is it tough to mould etc.

On the assumption the crapy soil is ok then the rest looks pretty much fine, i would go for at least a 200 deep slab thickness between the reinforced edge beams mainly to give you some room to put the reinforcement in. Also make sure the B385 mesh is the right way round (closer centred bars running shorter span).

 

[tiredofdigging]

Thanks for your advice static

The problem with the soil is that if you stamp on it, it sinks about 3 inches or so and the areas around it raise up. Its just like sponge! The soil is a mix of clay an soil so you can mould it but its got soil in it.

I've taken another look as the suns been on it all day but theres no real change.

So let me clarify what youe saying here to make sure i am doing this right.... This is what i was going to do...

I put my reinforced steel R10 cage and t16 bars in raised 200mm off the foundation floor. Get the steelwork approved by the BCO. Pour in the concrete up to a suitable level where the brickie can start bricking up to the damp course. (the steels will be half in the concrete and half out at this point). Pour the remaining infill to suitable level (and include all the B385, flexcell etc as per the foundation drawing.)

Oh, cheers for the tip on the B385 direction!... Its a good job you said!! just one question: (closer centred bars running parallel with the shorter span or parallel with the 8m length?)

 

[Static]

Pleasure to help,
Your best option is to dig out the areas of soft material down to something substantial and infill with lean mix to level required. This is assuming its a 2 storey extension if its just a 1 storey job then dont worry.
The rest of your plan seems fine, the b385 mesh will have the 100mm centred bars spanning the 3m and the 200mm bars running the 8m length.

 

[Roofer]

I know it's at the other end of the building from what I'm most used to but I have had some experience of footings and I've never seen any designed like that.

They don't seem to be either one thing or the other, but more strip than raft.

Has the designer really intended that the concrete is half poured, allowed to set to enable external leaf brickwork to be laid up to damp course and then more concrete poured? Doesn't that weaken the whole foundation? Would the brickwork even resist the push of the concrete? How on earth are you going to get the 200mm of compacted fill in the middle?

 

[tiredofdigging]

To be honest, im not sure on how i get the 200mm compacted fill in.

When the BCO came to see the footing depth, He asked "was the footing base fairly solid", I replied with a yes (i never actually noticed the sponge effect on the back of the hole until the massive amount of rain came! but the front and sides were fairly solid.

His words were, it looks fine... get the steelwork in and give me a call when its in.! No mention of lean mix, granular fill in the centre or anything.

Any advice on getting the 200mm granular fill in there (and having it compacted) would be good. Maybe i should take a different approach to making the foundations.. A good step by step would help too if anyone has any other ideas

Many thanks

Col

 

[hammerboy86]

Put some sterling board or plywood in the sides of your trench, then infill the area with type1, compact it down in 3" layers, carefully remove the boarding/shuttering, a little drop will probably fall back in, but as long as nobody jumps about on it etc should be ok until you pour concrete. I've done it this way a few times

 

[JohnD]

(i never actually noticed the sponge effect on the back of the hole until the massive amount of rain came!

What may also have happened, is that the subsoil will have been squashed firm and fairly dry until you exposed it and water lay in the bottom and turned it to mud.

 

[Static]

What may also have happened, is that the subsoil will have been squashed firm and fairly dry until you exposed it and water lay in the bottom and turned it to mud.

Also you said you removed some trees, this will cause the soil to become more saturated than normal.

 

[Mick Leek]

In my experiance there are 2 reasons for using a raft foundation instead of strip,

1, land has previously been disturbed, could be old delph or simular where the level has been risen, hence no solid sub base.

2, simular to above but additional risk of Methane gas. Have you been asked for any special gas membrane to be used ? If not then its probably down to the area suffering from poor sub base

I helped to lay a raft last year. we used 60m3 of concrete (was a bigun) and i have to agree with other coments about the way you are doing it.

You need to lower the centre and plan to pour the full raft in one go.

Whats it going to be ? looks pretty small. Is it a garage ?

 

[tiredofdigging]

Hi,

thanks for all your help here, its much appreciated.

I finally got going on it and have put in the steelwork in the foundations and am waiting for the BCO to visit this week.

The sponge area at the back was perfectly fine after i dug down a further 3ft and infilled with a lean mix.

The BCO took a look and said its was fine, he also told me to pour the 1st lot of concrete to the desired level for the outer brickwork skin from poured concrete level to dpc, then shutter off the inside of the base and infill with 100mm hardcore and remove the shuttering afterwards then pour concrete to desired ground level.

I spoke to quite a few builders too who all confirmed that its one way of doing it although many said they would not have done it like this.

I am still not sure about whether or not two pours instead of one will affect the strength of the overall foundation but if the BCO says its fine should i argue?

The only thingi forgot to do was dig down the middle to a suitable level (which in hindsight i should have done first) Still you live and learn!

I have another pressing question: On the drawings earlier in the post, I have a 25-50mm flexcell along the internal wall. After calling most builders merchants in the yellow pages none of them were sure what i was talking about and that the only thing they had available was an expansion board which was only 12mm x 100mm x 2.4m long. Nothing like what the drawing has. Can anyone help clarify what i need here?

 

[Static]

Dont worry about the 2 concrete pours as the reinforcement will hold them together.

 

[rangersman]

Dont worry about the 2 concrete pours as the reinforcement will hold them together.

Just don't float the area where the two concrete surface are to come into contact - leave the face as rough and "jaggy" as possible to ensure a good construction joint when the second lift is placed.

Also, damp the surface prior to the second pour - helps avoid shrinkage at the joint.