Negative comments for approved practice design of a raft foundation

[mar2805]

Hi people.
Plase look at the sketch attached.
Short description:
Virgin ground under the future raft slab is removed (aboth 20cm).
Beam trenches are dig out.
Beams are poured first and serve later as the "side formwork" for good hardcore compaction.
Virgin ground (slab part) is compacted and then filled with harcore compacted filling (fractions raging from 1-32mm).
An "mud slab" is poured (above the harcore fill) wich then serves as the leveling plane for the proper reinforecement placement.
Beams are poured first and later serev as the "side formwork" for good hardcore compaction.
Hidroisolation is placed on the raft surface as the first layer of the residential floor.

note: virgin ground level (left in green) and the down side of the RC raft slab are at the same elevation.

Im interested in your negative comments for this design.
Thank you.

 

[civeng80]

Why would you pour the beams first ?
Just form the slab and pour the beams and slab at the same time.

 

[mar2805]

I forgot to attach the reinforcement arangement.
I blue dotet line you can see the pouring break. (attached raft2)

I agree with you that the monolitic cast would probably result in a stiffer construction but I think that could be neglected due to provided reinforcement and links in the beams wich connect the two pour stages.
Advantage of beams beeing poured first that they can serve later as the "side formwork" for good hardcore compaction. You basicly made an "ground pool" with fixed edges wich wont allow any side movement of the fill during the compaction process.

If using one pour I could do it with forming an "skewd" compacted hardcore face (attached raft3)wich will also in return create an thicker "slab-beam" connection but I question the compaction at that face.

 

[mar2805]

Forgot the raft3 picture

 

[hokie66]

I would prefer the monolithic casting. I don't see the compaction of the granular layer as a difficult operation. I would do it all before beginning the beam excavations.

You called this a "raft foundation". By that, I assume you mean that there are a series of internal beams in both directions.

 

[mar2805]

"I would do it all before beginning the beam excavations"

Removing the virgn soil, compacting its exposed surface then filling the "hole" with granular material compacting it and then, excavating for ground beams would defineltly couse disturbance around the excavation trenchs and could couse corners sides caving in (depending on the soil properties).
Youve done projects like this?

"I assume you mean that there are a series of internal beams in both directions"

Theres the "all around" perimeter beam.
Internals only provided if the "bay ratio" is more then lx/ly > 2.
I want to have the feel of two way slab behaviour when using downstand beams with a thiner slab.

 

[hokie66]

1. Yes, I have done it that way, without undue difficulty. Detailed excavation is always messy at first, until the hand shovel guys do their thing.

2. Your terminology is strange to me. What you have is a slab on grade with perimeter footings (or beams). A raft foundation consists of a grid of beams or slab thickenings, and is typically used where there is a degree of volume change sensitivity to moisture.

 

[mar2805]

1.) Ill remain skeptical due to later excavations and possible ground distubance

2.) Why do you call this an slab on grade?
Raft foundation can be a solid plate element without any thickenings, or with them, can have downstand beams...there are various solutions. Typicaly used where the ground conditions are such that differential settlements are to be expected wich can couse structural damage to the superstructure. Like you said, swelling clays are also a bit concern.

What are you thought on the hydroinsulation position?

 

[hokie66]

I'm not familiar with the terms 'hydroisolation' or 'hydroinsulation', so I guess I can't help there.

 

[mar2805]

Hydro insulation = waterproofiing

 

[hokie66]

In that case, waterproofing goes on the side where the water comes from.

 

[mar2805]

Why do you think that the positions of the hydro insulation drawn in the previous attached sketch is completly wrong and it doesnt serve its purpose?

 

[hokie66]

What material is used for the "hydro insulation"? I thought the top surface of your slab would be the wearing surface, or at least a substrate for a finish of some sort. If you are trying to stop moisture coming from below, I don't see why you would want it trapped between the slab and an impervious layer.

 

[mar2805]

hokiee 66 sorry for the delay.
To answer your questions.
Please look at the picture attached wich shows the arangement of the residential floor layers.
This is something thats been bothering me for a while.
As said, this is a system that been used for over 30 years as the standard in insulating residential groundfloors resting on soil.
Material used for the water insulation is bitumenius paper wich is placed and fixed directly ON the RC slab.

Like you said water and moisture will come under the slab.
Placing the water insulation ON the slab will not prevent the slab of beeing exposed to water coming from the ground.
The PE folie (vapor barrier) is placed on the warmer side of the floor layer wich will prevent the moisture of condenstaing in the thermoinsulation.
But this is also contradicting the capillary action coming from under the slab becasue if the ground moisture penetrates the slab it wont have any problem wetting the thermoinsulation.

Maybe casting the RC slab directly on the vapor barrier...

 

[hokie66]

Maybe it has been used for 30 years somewhere, but I have not seen it done that way.

 

[mar2805]

Agree on that.
So, how did you see it?

 

[mar2805]

hokie66,
Im still interested to hear your thoughts

 

[hokie66]

I thought I had made myself clear...any material intended to prevent water or moisture penetration of a concrete slab must be placed on the side where the moisture exists. So for a slab on grade underneath, for a roof slab on top.

 

[AnaAnt]

..and the usual practice in USA is to place a vapor barrier directly under the slab, maybe also place an layer of granular material...

 

[mar2805]

Hi!
I read aboth these stuff a lot.
The placement of the vapor barrier seems to be crucial.