Raft slab supporting 4 storey building 1

[Contraflexure74]

Hi All,
Just got our SI interpretive report back for one of my projects. Geotech is recommending a raft slab on grade. He has calculated 43mm potential settlement. Loads on columns are 1000kN. I can't understand why he's not recommending piling and avoid any potential settlement. Any comments welcome.

 

[hokie66]

I would agree with you.

 

[jayrod12]

I would put the question back on the geotech.

Normally they're pretty keen guys when it comes to what works on a given site. Maybe the area is full of cobbles and boulders making the installation of piles difficult.

 

[WARose]

The raft will be at grade level? Doesn't that kind of defeat the purpose of a raft foundation? You may want to clarify that/get confirmation.

If the top of foundation does indeed need to be deeper......I'd do a cost comparison: it's been my experience with a (actual) raft foundation that once you get done with the excavation and so forth.....you'd be better off doing piles (as the others have suggested; if possible).

 

[Contraflexure74]

Hi WARose,

Just to clarify the raft is on grade...not at grade.

 

[hokie66]

Not sure I understand either of the last posts. To me "on grade" and "at grade" are essentially the same. And why would a raft slab be illogical on/at grade?
I still think 43 mm settlement is a bit much for a 4 story building. Perhaps the geotech can further expound on this advice.

 

[Contraflexure74]

Just to clarify so guys. On grade to me just means sitting on the ground at finished floor level.

 

[WARose]

[blue](hokie66)[/blue]

And why would a raft slab be illogical on/at grade?

Because that isn't the objective of a true raft foundation. Granted people use the terms raft and mat interchangeably (and that may be the case here)......but a true raft foundation is used where the soil has low allowable bearing pressure (or potential settlement issues) and (as a result) the soil is excavated down to a level where the weight of the structure is balanced by the weight of the soil removed. (I.e. you are not significantly increasing the net bearing pressure.)

Ergo your top of concrete elevation is well below adjacent grade level. You've made yourself a basement. That's why I brought up cost because it gets pretty expensive.

So when he said there were some issues with the soil at the site and he was using a raft.....that is what I envisioned.....but apparently this is not the case. (I.e. it's just a regular mat with the top of concrete near (adjacent) grade.)

 

[hokie66]

A matter of terminology, then. This is what is considered a 'raft' slab where I am. First Google hit.

https://www.cornellengineers.com.au/what-is-a-raft-slab/

 

[WARose]

I've seen it used interchangeably everywhere. Even in texts. Gupta (in his book: 'Raft Foundation Design And Analysis With A Practical Approach', p.6-7) refers to the situation as a "buoyancy raft".

Coduto (in 'Foundation Design Principals and Practices', 2nd edition, p.647) provides this historical perspective:

"The earliest documented floating foundation was for the Albion Mill, which was built on a soft soil in London around 1780. It had about 50 percent flotation (i.e. the excavation reduced Δσ^'_z by 50 percent) and, according to Farley (1827), "the whole building would have floated upon it, as a ship floats in water." In spite of this pioneering effort, floating foundations did not become common until the early twentieth century. Early examples include the Empress Hotel in Victoria, British Columbia, 1912; the Ohio Bell Telephone Company Building in Cleveland, 1925; and the Albany Telephone Building in Albany, New York, 1929."