When designing a raft foundation on piles, does the soil under the raft contribute? 6

If the soil is expansive, it might contribute to uplift, otherwise no. That's having your cake and eating it too. Rafts are usually designed as rigid as possible to span across the piles. In order for the soil to take load, the cap or raft would have to deflect in order to load the soil, so you would then have to decide how weak to make the raft so it would deflect just enough to load the soil to the "right amount". If you did that, then you have probably weakened the raft so much that it would no longer provide adequate stiffness to span across the piles. Is it really worth weakening the raft to pick up a tiny bit of bearing capacity on what is probably poor unreliable soil that might easily consolidate under the raft load in a year or two and completely lose all of its bearing capacity? I think not. Plus you would have to explain why you did that to everybody concerned and answer all their questions as they look at you with frowning "I don't think I agree" eyes.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

T2ioTD said:

When designing a raft foundation on piles, does the soil under the raft contribute to the load bearing capacity?

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Yes it does. But you cannot just assume a value of how much load the piles take compared to the raft. You have to run the numbers based on the ground conditions, pile diameters and spacing, raft thickness etc and iterate to get a result that works and is economical.

Prof Harry Poulos has done a lot of research in this area:

Theoretically.
Never did it, nor have I ever seen it done.
It reeks of false economy.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

1503-44 said:

Never did it, nor have I ever seen it done.

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I have seen it done a few times.

1503-44 said:

It reeks of false economy.

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Generally that depends on the ground conditions. Seems to work best when the raft is supported on dense sands.

I often use voidform beneath rafts on piles...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

To eliminate potential uplift?

Might be possible with lightly loaded stuff. My opinion is probably skewed. Other than pipe racks and the odd operator shack, I've mostly done very heavy refinery vessel foundations. Much better results with lots of piles and very thick and rigid mats and caps. Settlement is not well tolerated. Anybody even thinking of using soil bearing with piles would probably get run off in a heartbeat. And 800psf won't do much for a 500 ton vessel in the wind anyway.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

Despite Mr. (Dr.?) Poulos' study I would never do this on my own without some significant input from a qualified geotechnical engineer.

The problem of combining soil support under a raft/mat with some "strategically" placed piling is that you have to have a fairly high understanding of the relative stiffnesses of the soil vs. the piling.

If you are off on the stiffness numbers you risk having uneven settlements between high-piled areas of the raft and low-piled areas.
This could cause shears and moments within the raft that you didn't design for.

Just seems like a lot of work, money and risk to save a few piling.

JAE said:

I would never do this on my own without some significant input from a qualified geotechnical engineer.

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Absolutely agree - generally some Plaxis finite element analysis is required.

We have a highly plastic soil, and for exterior slabs we have frost heave...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

I was at a talk from Polous where he specifically said that not taking account from support from soil is overly conservative.

But in saying that I worked in a country where the municipality would not allow any contribution from soil in piled rafts for tall towers.

EireChch said:

he specifically said that not taking account from support from soil is overly conservative.

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That's a bit of a blanket statement that seems not to hold water in all cases. As Dik noted, the use of voidform under mat slabs is often used in certain regions of the world to avoid other issues (frost heave, expansive soils, etc.). Then there's the opposite, where you're anticipating consolidation of soils for various reasons, and a void develops below the mat slab.

Although I'm never married to my beliefs, I would need to see substantial evidence that it's worth the engineer's and owner's money (and insurance, headaches, etc.) to install fewer piles and rely on the slab subgrade to partially support the structure's loads before going down that road.

Sorry, I meant in situations where its applicable, of course in a heaving soil then you cant take any support from the soil. My response was to the general comments from 1503-44 and JAE, if tis favorable then you are being overly conservative by not considering any support from soil.

I don't think foundations are the place to 'skimp on costs'. Just a gut feeling...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

Right Dik. Concrete poured on dirt is the cheapest concrete there is.

Well Mr Polo's would probably say that, wouldn't he.

Tall towers have uplift, and need top steel. Soil support is no help. Bottom bars no help.
You need the dirt on top of the mat.

Of course it is Conservative to ignore any potential contribution from poor soil. That's the reason you are using piles. Being Conservative when you have poor soil is the name of the game. Let's play.

You say, "I will convince you that I will still have soil support after 2 years.
This book shows how it will work.

I say, The soil under the mat will tend to get drier and shrink. You will lose its minimal support.
You say, "The book didn't talk about that."

I say how much can you reduce bar size?
You say, I can't, but I can increase bar spacing from 12" to 13".
I say, They will use 12" anyway.

I will say, "How much can you reduce mat thickness"
You might say 2".
I will say, "Pile loads are high, doesn't punching shear govern mat thickness here?"
You will say, "Maybe you are right. I will check".

I will say, "let's get back to how much soil bearing will you have after a long hot, dry summer."





--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

That approach seemed to work for Millennium Towers in San Francisco...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

1503-44 your hypothetical discussion applies to someone like me. I have a little bit of knowledge of piled rafts, from books mainly with a fraction from experience.

But Mr Polous wrote those books, based on experience, having designed, constructed and instrumented and monitored piled rafts. So his experience isnt 'the book says xxxx'. Hes got the real world experience to give us guys the recommendations that we regurgitate as if they were our own!

All this experience has probably given him the title of 'godfather of piled rafts'

I'm sure Dr. Poulos is very smart and knows his stuff. For very large projects, with lots of design budget and time to correctly analyze the soils, a combination of piling and soil support could make sense economically.

For us regular peons, designing small to moderately large projects with limited budgets, I'm not sure how "risking" a combination soil/pile system makes sense, safety-wise or economic.

Now we are getting to the hard questions.

We are the last to get the design info and the first thing the field needs are the foundations.
Got everything except time. Refinery has $50 Million/day in sales revenue.

If the foundation breaks, you've gotta add interest on $5B capital investment plus lost sales revenue for how long to fix it?

This foundation will save me how much? Clock is ticking.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

1503-44

What if you make the foundation design so conservative that the construction period gets extended six months, delaying when the 50 million dollars a day starts coming in?