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."

Quote (T2ioTD)

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

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: https://www.icevirtuallibrary.com/doi/10.1680/geot...

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."

Quote (1503-44)

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

I have seen it done a few times.

Quote (1503-44)

It reeks of false economy.

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.

Quote (JAE)

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

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

Quote (To eliminate potential uplift?)

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.

Quote (EireChch)

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

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."

Quote (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.)

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?

Conservative also means as easy as possible to construct.

Not like this!





How would you design that wind turbine foundation?



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

I think you can caveat every situation to your specific needs and to make your point. But the question was can you consider support from soils and the sort answer is yes. If you decide to use a piled raft then you can consider some support from soil if its appropriate. Its something that does not only work in theory as 1503-44 said, its been proven to work.

But to JAEs point, of course you need appropriate goetech data, that applies to all foundation systems.

Funnily enough Polous is giving a talk today on the very topic!! I will be attending, you can sign up here. https://www.eventbrite.com/e/geotalk-005-piled-raf...

I think I said it works in theory, but perhaps not in practice.

If he gives any examples that were actually constructed, it would be interesting to hear the details. I still don't understand why anyone would do that. I mean, how much money are we talking about, 5 or 10% of bottom bar cost? What's your saving % estimate? Im still still smelling high risk and false economy here.

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

To reiterate, "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

EireChch - How was the presentation? Did you gain any insights or have any newer references to share?

Dik - I agree with not skimping on costs. Still, unfortunately, we have to have these conversations because some engineers and contractors want (or will want) to implement this approach to save some time and money (in theory) and look like a hero. Many have lost the battle against ground improvement via rigid inclusions and aggregate piers, which seem to have problems on some jobs, more than pile foundations in my experience, but then perform fine on others and end up saving the owner big money. These newer technologies seem to work themselves out over time through lawsuits, refining the design of approach in the industry, and sometimes regulation/codes (which there is little of for pile-enhanced rafts, rigid inclusions, and aggregate piers). It just seems like we could be in the infancy stage, even if the technology is decades old.

I'm glad we're having this discussion.

MTNClimber - Could you elaborate on issues you've experienced with aggregate piers or rigid inclusions? Or link to such info? We've recommended them on several projects in recent years and other than equipment breakdowns and occasional mis-location haven't experienced any problems that I'm aware of.

I'd like to know if we should be anticipating problems or considering other factors. Thanks.

Jrit - I've heard many nightmare stories from ground improvement contractors talking crap against their competitors. "Did you hear they installed rigid inclusions in (insert city) and the building settled 8 inches?" type of stories. No real specifics, but they all say it about each other. One of the more public ones is the 295 Retaining Wall Failure in Bellmawr, New Jersey. I don't love RI's and AP's but have accepted them as good cost savings on specific projects. That doesn't mean I don't lose sleep over them.

EDIT: I couldn't find the report, but this website summarizes the report in some detail.

MTNClimber - Thanks for the anecdotes and link. Interesting read. I hear the same types of things from contractors. "Can't believe they used that system...."

Quote (1503-44)

If he gives any examples that were actually constructed, it would be interesting to hear the details.

These 40 and 50 storey towers on the Gold Coast (Australia) were constructed with a piled raft. The geotechnical engineers presented at a conference about it.

https://australiangeomechanics.org/papers/design-o...

I understand the savings on piles amounted to millions of dollars.

Millennium towers was also based on geotechnical information...

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

-Dik

The Gold Coast towers were done by geotechnical engineers who actually knew what they were doing (Golder).

https://www.youtube.com/watch?v=vxdROvBFXk8

MTN - the talk was good, but I did drift in and out as I was trying to get something out!

But yeah, there was an example of saving of 3.3million dollars for project in Gold Coast so could be the ones mentioned above.

I would think that these loads would be quite high and the soil was relatively competent by itself, in which case I could see potential there for combining piles to increase capacity further. I don't think combination is feasible when the soil is too thick to drink and too thin to plow. Most of my cases were at refineries next to the Mississippi River and cogen turbine structures adjacent to river mouths and harbors with SB of 800 psi.

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