In need of a definition 1

[Zambo]

I am looking for a definition, but need some substantiation (a quote from a text book or other source)

Is a raft foundation a structure supported directly by the underlying soil or can it also include a foundation supported by piles?

Any help appreciated.

 

[BigHarvey]

Soil Mechanics in Engg Practice 2nd Ed by Terzaghi & Peck chapter 9 p 472 in the Wiley edition : " if a single slab covers the supporting stratum beneath the entire area of the superstructure, the foundation is known as a mat or raft ".

A piled raft is a foundation where some the load is distributed between the piles and the soil beneath the raft.

 

[BigH]

Can't get better than T&P - Conduto in his foundation book says a "second type of shallow foundation is a mat foundation, . . . A mat is essentially a very large spread footing htat usually encompasses the entire footprint of the structure. . . . somtimes engineers use ple-or shaft-supported mats, . . . These foundations are often called piled rafts, . . ." (p352-353)

 

[ishvaaag]

Even if what said above stands correct, for what I have read the more common use is raft for a mat without piles, and piled mat when it has.

 

[Zambo]

thank you ishvaaag,

my understanding is as you suggest. In which case if I price for a raft foundation I don't expect to have to excavate around piles, I also don't expect to be told that I have to cut down someone else's precast driven piles included my raft foundation price.

But as mentioned above there is some lack of clarity in the usage of the term "raft" and that with an absence of a method of measurement is enough to cause a confusion. As is fairly common the owner's consultant suggests we just do the work and "sort it out later".

 

[Ron]

Bowles does not differentiate between pile-supported mats and soil supported mats. He call them all "mats".

Sowers doesn't contemplate pile support for mats. He considers them to be soil supported.

Lambe doesn't clearly define a mat, but considers it just a shallow foundation variant.

 

[Zambo]

Ron,

thanks. This lack of clarity means that I am now faced with a difficult situation in Vietnam. The owner's consultant says we should have realised from the borehole logs that piles would be required (we are not responsible for design) and therefore we should have allowed for excavation around the piles and cutting at cut-off level.

We didn't allow for this we just priced an itemised BQ for the works based on 2 tender drawings.

 

[Ron]

Zambo...I assume you are the foundation contractor on the job.

Given that, the approach I would take is that the plans should have shown piles if they were required. To show only the mat foundation and provide boring logs that they expect YOU to interpret and offer further foundation design is absurd. You are the contractor. You are not required to design the project (unless of course you have a design-build contract).

 

[hokie66]

The Bill of Quantities should have included an item for detailed excavation and trimming piles. Sounds like the bill was not sufficently detailed. What does the QS have to say about the situation?

 

[BigH]

Was this not brought up at the tender meeting? Usually they have a clarification meeting . . .

@ishvaaag: seems that this is not the case - most use piled-raft foundation; probably depends on the country (remember our discussions of what is a bored pile or a drilled caisson, etc). See:

http://www.icevirtuallibrary.com/do...est&checksum=303A078900BBEBEAB7505ECB5F31D2CE

http://www.ejge.com/2009/Ppr0906/Ppr0906s.pdf

Varghese (Indian) says, "A raft or mat foundation consists of a large concrete slab or a slab and beam system resting on soil or rock and supporting all the lods through a num ber of columns or walls. When the raft is supported by piles it is called a piled raft.

I've always considered "raft" and "mat" to be interchangably the same.

 

[Zambo]

When we priced we just had a GA which had a simple outline of the foundation with the label "raft foundation". The BQ does not mention piles just excavation, blinding, rebar, formwork, concrete.

There is no QS but the owner's engineer says piles weren't mentioned because they are not in our scope of work. However he is of the opinion that as we would have known there would be piles by looking at the borehole logs. He also states that as it is common practice in Vietnam that the foundation contractor cuts the piles to level then we are deemed to have included this in our price.

This is not a design and build project.

 

[Zambo]

BigH,

at the time of pricing there was no discussion about piles. But some time has elapsed now and in the interim we have become aware that piles were being driven by another contractor. There are 2 issues.

Firstly cutting the piles and taking away the cut off lengths which we originally assumed the piling contractor would do, secondly excavating around the piles which we have only just realised as a valid claim for an increase of rate. The second item is negotiable and depends whether they intend to reduce our excavation quantity due to the volume of the piles.

 

[Zambo]

BigH,

please give the title of the book by Varghese (as a useless bit of info I recall that that is a Christian family name)we do seem to be coming to the conclusion that a "raft" is a raft and a "piled raft" has piles.

 

[hokie66]

The definition of raft and mat must vary with location. In Australia, a mat foundation is invariably for a big building, and can be soil or pile supported. Raft is the term used for small buildings, generally residential or small commercial. Rafts normally have ribs, and are used on moisture sensitive clays, but are not pile supported.

It is quite a stretch to expect a subcontractor to infer that a building is to be pile supported based on boring logs. If you were given a complete geotechnical report which excluded shallow footing systems, the owner's engineer might have an argument.

 

[Ron]

A couple of issues:

1. The piling contractor should cut the tops to the required elevation. They are his piles and you do not want to get into further liability of potentially damaging his work by cutting them yourself. This should not even be an issue on your part...not your job.

2. If piles were not anticipated in your bid, and you had nothing other than boring logs and insufficiently detailed plans, you should have a valid claim for extra pay for the more tedious and time consuming excavation around piles...again you have to be more careful not to damage the pile contractor's work.

As for the mat/raft designation, I've heard them throughout my career to be interchangeable; however, BigH's statements make sense...I'm just not sure that most use the vernacular in that manner. As for pile support, it generally is stated if they are pile supported (piled mat, piled raft, pile-supported mat, etc.).

Is there anything indicative in the design of the mat reinforcing that would lead you to conclude that there is a pile interface?

 

[BigH]

All - the problem Zambo has is that the work is not in North America, is not in Europe and is not in OZ/New Zealand but is in Southeast Asia. We may not assume that the claim process in his neck of the woods will be the same that most of us in the areas I noted above would presume as a matter of course. Zambo - I'll get you the full book title tomorrow - it is at work.

 

[Ron]

BigH...excellent point. I guess they should adopt one of Florida's old marketing slogans...."The Rules are Different Here"!

 

[BigH]

Used to see "FIDIC with XYC (country) Characteristics."

Zambo:
P.C. Varghese, Foundation Engineering, Prentice-Hall of India, 2005. ISBN-81-203-2652-0/

 

[Zambo]

BigH, thanks I found the page on google books so using print screen can get a print out.

Vietnam is a bit "wild west" but with a few facts gathered together the owner will probably see sense.

 

[csd72]

RE:'the owner's engineer says piles weren't mentioned because they are not in our scope of work.'

Bingo - free kick. If they are not in your scope of work then you are not expected to do any work related to them.

I really cannot see how they can wriggle out of that one if you have it in writing.