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Mat (Raft) Foundation Design

Mat (Raft) Foundation Design

Mat (Raft) Foundation Design

Mat Foundation Design
(We call it Raft Foundations in South Africa”

I have to design mat foundations for low cost housing for the previously disadvantaged communities in South Africa, the poorest of the poor.
A typical mat size is 6mx7m.
If one has to design one small mat like this and you over-design, the implications are not too serious, but if 2000 houses are to be built then one is actually wasting vast amounts of money that could have been used elsewhere on the house. These houses are very rudimentary and every cent we can save by not over-designing is very important. “Deemed-to-satisfy” specification are this case very unsatisfactory, in my opinion.
Designing for heave is a problem for me because:
1. The effect of the structure on the soil is small in the case of lightly loaded structures like these but the effect of the soil on the structure in the case of heave is critical.
2. I don’t seem to be able to get a grip on understanding the soil-foundation interaction in this case
3. Some proposals have been made in the past by e.g. RL Lytton in 1972 during the Proceedings of the 3rd Inter-American Conference on Materials Technology. I can not find his complete lecture but what it boils down to is that he gives a formula for the shape of the idealised dome (inverted saucer) effect of the soil below the structure. He also gives some formulas for the bending moments that develop in the mat foundation due to this. Account is taken of modulus of subgrade reaction, dimension of structure, moment of inertia of the mat and depth of active layer.
I find it rather unsatisfactory to work with such “old” information that is not backed up by field studies etc and where I just have to accept the formulas and equations without understanding the underlying assumptions, principles and theory.
4. Some designers apparently use this idealised dome shape to establish what length of the mat foundation will be unsupported at the extremities and the design for these cantilevers bending moments.
5. I don’t seem to be able to find authoritative, practical literature on the subject.
6. In finite analysis the upward pressure can be idealised as springs acting at selected nodes. This is a very rough approximation because the shear strength of the soil is not taken into account.
7. Some designers apparently use the beam on elastic support theory. Clearly such a two-minensional
analysis is not the answer
I realise this is a complex issue but is there anybody that can give me some sort of substantiated advise, point me in the direction of where to find literature, propose some design software etc?

Any help will be greatly appreciated.

Thys Cilliers
Pr. Eng

RE: Mat (Raft) Foundation Design

In expansive clay regions, we have used a document prepared for the Wire Reinforcement Institute and the Concrete Reinforcing Steel Institute, dated August 1981.  The procedure was developed by a Mr. Walter L. Snowden, P.E., Austin, Texas.  This method was presented in a "Building Research Advisory Board (BRAB) Committee and seems to be similar to what you outlined.  It's called "Criteria for Selection and Design of Residential Slabs-on-Grade", BRAB Report #33.  The references to this document include Mr. Lytton.

The method and resulting design is affected by the actual soil properties so I don't know how you could just design one slab and use it all over the place.  

You might try a web search for the above organizations and names to see if any recent work has been accomplished.

RE: Mat (Raft) Foundation Design


Thanks for your response.
I'll try and locate the document you referred to.
What I have outlined is typical. The site is obviously tested and zoned according to geotechnical properties and a raft designed for each zone.


RE: Mat (Raft) Foundation Design

The American Society of Civil Engineers has numerous papers and publications that deal with the construction of houses and other structures on highly plastic clays.  I suggest you contact them for more information.  Their website is www.asce.org. ; Their phone number is 800-548-ASCE.

RE: Mat (Raft) Foundation Design


Thank you, I'll Try.

I came across http://books.nap.edu/books/NI000261/html/1.htmlCriteria for Selection and Design of Residential Slabs-on-Ground (1968)
(downloadable with Design Tables etc)

Wonder how valid this still is?


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