5 May 01 12:41
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.