Expert: Arshad Khan - 4/12/2010

Question
QUESTION: Dear Arshad Khan;

I am a structure student, studying a model on ground engineering. I have to build the pisa tower as being build new and perfeably with leaning. I am have done some background study and suggesting that the pile raft foundation might be suitable. My question, Is it a suitable foundation for Pisa tower? Are u suggesting any other foundation? How I am going to design the raft pile foundation I am finding diffculty with raft design. Your help will be appreciate.

Kind Regards

Murid

ANSWER: Hi Murid,

Yes raft foundations are a bit complicated to analyse and design, and so are piled rafts. For your problem, I would suggest that you have piles directly under the footprint of the outer walls of the circular tower, with a capping beam connecting them all up. Then you will need to make the piles longer on the side that the tower is tilting, so that all along the circumference you comply with Newton's law action=reaction, for gravity loading only. Of course for transient wind loading you will have different loading on the piles in any direction, but that can be part of your design for the ultimate pile capacities/lengths (or diameters).

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QUESTION: As suggested by you piles with capping beam, there is a problem with special pancone clay layer if the pancone layer is distrub the soil loses its strength. first 10m is sandy clay silt and from 10-20m is upper clay which is pancone clay. will this foundation suitable or let me know what foundation you think is is suitable with keep in pancone clay.
Your help will be appreciate.

Answer
Hi Murid,

This then depends on the pile capacities vs length and diameter. If you can cater for all loading without piles goind down into the pancone clay, then you can use piles.

If not, the alternative is to use a wide circular strip footing, which is wider on the tilting side, so that it maintains uniform pressure with dead + gravity live loading only.

If the maximum pressure exceeds allowable, then you are back to square one- a raft foundation! In that case, use a circular raft and place it eccentrically under the tower so that the net pressure for DL+LL is uniform throughout, otherwise the lean will keep growing (like the real Tower of Pisa).