Pile Supports to Heavy Raft Foundation

[MPHenry]

Consider the following hypothetical situation. I have a heavy reinforced concrete raft foundation pad, 400mm thick which will be constructed on a loose to medium dense sand. The Geotechnical Report recommends that the foundations should be constructed on piles to reduce differential settlement. Is it possible to cast the raft on a series of bored concrete piles without using pile caps or directly connecting the piles to the raft itself? Lateral stability will be provided by passive soil resistance as the raft foundation pad is constructed at basement level, 3m below final grade.

Regards

Mike

 

[TYL]

I don't see why not. You could design the slab as a flat slab with 'hard points ' at the bored piles. You should cut back all projecting pile reinforcement, perhaps place some load dispersing sheet material between the pile and the slab. Design the slab for high shear around the pile heads and the pile supports as hinged to the slab.

 

[dgb]

MPHenry,
In most cases, people don't like to rely upon the passive resistance as the sole means of resisting the lateral forces. This is because the passive pressure is not fully mobilised unless there is a large lateral movement of the member (the raft in this case) & in turn of the soil. Generally such a movement is not tolerated/accepted by the structure or the owners/users of the structure.
Of course, if you are utilising a very small fraction of the available passive resistance, your scheme could be an acceptable option.
If no, you would still end up anchoring the piles into the raft to get a safe design. Although passive resistance IS still available, it is an additional factor of safety for the owner not the designer.
On second thoughts, why don't you design the raft taking only the at-rest pressures (with k0 instead of kp)? Though it would still be an unconventional design, a soil resistance to that extent should be available without a ground movement.
dgb

 

[BigH]

First off - loose to medium sand. What is your bearing pressure and do you really have sufficient settlements - total and differential? One reason, I always thought to build rafts is to minimize impact of differential settlement between columns, etc. Raft rotates as unit - of course, if you have requirements of equipment to maintain horizontal, this might still be problematic.

See Zeevaert's book on Foundations of Difficult Soils - I believe he describes building of foundations where piles driven into ground, a granular mat is placed above to provide a flexible but somewhat stiff "bridge", then the foundations atop. (It was either Zeevaert or some paper in Mexico 1969 conference - if memory is not fleeting.).

Best regards and

 

[Focht3]

BigH hits the source (if not the citation) on the head. (He may have gotten that right, too!) Zeevaert used some novel approaches to difficult site conditions. And he is right to raise the settlement issue. Where is the site? Are loose sands really present below 3 meters in sufficient thickness to justify the use of piers or piles for support?

A 0.4 meter (16 inch) thick raft, while thick, may not be that stiff. It depends on the mat size, loads, allowable movements, etc.

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