Pile Supports to Heavy Raft Foundation
Pile Supports to Heavy Raft Foundation
(OP)
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
Regards
Mike





RE: Pile Supports to Heavy Raft Foundation
RE: Pile Supports to Heavy Raft Foundation
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
RE: Pile Supports to Heavy Raft Foundation
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
RE: Pile Supports to Heavy Raft Foundation
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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