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Re: Pile Caps vs Pile Raft

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I doubt you will be able to do it without a pile-raft. If the piles are too
close, their capacity gets reduced (confirm this with your geotech). If they
are far apart, there is no space left for the slab. Plus there will be
water-proofing problems as your 300 slab tries to resist two basement water
pressure. The slab is too flimsy compared to the caps and will easily
tearoff if the groups had even a small differential settlement relative to
each other (check that).

Best regards
Syed  A Masroor
Karachi, Pakistan.


----- Original Message -----
From: "MAM" <mam(--nospam--at)cyber.net.pk>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, August 23, 2006 8:55 PM
Subject: Pile Caps vs Pile Raft


> Members' comments solicited on the following issue:
> 50 story building  with two basements; Seismic zone 2B (UBC);
> Columns at 9.0m grids each direction;
> 1200/1500 mm dia bored piles  drilled in weathered rock (25%RQD) with
> 4 piles under each column ; Ground water level 1.0 m below surface;
> Option 1:  Provide a continuous pile raft ( 3000 mm thick) over the
> entire area,
> Option 2: Provide individual pile caps under each column, capping a set
> of 4 piles
>   and  inter-connecting the pile caps with tie-beams along column
> centerlines with
>    a 300mm thick monolithic slab on top (actually extension of  the
> upper 300mm
>     of the pile cap thickness) to cover the entire area upto the
> basement retaining walls.
> Option 1 will be very costly in terms of concrete and steel ( tremendous
> quantity of
> continuous concrete pour required), while option 2 will not only be much
> cheaper
> but also would be very convenient to construct,  pile cap to pile cap
> with a horizontal
> construction joint just 300mm below the pile cap top .
> Of course all the requirements of equilibrium and stability would be
> fullfilled alongwith
> water-proofing.
> My question: What are the merits and demerits of  each of the options?
> Why not go with option 2 if it fulfils all the code requirements?
> Your comments would be greatly appreciated.
> Thanks,
> mam
>
>
>
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