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# Re: Required Compaction for Spread Footing Sub-base

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Required Compaction for Spread Footing Sub-base
• From: "Lloyd Pack" <packman90(--nospam--at)qwest.net>
• Date: Sun, 28 Jan 2007 09:53:31 -0700
• Priority: normal

```On 28 Jan 2007 at 4:41, Romel Gomez wrote:

>
> Hi,
>
> Can anyone show me by computation that spread piles supported by bored
> piles need not have its sub-base soil be compacted since it is
> supported by a bored pile.
>
> The size of the bored pile is 800mmØ and the spread footing is 2m x 2m
> x .80m. The structure for this spread footingsis a concrete pipe rack
> structure.
>
> Thanks.
>
> Romel

Only if the pile is designed to use skin friction to resist the load and not
only tip bearing.  If skin friction is used, then the density of the soil near
the top affects the friction factor.  Generally, once the depth of the soil
around the pile gets to about 20 pile diameters, then the friction factor
is pretty constant for the rest of the pile's depth.  So, the density in the
upper 20 pile diameters can affect the friction factor.  If the pile is a
driven pile, then the densifying is usually accomplished in the driving
of the pile, unless the soil is very loose.  If the piles are drilled or jetted
then the soil will probably be looser than in situ conditions.

Also, if your footing is designed to use the soil bearing capacity then
yes, the soil must be compacted to have the needed capacity.  If the
footing is only designed to have support from the piles, then the only
issue is the method used for pile design.

Take Care,
Lloyd Pack

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