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Re: surcharging a site

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Please see my comments below.

Mark Trimble wrote:

> we have a site with about 12 inches of top soil\vegetation and about 12
> feet below that of fatty clays.  below the clays is fine sands. bedrock
> is about 50 feet deep.
>
> we are looking to erect a warehouse and plant facility on this site.
> loading is as such:  typical columns - 120 kip or less, floor load - 400
> psf, mezzanine column loads - 350 kip.  the concerns are the column
> foundation types and the floor loads.
>
> the recommendation is to surcharge the site for 3 months, the lighter
> columns could be on spread footings, the slab would be good to go (using
> a subbase of granular) and the high column loads go with auger cast pile
> into the sand (friction pile) or to bedrock.
>
> i have some questions.
>
> 1.  will the fatty clays swell after surcharge is removed with ground
> water within about 4 feet

Yes, fatty clays will swell when it comes into contact with water.
Surcharging the soil (also called pre-loading) does not remove swell
characteristics.  If the swelling potential is very high, the soil testing
company may be asked to recommend the thickness of a cohesive non-swelling
soil (CNS) that will compensate for the swell.  In this procedure, the
recommended thickness of existing clay is removed and replaced with CNS.
However, please remember that a swelling soil also has a shrinkage potential
when it loses moisture.  While CNS will negate problem due to swelling, it
won't prevent shrinkage of underlying natural subgrade.  So during
construction, the underlying subgrade should kept staurated. Make sure that
the floor slab is designed as a floating slab. That is, no structural
connections between the slab and the columns, grade beams etc.

> 2.  do we need to strip the organics prior to surcharging

No, not necessary.  However, it should be stripped prior to final
construction.

> 3.  can we put the deep foundations in while surcharged if downdrag is
> taken into account.

Theretically, yes.  Practically may not be feasible to install piles with
surcharge in place.  Depending on the recommended value of allowable bearing
capacity, the volume of surcharge can be fairly high.  Also you will have
settlement measuring devices at several places on the surcharged area.
Settlement measurement will be required for an extended period to ensure
that incremental settlements between successive measuring periods are within
acceptable limits.

> 4.  any thougts about the risk of being into fatty clays with spread
> footings,

See my notes for item 1. above.  Also, I would recommend that you provide
for grade beams.  Surcharging will not completely consolidate the soils that
can cause settlement.  After construction, some small settlement would
continue.  If architectural features of the building  cannot allow any
relative settlements between various structural elements, it will be well
advised to provide grade beams.

Rajendran