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Underpinning

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>	Bill Allen,
>The values in the code foundation section are low values for
un-investigated conditions.  An alternate
>to the underpinning would be a complete geotechnical investigation to
determine actual
>bearing capacities.  If the problem is from a "soft" spot in the soil or
damage to a continuous footing
>the underpinning is required.  If the problem is slope creep or failure
then an extensive soils
>investigation is needed prior to the engineering of any underpinning. 
Most underpinning situations
>are generally expansive soils and the bad things they do and who they do
them to and why they do 
>those things they do and how to stop them from doing the things they do.

	Bill Warren



What if the analysis of the existing foundation was based on an "assumed"
soil bearing pressure, say 1,000 PSF and, based on that analysis, the
loads imposed on the soil exceed 1,000 PSF. Is it a reasonable solution to
obtain a geotechnical report that *may* provide higher allowable soil
bearing
pressures thereby mitigating the expensive footing improvement?

Regards,
Bill Allen
-----Original Message-----
From: Williston Warren IV <billw4(--nospam--at)pbs1.com>
To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org>
Date: Thursday, November 27, 1997 7:57 PM
Subject: Re: Copy of: Underpinning Ex


>Underpinning of foundation elements is a large part of the work that our
>office provides throughout the western US.  There are several options and
>the choice depends upon the problem.  If you have a simple soil settlement
>or over loading of the soil's bearing capacity, installation of new
>continuous grade beam beneath the existing or installation of new pads
>beneath the existing continuous footing.  Thc choice between these two is
>dependent upon the state or capacity of the existing footing and it's
>ability to span between new pad footings.  Re-leveling of a structure that
>has suffered differential settlement is another topic and should be
>seriously considered.
>
>Another type of underpinning available for foundations in good condition
>and when the soil is creeping or slope failure.  In this case you want to
>install "piles" to vertically support the building and possibably the
>lateral forces due to slope creep or failure.  Depending upon  the level
of
>soils investigation and engineering properties determined, we design a
>"hybrid" system for installation.
>
>The system that uses a screw element assumes that you install this like a
>cork screw into the soil and support the vertical load of the structure on
>the flukes of the screw along the length of the pile, assuming the soils
>along the length would support the vertical load.  Another system is a
>steel mini pile that is driven to refusal or end bearing.  The classic
>solution is a cast inplace reinforced concrete pile that is placed within
a
>drilled excavation.
>
>The different methods have their positive points and drawbacks, careful
>consideration should be a part of underpinning foundations.
>
> Bill Warren