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Re: Underpinning

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No doubt all of your points are valid and should be considered if the
structure is not performing geotechnically. However, if the original problem
was that the original, one story structure was built on a "typical"
foundation and the only reason the original foundation appeared inadequate
was due to the additional loads induced by the second story addition,
wouldn't a feasible option be to investigate the actual soil conditions and
obtain a report from a qualified geotechnical engineer stating that the
allowable soil bearing pressure is greater than that assumed in the original
analysis? If I recall from the original question, the foundation in question
was just "slightly overstressed". I had assumed that this was based on a
soil bearing pressure of 1,000 PSF. If this is true, I have not had trouble
getting a soils report stating 1,500 PSF was acceptable in _most_ cases.

Bill Allen
-----Original Message-----
From: Williston Warren IV <billw4(--nospam--at)>
To: seaoc(--nospam--at) <seaoc(--nospam--at)>
Date: Friday, November 28, 1997 12:49 PM
Subject: Underpinning

>> 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
>pressures thereby mitigating the expensive footing improvement?
>Bill Allen
>-----Original Message-----
>From: Williston Warren IV <billw4(--nospam--at)>
>To: seaoc(--nospam--at) <seaoc(--nospam--at)>
>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
>>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
>>drilled excavation.
>>The different methods have their positive points and drawbacks, careful
>>consideration should be a part of underpinning foundations.
>> Bill Warren