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Re: underpinning

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Bill Allen wrote:

"It's been my experience that, in my area (SoCal), it is relatively easy to
get a soils report that provides an allowable soil bearing pressure of 1,500
PSF
which may mitigate expensive foundation underpinning. The cost of the soils
report (BTW, it sounds like geotechs get paid better in your area than mine)
is much less than the foundation underpinning costs....

Certainly, these issues need to be considered. However, one should consider
that the existing foundation has settled and probably has its best bearing
condition it will ever see. Once the contractor excavates for underpinning
(difficult to recompact an underpinned footing), actual foundation
capacities
(in PSF) may actually decrease"

[Bill Cain]  My experience has been very similar to Bill Allen's.  The
caveats I listed in my previous post are important and must be considered.
 But I have found numerous projects where a new foundation was not required.
 The basic UBC footings are "very conservative" for reasonably competent
soils.  Also, careful observation of the existing structure and those in the
immediate area will give a good indication of potential problems.  

Most residential clients have limited funds to achieve their dreams as well
as securing a structure that will perform adequately in an earthquake.  I
believe they get more bang for their bucks by putting it into needed seismic
improvements (at least here in Northern California along the Hayward Fault)
than in strengthening foundations that do not need to be strengthened.
 Remember that the job of the foundation is simply to transfer loads to the
soil while meeting serviceability and load requirements.  Anything beyond
that wastes the clients money.  

Bill Cain, SE
Albany, California