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Re: underpinning

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-----Original Message-----
From: Williston Warren IV <billw4(--nospam--at)pbs1.com>
To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org>
Date: Saturday, November 29, 1997 1:21 PM
Subject: underpinning


>Bill Allen,
>
>The addition of a second floor could be included with a geotechnical
>investigation that provides greater than the code value bearing capacities,
>but to obtain these "higher" values a rather greater than normal
>investigation could be required.  Our experience has been that goetechnical
>investigations for residential work, that the owner wants to pay for, are
>about $3,500.  This fee really does not allow enough investigation for the
>Geotech to provide significantly higher bearing values.

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.

>
>Our experience has shown that this $3,500, larger than the structural fee?,
>includes a couple of shallow test pits and testing of the recovered
>material.  Chances are, depending upon the date of the original
>construction, with the higher design bearing values the footings now are
>inadequate for bending criteria.  Consider the assumed maximum span of a
>typical code prescribed footing (reinforced or not) would have over a
>"soft" spot for one story loads, and now consider the impact upon the
>footing when the load is increased and the implied span is still required.
>
> Bill Warren, S.E.
>
>

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.

Regards,
Bill Allen