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Re:Short and long term settlement

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John,

Just to be clear, I assume the settlements estimates you are relaying are
defined as total settlements in the geotechnical (or "soils") report.  Are they
saying differential settlement during construction will be a 1/4-inch?  Most
geotechnical reports clearly quantify differential settlements (at least in
California).  If not in the report you have, I would ask the project
geotechnical engineer (the geotechnical engineer of record) to quantify
differential settlements both short and long term.  I would go so far as to say
it is the standard of practice to provide both total and differential settlement
estimates in a final design geotechnical report in California, and probably most
of the United States.  Differential settlement estimates are a function of site
specific geotechnical conditions such as variations in the soil (earth material)
compressibility, fill thickness variations, depth to "bedrock" variations,
and/or variations in your structural loads across the footprint.  However, for a
flat, homogeneous site with a uniformly loaded structure, it is not unreasonable
to assume that differential settlements will be half of total settlements.  Your
geotechnical engineer would have to have a lot of subsurface information to be
confident that differential settlements would be only 1/4-inch, when your total
settlements were on the order of 2 inches.

In most cases, it is possible to reduce the construction total settlement by
improving the ground (overexcavate and recompact, surcharge if you have time,
and other more exotic alternatives such as dynamic compaction, etc.).  This may
not be cost effective for your project, but you may ask your geotechnical
engineer if these alternatives were considered.  I am at a disadvantage since I
do not know your site, jurisdiction or the structural configuration, so in
summary, don't hesitate to talk to the project geotechnical engineer.  He'll
probably be happy that someone actually read his report.
  
Tom Benson at Lowney Associates
251 East Imperial Highway, Suite 470
Fullerton, CA 92835-1063
(714) 441-3090
FAX: (714) 441-3091
see: www.lowney.com
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____________________Reply Separator____________________
Subject:    Short and long term settlement
Author: seaint(--nospam--at)seaint.org
Date:       10/24/01 11:42 PM

I am designing a large concrete industrial structure. The soils report
states that settlement during the construction phase will be 2 to 2 1/4" and an
additional 1" long term. I am considering using a combination mat and strip
footings. I am a little worried about differential settlement. For the long term
it is acceptable (assuming 75% of 1") but how much do I have to worry about the
differential settlement during the construction phase?

Thanks in advance for all answers.

John Holliday P.E.


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