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RE: Geotech

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If you think Southern California is bad, you should see the reports for
Southern Utah. The soils are so expansive that the report I received today
made the following recommendation:
"Minimum 8-inch thick mat foundation or narrow "slot" footings are expected
to be the most economical and safe foundation system for the proposed
structure." This consisted of the following recommendation:
1. Narrow "slot" footings placed a minimum of 4 feet into native clays can
be designed for a maximum soil pressure of 3,500 psf..... Under this
pressure the total footing settlement is expected to be about 1-inch.
2. Footings should be kept as narrow as possible and should be poured "neat"
against the excavation.
3. Continuous foundation walls should be adequately reinforced both top and
bottom. As a guide, we suggest an amount of steel equivalent to that
required for a simply supported span of 25 feet.
4. A 4-inch thick layer of "expanded" foam under the slab AND
5. The mat, if used, should be placed on a minimum of 2-foot of
non-expansive impervious structural fill compacted to at least 90% the
modified Proctor maximum dry density.... Mat edges should be thickened and
extended a minim of 2-feet below the proposed final grades"

I've designed in Los Angeles and even Simi Valley but have not come upon
expansive soil this bad that required such a foundation for a small one
story commercial structure (less than 5,000 square feet).

Dennis Wish PE
-----Original Message-----
From: Paul Feather [mailto:pfeather(--nospam--at)san.rr.com]
Sent: Wednesday, November 04, 1998 7:12 PM
To: SEAOC list message
Subject: Geotech


Here is a topic I would like to offer up for discussion.

It seems like no matter where the project I am working on is located, the
soils report recommendations are almost a foregone conclusion. Low bearing
values and limited information. Apart from the obviously poor sites where
piles or caissons are recommended, the typical values for a traditional pad
footing foundation are equivalent to UBC minimum values (a straight 2000
psf).

Every soils bearing relationship I have ever studied, the bearing capacity
is dependent on the configuration and depth of the foundation, but only once
has a soils report provided equations for bearing with site specific
coefficients to allow for increased values due to width and depth.

Is this a typical condition for the rest of you, or is Southern California
primarily lousy soil?

Are these values realistic, or are the Geo-tech reports heavily influenced
by the 267,000 lawyers in California?

Structural design forces per the 1997 UBC are heavily soil dependent. All of
the new base shear equations, zone factors, and period values are
interrelated with the parameters for the top 100 feet of soil. The soils
report will have a large impact on the configuration and economy of the
final design.

I would be interested in hearing your opinions.

Paul Feather PE