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Re[2]: Retaining wall question

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A comment from a Geotechnical Engineer (GE) on your point 2.  In California (and
many other jurisdictions), it is a common geotechnical practice to reduce the
calculated soil sliding friction coefficient by a "factor," typically 1.5.  Many
call this a factor of safety, although I see your semantical point.  It, of
course, can be derived independent of the "structural" design of the wall.

If you cast concrete on sand, it can be argued that the concrete will be
cemented to the surface of the sand, so that sliding will actually occur on a
soil to soil slip surface (not the case for a slab-on-grade over a polyvinyl
vapor barrier).  Understand that for a dense sand, a direct shear test will
commonly measure a friction angle on the order of 40 degrees.  For cohesionless
soils, the strength can be calculated as the overburden or normal load
multiplied by the tangent of the friction angle (TAN(40)=0.84).  Hence, the
ultimate or unfactored sliding friction coefficient for a cast-in-place concrete
wall bearing on dense sands could be 0.84.  If we divide that by 1.50, we get
0.56.  This is a little bit closer to the 0.35 listed in Table 18-I-A of the 94
and 97 UBC (and 98 CBC).  Consider a loose sand (TAN(25)/1.5=0.31).  I think it
was the intent of the UBC to approximate a lower standard deviation of the soil
type variation, reduced by a factor (call it what you will) of 1.5.  A good
geotechnical report (as cited earlier in this "thread") will point out to the
Structural Engineer what "factor" was used.  Of course sliding friction becomes
less when the concrete is not cast on sand.  We also prefer to use adhesion (not
friction) when footings bear on cohesive soils (clays).  Bottom line, talk to
your project geotechnical engineer (GE).

By the way, you have an interesting point about the passive resistance.  It
often is a function of the means and methods (and sequencing) of construction. 
What is also overlooked is that some deflection is required to mobilize full
passive resistance in soil.

Tom Benson at Lowney Associates
1785 Locust Street, Suite 10
Pasadena, CA 91106-1614
(626) 396-1490, FAX: (626) 396-1491
Geotechnical and Environmental Consultants

____________________Reply Separator____________________
Subject:    Re: Retaining wall question 
Author: <seaint(--nospam--at)>
Date:       3/31/00 1:41 PM

Just two points:

1.- We generally don't use the passive pressure in the sliding calculations
because in nearby all cases that value is reduced under the assumption that
during construction, or after, the soil cannot be expected to remain undisturbed
at its final designated elevation. A better solution is to use a base shear key,
if necessary.

2.- The friction coefficient is a mechanical property of the materials involved,
regardless of the design method used. The safety factor is the ratio of the
resisting and the applied loads. How can the soil report say the safety factor
if the design has not been done yet?

Javier Encinas
ASDIP Structural Software
-----Original Message-----
From: Tarek <Tarek(--nospam--at)>
To: seaint(--nospam--at) <seaint(--nospam--at)>
Date: Friday, March 31, 2000 12:09 PM
Subject: Retaining wall question

>Hi all;
>I am designing a 10' retaining wall where the soils report gives
>a passive value of 400 psf and a coef. of friction of 0.3 for bedrock.
>The report states that these are allowable design values and have a factor
>of safety of question is whether this is the same safety factor
>required for sliding by the ubc, or are the ubc factors in addition?
>Tarek Mokhtar, SE
>Laguna Beach, CA.