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• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
• Date: Wed, 29 Jan 2003 12:15:49 -0500

```I often have designed for at-rest lateral soil pressures (equivalent fluid
pressures) on the order of 90 pcf. This value includes the effect of
hydrostatic pressure when a structure is below maximum ground water level.
For example, using a lateral coefficient of 0.50 for at-rest conditions in a
soil of 120 pcf density: 0.50*(120 pcf - 62.4 pcf) + 62.4 pcf = 91.2 pcf.
Without hydrostatic pressure, 60 pcf is a reasonable at-rest soil pressure.
I have been given equivalent fluid pressures by Geotech's as high as 125 pcf
for clays (including hydrostatic) - this means the clay acts essentially as
a fluid.

I feel that "active" earth pressure is used in too many situations where
"at-rest" earth pressure would be more appropriate. Unless the amount of
movement required for active earth pressure can be numerically justified, I
use at-rest earth pressures. Most box shaped structures surrounded by soil
can't move laterally by a significant amount and thus should use at-rest
pressures.

William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

-----Original Message-----
Sent: Tuesday, January 28, 2003 1:50 PM
To: 'seaint(--nospam--at)seaint.org'

Tripp --
The 85 pcf to 110 pcf is not a soil density -- it is an equivalent fluid
weight.  In other words, it already has the at-rest coefficient factored
into it.  They typically give another value for in-place density, often in
the range of 120 - 135 pcf.  Using these values, that's saying that the
at-rest pressure coefficient is about 0.75 -- pretty high.
To give you an idea of what we commonly see, here's a snip of a geotech
report from a project I did a while back:
--------
Lateral earth pressures acting on the below grade walls will depend on the
type of backfill material used.  These walls should be considered rigid and
designed for at-rest earth pressures as presented below for a level backfill
and a drained condition.
EQUIVALENT FLUID PRESSURES
Backfill Material                  At-Rest (pcf)
On-site soils                          110
Select fill, with LL<35 and PI<15      65
Granular backfill w/ <3% passing
No. 200 sieve and <30% passing
No. 40 sieve, non-plastic          45
--------
Obviously, for large project like a culvert there is no way possible to
economically replace the on-site soil with granular backfill, so we're stuck
using the 110 pcf -- and designing a bomb shelter.
-- Joel

-----Original Message-----
From: Tripp Howard [mailto:tripphoward(--nospam--at)yahoo.com]
Sent: Tuesday, January 28, 2003 1:45 PM
To: seaint(--nospam--at)seaint.org

Joel,
You aren't using a lateral load of 85psf - 110psf are you?  What you get
from the geotech is the soil density that has to then be multiplied by
either the active or at-rest coefficient (usually around 0.33 and 0.50 for a
good sand).  This would give you about 37psf - 55psf lateral load on the
walls (which correlates very well with what AASHTO prescribes).  From what I
understand, the AASHTO pressures already include the active or at-rest
coefficient.
Tripp Howard
The recent thread about culvert design prompted a related question.  We have
an ongoing quandary at out firm regarding what lateral soil pressure to use
for design of culverts, retaining walls, junction boxes, and all sorts of
other buried structures.  Take culverts, for instance:  If designing to the
AASHTO standard, we can get a geotech report or we can simply use the
code-dictated 30 pcf or 60 pcf equivalent fluid weight (depending on the
load case) to determine the lateral pressure.  TxDOT, in developing their
culvert standards, used 40 pcf for the equivalent fluid weight.  However,
when we get a geotech report for a project, we typically get an equivalent
fluid weight of 85 - 110 pcf for calculating the at-rest pressure.
So, you see our problem.  If you take the "show me the bodies" approach, the
TxDOT values are adequate, since there are hundreds of miles of TxDOT
standard culverts all over the state, and there probably isn't one of them
that has failed due to excessive lateral soil pressure.  But in Texas we are
considered negligent by the PE Board if we design a foundation or a buried
structure without a geotech report.  So, we dutifully go get our geotech
report, find out that the soil at this site "really" exerts 100 pcf of
equivalent fluid weight, and design our culvert accordingly.  We're then
caught in the difficult position of having to explain why our culvert looks
like a bomb shelter.  This is especially troublesome when the client is
TxDOT, and we have to try to explain why we can't use their own standards.
I'm wondering if any of you have faced this, and how you have handled it.
What values do you typically use for lateral soil pressure on these types of
projects?  How would you approach our dilemma?
Thanks.
-- Joel
-------------------------
Halff Associates, Inc.
-------------------------

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