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• To: seaint(--nospam--at)seaint.org
• From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Wed, 29 Jan 2003 14:26:04 -0500 (EST)

```I might be misunderstanding something, but from what I recall hydrostatic
pressure (i.e. equivalent fluid pressure) is NOT constant with depth.  The
soil density (or water density in the case of 62.4) IS constant with
depth, but the lateral pressure either from water or soil will vary with
depth.  I KNOW that fluid pressure based upon the water density of 62.4
DOES vary with depth...this is why there is a "crush depth" for
submarines...the deeper you go in the ocean the more pressure the
submarine depth must sustain until it crushes like an aluminum can under
my foot.

HTH,

Scott
Ypsilanti, MI

On Wed, 29 Jan 2003, Eric Ober wrote:

> Just remember that hydrostatic is constant with depth, so you have it
> all the way up to the water table.  For illustration, if the water table
> was at ground surface, then you'd have 62.4 instead of zero at the top.
>  This can obviously affect the maximum pressure at depth, as well as the
> distribution with depth.  It might be worthwhile to get the geotech to
> discuss his pressure diagram.
>
> Eric Ober
> Cagley and Associates
>
> Sherman, William wrote:
>
> >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
> >-------------------------
> >Joel Adair, PE
> >Halff Associates, Inc.
> >-------------------------
> >
> >
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