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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 14:29:44 -0500

```Not true - hydrostatic pressure varies with depth equal to 62.4*depth. Thus
it is zero at the top and increases with depth, the same as a lateral soil
pressure given as an "equivalent fluid pressure".

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

> -----Original Message-----
> From: Eric Ober [mailto:eric(--nospam--at)cagley.com]
> Sent: Wednesday, January 29, 2003 11:10 AM
> To: seaint(--nospam--at)seaint.org
>
>
> 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
> >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
> 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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