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RE: Retaining wall design pressures

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> -----Original Message-----
> From: Robert McGhie [mailto:robert0(--nospam--at)]
> Sent: Sunday, 30 August 1998 4:40 pm
> To: seaint(--nospam--at)
> Subject: Re: Retaining wall design pressures
> > Saturated condition and submerged condition are two different
> > scenarios.  In saturated case, there is just enough moisture in soil
> > to fill all the voids.  Obviously, the unit weight of saturated soil
> > would be greater than dry unit weight.  However, the moisture is not
> > large enough to develop hydrostatic pressures. In this situation,
> > pressure on wall is equal to (Coeff. of lateral earthpressure X
> > saturated unit weight of soil X depth under consideration).
> >
> > In submerged situation, there is sufficient free water to develop
> > hydrostatic pressure.  Therefore, the pressure on the wall will be
> > due to two components:  Pressure due to submerged soil (use
> > submerged unit weight and coeff. of lateral earthpressure) and
> > hydrostatic pressure.
> >
> > Rajendran
> What happens when you add one drop of water to the saturated
> soil? Does the pressure on the wall suddenly change or are the two
> conditions essentially the same?
> Robert
Robert, the issue is not whether one extra drop of water beyond the
saturated point produces hydrostatic pressure.  Simply, if the water behind
the wall is not "free-draining", then the wall is being depended on to
retain it.  If drainage can occur, then only that moisture trapped within
the voids of the backfill apply additional pressure to the wall and this is
taken care of using the submerged density.  Below the water table, the water
clearly pushes against the wall and hydrostatic forces will need to be
considered in addition to earth pressures.  Above the water table, the
saturated soil applies pressure to the wall but not hydrostatic pressure.

Note that below the water table, the earth pressure is calculated using a
density based on the saturated density minus the water density, and this
pressure is additive to the hydrostatic pressure.

Dave Meney
Structural Engineer

Yenem Engineering Services
"Providing structural engineering services to the mining, process and
commercial industries"

Perth, Western Australia