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# Re: passive pressure for retaining wall

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: passive pressure for retaining wall
• From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
• Date: Wed, 3 Sep 1997 14:19:24 +0500

```As Stan Johnson pointed out, "I would think that a few limitations should be
looked at" before calculating passive pressure using the methods in the
example in REINFORCED MASONRY ENGINEERING HANDBOOK BY James Amrhein - I also
suggest that most designs using a keyway to develop passive pressure are based
on an invalid free body diagram.

Regarding limitations of Amrhein's method, in my opinion Amrhein presents one
such limitation on sliding resistance in the "Alternate Design of Shear Key"
shown just after his special method being discussed (presented on page 256 of
the fourth edition).  Again as Stan pointed out, "His approach does seem to
have some logic to it.  Essentially he is saying that the pressure from the
footing above is providing a confinement pressure for the soil".  It may be
true that the soil immediately behind the keyway has enhanced passive
resistance due to confinement pressure, but the overall wall does not have
such confinement beyond the toe of the footing - thus the "Alternate Design of
Shear Key" which considers passive pressure at the toe (without confinement
pressure) should actually be viewed as a "limitation" of passive resistance
for the overall wall, rather than as an "alternate" design.

It should also be noted that Amrhein's examples do not include a factor of
safety against sliding (typically 1.5).

The "alternate" design method presented by Amrhein is basically the method I
typically use to analyze sliding resistance with a keyway - with at least one
essential correction.  Since the passive pressure is considered to the bottom
of the keyway, the active pressure must also be extended to the bottom of the
keyway.  Although most design examples I've seen for sliding resistance with a
keyway do not address this added active pressure, a proper free body diagram
would include the soil pressure on both sides of the keyway.  Consider for a
moment if the entire footing were thickened 1 ft rather than just a keyway
extended down 1 ft - there would be no question that the active pressure
diagram should be extended to the bottom of the footing in an opposing
direction to the resisting passive pressure.  The fact that only a portion of
the footing is extended down as a keyway should not alter this load effect.
(Can anyone explain why this added force is neglected in many design
examples??)  And this is a significant effect - for example, with a 10 ft high
retaining wall with 2 feet of passive soil resistance, the active soil
pressure at the top of the keyway with 40 pcf EFP would be 400 plf - the same
as the passive soil pressure at this point using 200 pcf EFP.  Thus the active
and passive soil pressures would approximately balance, eliminating the
benefit of the keyway in developing passive soil resistance.  Thus I rarely
use keyways since they are not very effective in increasing passive soil
resistance.

The main benefit a keyway seems to provide is that the sliding plane is moved
from the concrete-soil interface to a plane within the soil.  Per Amrhein's
example, a higher coefficient of friction may possibly be used at this
location, although geotechnical recommendations rarely address this improved
sliding coefficient below the base.  Additionally, I would include the weight
of the soil wedge mobilized between the face of the keyway and the toe of the
footing in the vertical force used to determine frictional sliding resistance.
Thus a keyway may provide enhanced sliding resistance due to added frictional
resistance rather than due to any gain in passive resistance.

Regarding the value used for passive soil pressure, it is generally recognized
that greater wall movement is required to develop full passive pressure.
Since I typically don't want much movement in the walls I design, I use a
conservative value of passive pressure which can be developed with little wall
movement.  For example, I would use a lateral passive coefficient of 1.00 for
sand in lieu of 3.00 (see Figure 1 in Chapter 3 of NAVFAC DM-7.2).  This
further limits any gain in passive pressure from keyways.

```