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comm#780--Re: sloped retaining walls

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If the slope of the backfill is made flatter than the angle of repose of the
soil, the lateral force it exerts goes to zero.  The structure is then
called "slope paving", and you just have to toe it in at the bottom of the
slope to prevent the paving from sliding.  We usually use a vertical
foundation wall with no particular design force, a few feet deep (around
here, we try to get it below frost).

For slopes greater than the angle of repose, you're back to a retaining
wall, but with reduced lateral forces.  Also, as Stan noted, the wall itself
is more effective because its weight is in a better position to reduce
overturning.

As to determining the lateral forces, go back to first principles, i.e. a
sliding wedge.  Since the front face of the wedge is reduced, it is lighter
and thus exerts less force in sliding.  Most soils textbooks explain this
adequately; I like Bowles, "Foundation Analysis and Design."  We usually
batter walls (slope them backwards) perhaps 1:24, then design them as
vertical, both because they look nicer, and because the actual forces
experienced by the wall are reduced.

The slope effect, by the way, doesn't stop at vertical.  Around here we see
a lot of modular block walls built to cut a parking lot into a slope.
Sometimes the basic engineering principles and assumptions (i.e. anchorage
and drainage) are pretty much ignored by the builders, and after a few years
they start to bulge and shift.  If you see such a wall with negative batter
(top overhanging the base), and it looks like rain, don't park under it.

Michael Hemstad, P.E.
  phone: 651/292-4468
  fax: 651/292-0083
  e-mail: hemstad.ml(--nospam--at)tkda.com
Toltz, King, Duvall, Anderson & Assoc., Inc
1500 Piper Jaffray Plaza
444 Cedar Street
Saint Paul, Minnesota 55101-2140