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History of Retaining Wall Sliding

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On 4/3/2003 2:09 PM after considerable forethought (or
so says Steve Privvet), Jonathan Mallard wrote:

>Back to my bridge built in 1930....
>
>On one of the approaches, there is a retaining wall
adjacent to a cemetery.
>Rough dimensions are 13'-4" tall, 6'-0" wide at the
base and 2'-3" at the top.
>
>We're thinking about cutting the wall slightly below
the top, adding a moment
>slab, and raising the grade about 7.5 feet.  At this
point, sliding becomes quite
>a problem!  We then went back and analyzed the
existing wall using the plans
>we have, and calculated a factor of safety against
for sliding of 1.08 - a good deal
>less then 1.5.  
>
>Looking at pictures of the existing wall in place, it
doesn't appear to have moved
>much in 70 + years.  Does anyone know if they even
checked for sliding
>back then?  
>
>And other than taking out the existing and starting
over, does anyone have any
>stories about similar additions to older retaining
walls that they can share?
>
>Thanks!
>
>Jonathan Mallard, P.E.

Jonathon,
My guess is they did check sliding back then, at least
the engineers who knew what they were doing.  The fact
that the wall hasn't moved is promising on that front.
 

In adding that much surcharge load, I'm guessing
you'll have problems with sliding, overturning, toe
bearing pressure, stem bending, and stem shear. 
Possibly footing shear or bending, too.  (Or is it a
gravity wall?)  In one sense sliding may be the least
of your problems.  Bearing and overturning will be
progressive in nature, so once they start they aren't
likely to self-arrest.  In other words, any tipping of
the wall makes it more likely to continue tipping.  If
it's a cantilever wall (doesn't sound like it) stem
bending, and especially stem shear, are fairly
catastrophic failure modes.

In any event, it seems to me that unless you know a
lot about the wall and its backfill and foundation
soils, and unless the wall was grossly overdesigned in
the first place, you will need to help it out.  There
are a number of ways to do this.  If the backfill is
fairly granular, you may be able to use soil
solidification to reduce (actually, pretty much
eliminate) any further lateral pressure on the wall. 
Failing that, you may need to install tiebacks (soil
anchors)reaching down behind the wall.  How close are
you to the cemetary?  (Well, you never know).

HTH,

Mike Hemstad
TKDA
St. Paul, Minnesota

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