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RE: Assistance With Retaining Wall, Wood vs. Block?

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Dear Rick,

I designed a Keystone retaining wall of a size very similar to yours in a
similar location adjacent to a stream bank.  This was in 1997 and it ended
up costing over $100,000.  This one was so expensive because the stream had
to be routed away from the base, and it required a concrete footing.  It
also required two or three layers of geogrid (plastic mesh) that ran about
ten feet behind the wall.  So, at least 8 feet high by 10 feet wide of soil
had to be excavated and backfilled along the length of the wall.  I would
think that your wall would require geogrid reinforcement as well.

For a timber wall of the size you are describing, I would think deadmen
would be required.  Pinned to the face of the wall, to the posts and/or
whalers, will be timbers or steel rods that run horizontal and perpendicular
to the face of the wall, back into the retained soil.  These timbers or
steel rods will be connected to other timbers, also horizontal, that are
parallel to the wall.  The parallel timbers are "deadmen", and the
perpendicular timbers or rods are "tiebacks".  In order to get the posts
deep enough, you will need some form of powered auger.  I believe there is
an attachment for a Bobcat that will do this.

Retaining wall failures are fairly common, and, since your new wall will be
fairly expensive, I encourage you to hire a local structural engineer.  I am
sure you are familiar with the "penny wise, pound foolish" addage.  In
evaluating cost, you also need to look at life-cycle.  If you install a
Keystone wall properly, it will probably out-live you.  If you install a
timber wall that is cyclically wet and dry, it will likely require
replacement, or at least significant repair, in 10-20 years.

Mi dos colones

Donald R. Bryant, PE
518 Bushnell Drive
Virginia Beach, VA  23451
fax 757-428-6473

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