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

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I will be the one to state what you already know.  Wood in direct contact
with soil is generally a bad thing.  Significant rot and insect damage are
possible, even with the best treated lumber, if left permanently exposed to
moist soil.  How do you plan on maintaining the soil side of the wall?  Will
you dig it out every year and apply a new coat of paint?

Secondly, a post will provide very little support for you wall.  Have you
ever pushed on a fence post and watched the soil around the post move?
Usually fences at least have a bag or two of post mix.  The dirt is much
stronger than you and won't relax.  It will keep pushing on your post until
the wall fails.  It is entirely possible that you might make it work by
putting posts at a very close spacing.  You may need to place them as close
as 12" on center (or closer).  I haven't done the math for any of this; just
know that I think the concept should be rethought.

If you really want to pursue this, I recommend you contact a local engineer.
He/she will be happy to repeat this advice.

Jake Watson, P.E.
Salt Lake City, UT


-----Original Message-----
From: Rick Reineman [mailto:rick(--nospam--at)reineman.com] 
Sent: Saturday, January 01, 2005 12:33 PM
To: seaint(--nospam--at)seaint.org
Subject: Assistance With Retaining Wall, Wood vs. Block?

I stumbled across the seaint site while researching wood retaining 
walls.  I am not an engineer but judging from the archives this looks 
like a great place to ask for some engineering suggestions.  I am a 
computer geek by trade with a background in home construction and 
remodeling.

The issue is that I need to build a retaining wall that is approximately 
80 feet long by 7 feet high.  The wall is in my back yard, is typical 
Northern California clay/silt sort of stuff and has no load above.

The foot of the wall will sit on a graded area that extends out to a 
slough (a one ended river in the Sacramento River delta).  The seawall 
is a CMI wall (http://www.cmilc.com/products/sheets.htm) and is about 20 
feet from where the retaining wall will sit.  The retaining wall will 
not sit in water but the water is not far away (I assume this is 
significant).  Water will occasionally flood the area during a very high 
tide but it is not long lasting.  The area may be wet but not what I 
would call standing water.

I have looked into the systems such as Keystone.  Keystone or equivalent 
appears to be an excellent solution and they provide enough engineering 
detail that anyone reasonably competent should be able to build a wall 
such as I need.  It's pretty expensive though, around $10,000 for 
material alone.

I got to thinking about how far the technology for treated wood products 
has progressed.  I can get the materials for about half the cost if I do 
something like 8x8 posts on 48in. centers with 2x10 planks across.   The 
labor cost will be significantly lower also.  The real problem is there 
is not much engineering information readily available.

I ran across some info that suggested the wood posts should be buried 
the same distance as the height.  I can't begin to imaging how I can dig 
a 7 foot deep post hole.  The use of a deadman system is frequently 
mentioned but no useful details for layout.

I will appreciate any comments, opinions or suggestions.  I have a gut 
feeling that wood is not the way to go structurally, the interlocking 
block solution is just so darn expensive.

Thanks,
Rick Reineman

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