[Greenbuilding] Fence posts--gravel or concrete

[RT]

On Fri, 22 Jul 2011 22:55:24 -0400, Benjamin Pratt  
<benjamin.g.pratt at gmail.com> wrote:

>  I was only able to bury the posts about 18
> inches. (8 foot posts that needed to be 6.5 feet high). Put at least a
> foot of gravel beneath them, gravel beside them, and where the gates
> are, a 3inch "washer" of concrete above the gravel.

The "Rule of Thumb" for burial depth of posts (for pole-frame buildings,  
simply from a structural perspective (ie no consideration of frost action,  
viewing the post as a "vertical cantilever") is 1/3 the length of the post.

That recommended burial depth is intended to provide the necessary  
resistance to wind loading of the cantilevered (ie above grade) portion.

Obviously, for a backyard fence, whose life expectancy is probably limited  
to 40 years at most , the structural considerations wouldn't be as  
relevant.

Where frost action is a consideration, the potential for frost heaving can  
be minimised by eliminating frost-susceptible soil in the vicinity of the  
post and replacing it with coarse granular fill (as Ben did).

But in order to ensure that this strategy works long-term, one needs to  
provide a barrier (ie "filter") between the coarse granular material and  
the surrounding soil to prevent fines from the soil migrating into and  
clogging the interstitial voids (ie the frost expansion "chambers") of the  
coarse granular material ... and to prevent the surrounding soil from  
"eating" the coarse granular material (thereby causing the lateral support  
to diminish and disappear over time.)

Geo-textile fabric is the official material that one would use for the  
filter material but those woven poly tarps that are used to protect skids  
of kiln-dried lumber during shipping from the mill to the lumber yard and  
which are usually tossed into the dumpster at the lumberyard, work fine.  
Place a piece of the filter material in the hole to make a "sock" before  
filling with stone and post, fold over to wrap top of stone before placing  
concrete for cap.

The "3 inch washer" cap of concrete that Ben mentions is pretty important  
to include. It should be  (truncated ) conical in shape to shed water out  
and away from the post and it's a good idea to insert some shingles  
between the post and the top of the concrete to form a caulking groove,  
removing the shingles after the concrete has set and then sealing the gap  
with a forever-flexible sealant.


-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at  Y a h o o  dot  C A >
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