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Re: Stone Retaining Wall

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Gail,

It depends.  If it does not matter if the wall is battered (sloped
backward), then it certainly can be engineered (in general refered to as
a breast wall).  It becomes a basic slope
stability problem of which you are bascially dealing with a nice simple
statics problem except that the soil properties will certainly affect
that basic statics problem, which I have encoutered in a class in slope
stability.  If the wall is to be perfectly vertical, then again it could be
engineered.  There are currently no code provisions that are
intended to directly apply, but one would apply the unreinforced masonry
provisions in the MSJC (or some other code) since it is intended to be
the code for ALL masonry (just does not have direct provisions for that
yet).   It would be necessary to either "assume" a conservative f'm for
the stone masonry wall OR if you really wanted to "do it right" you could
have a prism test created to determine the f'm.  Now, the ASTM prism
method may not directly apply to the use of stones (don't recall if it
could be stretched is such a manner and still actually be per the ASTM
method), but it certainly could be adjusted to "work", even though it
might not directly be applicable.

The decision of concrete vs. mortar becomes both a cost issue and a
materials issue.  As far as cost, as you are likely aware, the primary
reason for the use of larger aggregate is that it typicallly will make the
"concrete" (term used very generically here) cost less.  On the material
side, it depends on things like how much shrinkage can you live with in
your gaps as the mortor will likely shrink more than the concrete will.
I would say how ever, that you would likely still use a mortar rather than
a concrete, unless you were able to get a extrememly small small stone
aggregate.  Even 1/2" aggregate might be too much as not all the gaps will
have any significant size, so if you want to do those then a mortar might
be necessary.

Now, the issue becomes a little more murky if you want to dry stack the
rock.  Once again, a breast wall would work in this situation and could be
engineered.  A pure vertical wall would likely no longer be feasible.
There are no required or suggested design provisions for such a case that
I am aware of, although it is certainly possible.  I will mention that
this issue has be discussed within the MSJC code committee, but I believe
that for now the issue has been placed in the hands of The Masonry Society
to have a committee work on it if desired, even though there were many
that thought that it out to be the providence of the MSJC (it is masonry
after all).  The direction it seemed to be headed the last that I heard
was that at some point a TMS committee would take it up, and then
eventually could have recommeded procedures that could eventually become
the basis some code provisions in the MSJC (working of the 318 model of
feeder committees that determine recommedations then the main code
committee working with the committee's recommendations to put it into the
code).

Now, the truth is that in all likelihood it would not be engineered at
all.  If done as a breast wall, it could be "emperically" designed as it
has been done for many, many moons.   A pure vertical wall (if done with
mortar) would be tougher to do without some basic engineer stuff, but
could still be done as "emperical" as we are only talking about 3 ft of
height.

Hypothetically HTH,

Scott
Ypsilanti, MI

On Wed, 12 Mar 2003 GSKWY(--nospam--at)aol.com wrote:

> A question way out of my expertise.
>
> Hypothetically,  say one wanted to build a small retaining wall to retain a
> front yard which is 3 ft above the sidewalk.  The wall is going to be made of
> random pieces of stone (large cobbles) on the order of 50 - 75 lbs,  mortared
> together. (I.e. not stacked flat stone).
>
> How does one do the engineering on it?  Does one do any engineering?  Do you
> treat it  as an unreinforced CMU wall?  Are there some rules of thumb to use?
>
>
> Also, since the voids betweens the rocks might be quite large, would you
> actually be better off using concrete (mortar plus large aggregate) rather
> than just mortar?
>
> Gail Kelley
>
> "Experience is the name every one gives to their mistakes."
> Oscar Wilde, Lady Windemere's Fan.
>


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