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Re: Stone Retaining Wall

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Gail ---
 
I do have some experience at old - not new - fieldstone retaining walls.  In the 1920's and 30's, several local homes were constructed of fieldstone, and some had retaining structures of similar construction.  One client had old photographs of workmen mixing mortar by hand and placing stones for a retaining wall.  That wall retained about 18 feet of eartha and stone, and was designed as a tieback structure with concrete deadman anchors, tied to the wall with steel cables.  The wall had stood for nearly 70 years before distress of the unreinforced masonry facing became of concern.
 
In a second case, a home had been constructed with all exterior walls and foundation of unreinforced fieldstone masonry.  As it had been condemned by the local County building officials, it was necessary to study in depth in order to design corrective measures for the settlement distress.  Cores of 8 inch diameter and up to 30 inches long (foundation level) were cut and tested.  Estimated effective bond lines between mortar and stones was over 90%, and the assembly tested with strengths similar to low-strength concretes (1500-2000psi compressive).  Mortar samples were tested at up to 4,300psi compressive.  Anchor tests were conducted with a through-thickness rod backed with a Simpson mansonry retrofit washer.  The Simpson washer failed, but the masonry (12 inches thick at the test location) remained undamaged at loads up to 13 kips.
 
My recommendations for your project based on these mostly successful structures:  Use a rich cement mortar with sand and pea gravel.  Excavate to good soils (say 12 inches minimum), and lay the base stones on the earth "dry".   Then place a generous quantity of fresh, stiff mortar (filling all voids) and lay the next course of stones immediately into the fresh mortar.  Continue upward, tapering the width to the final elevation, where the width will be at least 12 inches (adjust to the stone size available).  The masons will not be able to effectively point the mortar, so the outside 4-6 inches of each face will be ineffective for any bond strength. (none of our local structures had pointed mortar, although the repair folks were able to somewhat point using rubber gloves and fingers)  Design as a simple gravity structure, where strength is not critical.  It's just a heavy lump that won't turn over.  FS > 1.5 should suffice for any surcharge loads the owners may abuse it with.   My research showed that the density could be calculated assuming the "core" to be 70-80% stone.  It may be reasonably water tight, so check for saturated soil conditions.  The only strength calculation would be horizontal shear through the masonry.  You may find that friction alone between the stones is adequate to retain the soils.   It will be very rustic, very heavy, and will likely last longer than your engineering practice.   Oh yes, you may opt to select the cap stones for appearance, and discount them in your calcualtions (they get dislodged with time and abuse).
 
With this simplified approach, you may conclude that it is easily within your expertise.
 
BTW, if you want to search out a similar construction material in literature, check out the term "cyclopean concrete".
 
Russ Nester, SE, GE
 
 
 
 
On Wed, 12 Mar 2003 00:24:26 EST GSKWY(--nospam--at)aol.com writes:
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.