The "boulder" wall that you are describing sounds like a gravity wall and normally these walls are sized so that geogrids are not necessary. The walls that utilize geogrid reinforcement generally use blocks that are specifically manufactured for that purpose, such as Keystone, Redi Rock, etc.
this may be somewhat off the subject (perhaps parellel to the issue), but i'm not found of either rubble or mse walls since the designers in my part of the world disclaimer liability for the walls eighteen different ways and leave the owner or someone else holding the bag. i would like to see the building code, code officials, architects, etc refuse the ridiculous language that usually accompanies the designs. my favorite line from designers is "well, i'm not a geotechnical engineer so i don't know what the testing requirements should be"...yet they design geotechnical structure. i've never seen a structural engineer not know what testing frequency he/she wants their concrete tested at...and casting cylinders for that concrete sure as heck doesn't imply that the testing firm is responsible for the engineers design and acceptance of the material. the designers must specify which tests and which frequencies they want since they're responsible for the wall design and its construction. i'll get off my soapbox now...thought i'd throw it out there for fjciii...
I started designing shot rock (not round rock) walls in the early 1980's. The rocks were 2 to 4 feet in size.
The base width of the wall constructed of rocks, carefully placed in a brickwork fashon was designed as a gravity wall. The backfill was always fine shot rock in the minus 12 inch size with the coarser rock placed closest to the wall. I did walls up to 30 feet high supporting condos.
In the late 1990's we added geogrid with only a single facing rock width. We only use HDPE because the backfill (fine shot rock) can easily damage polyester grids. We used a relatively high reduction factor for damage and generally didn't use anything smaller than UX1500.
This is an economical retaining wall where there is a large cut and fill required in granitic bedrock.
The walls you are referring to must be Bob Barett's or a knock off of his ideas. The guy is a complete nutter (in a good sense), a real missionary, but he's very innovative and, from expereince, knows what he's doing.
For these "boulder walls" its not always easy to make the calculations stack up, especially with respect to strength at the connection with the "facing." The best you can hope to do is figure out what contact area you need to get 100% of the factored design tensile stress (since you'll never get 100% of Tal of the grid, use what's needed not what's available) and specify the contact area in the construction spec.
In summary, its not entirely conventional, but it works and you should be able to figure out a way to make it work on paper as well.
A final word - over 6' and less than 20' is probably okay but I'd start to sweat a bit higher than 20'.
Our firm has designed many "rockery" walls (as we call them) with geogrid reinforcement and they can be very effective where the walls heights are between 6' and 20'. I have been with this firm for less than a year and since the housing market has dropped, I have not had a chance to personally design one of these walls. I do know, however, that we recently put together a manual w/ FHWA on designing these rockery walls. I dont know if the manual covers geogrid reinforcement or if it is just for gravity walls, but it may be worth looking at.
jdmm - Polyester is more durable to installation damage than HDPE.
Industry standard per NCMA and FHWA is to consider geogrid for anything higher than 4 feet and include geogrid for anything higher than 6 feet. That is the industry standard. There are some larger block that can go higher, just make sure the gravity mass is properly designed.
I have a client that insists on an Ecology Block wall for a 10 to 12 foot high application with vehicle parking above. Are there any links to the design of this system with design examples similar to the link above using geogrid?
This thread discusses the use of arranged boulders for the facing, which causes a problem with designing in geogrid, because the connection between boulders is not an engineered joint that can serve as the mechanical connection,... Or can it.... We've been wrestling with this point with boulders on a current project. We gave up on really engineering it and looked for other ways to reduce the wall height so we could use the FHWA documents as a guideline.
I looked at a photo of a "ecology block" in wikipedia and it seems similar to some blocks i've used. Try taking a peek at
and see if these materials are similar to what you're interested in. The website has sample designs with some of the comps for a large variety of different large concrete blocks (reinforced and unreinforced) and in different loading conditions and soil conditions.
