I recently received this question about using a particular commercial product for making retaining walls:
“I’m ready to do an earthbag-style retaining wall project at the end slope of my front yard. I’m looking at an earthbag-style product called Flex MSE, which is basically UV-safe PPE material that can be sliced open to insert seeds or native plants. Flex MSE apparently strengthens as a retaining wall as vegetation becomes established. But the bags are specially formulated and have aforementioned UV-blocking properties, which is opposite of the much more affordable PPE earthbag that has to be shielded from the sun. The result of putting plant matter in a small slit in the Flex MSE bag is that the vegetation spreads and covers the bag, which makes UV protection unnecessary so long as it is fully developed in cover. Do you know of anyone who has cut small slits in plain earthbags for inserting plant matter? Would you recommend against it? I assume that plant coverage could protect a plain PPE bag from the sun, just as long as it is a vine or perennial that doesn’t recede during winter months (Zone six Western Pennsylvania here).”
“Plain earthbags would save us hundreds of dollars here on our retaining wall, as the Flex MSE product is just about $4 a bag; we need 128 for the project, plus a proprietary interlocking plate product that would act as the barbed wire does in a traditional earthbag setup to hold the bags tight to one another, working in tandem with gravity. The plates are about the same price per unit and are used in a 1:1 ratio with the build style we’re using for our retaining wall, although I feel that using barbed wire instead would be just as effective and so much cheaper. Let me know what you think.”
So I wrote back:
I don’t believe that there is a completely UV-proof Polypropylene material. The coatings that are used do have a lifetime, and they are usually measured in a matter of weeks, or months at the most. I would not trust the coating to protect the bags for very long. On the other hand relying on plantings to ultimately protect the bag material from sunlight is equally uncertain; as you know, nature never provides complete and uniform cover.
As for slitting the bag material in order to directly seed the soil sounds counter productive to me. Such cutting of the bags can only weaken the integrity of the matrix that creates the monolithic nature of the earthbag retaining wall. And, as you point out, replacing the barbed wire with specially fabricated connectors just adds extra cost to the project.
If I wanted a green retaining wall, I would use standard earthbag methods and protect them with stabilized plaster, then add a secondary commercial green living wall system, similar to what is shown in this article. By doing this you would create a more durable and likely less costly retaining wall.