The only sunny, level piece of ground on our lot is in the front yard, next to the driveway. Despite my well-reasoned and insightful explanation of why my new greenhouse should go there, my wife vetoed the idea. So, the only other location with a bit of sun was at the top of our lot, in an area the utility company keeps cleared of trees. Unfortunately, this location was far from level. To solve the problem and make the site usable, I decided to play in the mud.
To make a pad large enough to accommodate the 13-foot-long hobby greenhouse, I needed to raise the elevation on the lower edge of the site about 42 inches. Because the site is visible from the rest of my shade garden, I wanted something that would blend in and seem to disappear. Also, because there is no access to the area, all building materials had to come up the steep hill via wheelbarrow.
Early Construction Techniques
After considering various options and rejecting them because they were too costly, too temporary or too ugly, I finally decided to build the wall using a technique I call “mudcrete” construction. I saw a mention of this technique years ago in an old book describing early roadway construction techniques. Apparently, in the early 20th century some secondary roads were constructed by working a cementing agent directly into the soil and then packing it down.
Last spring I visited a site in southern California that used a similar technique to build a 300-foot-tall dam using an almost dry concrete mix and heavy construction rollers to pack it in place. One equipment manufacturer sells a machine, apparently intended for the desert southwest market, that makes what it calls “adobe” blocks by blending native soil with a cementing agent and then forming the blocks under pressure. So, the technique is still used, but is not at all common.
Sedimentary rocks (sandstone, limestone, shale) are, after all, just pieces of small mineral fragments that are slowly deposited and become solid in the presence of pressure and a cementing agent. My little project just shortened the timeline of rock formation.
A Solid Combination
Portland cement is the binding ingredient that transforms clay into what is essentially a manmade rock.
Mudcrete is made by combining Portland cement and the existing soil to form what is essentially a manmade rock. My first attempt at making mudcrete was about 10 years ago when I needed a low, level retaining wall to support one edge of flagstones for the patio in my Japanese garden. The wall was only about a foot tall and mostly hidden by plantings so I wasn’t especially concerned if it cracked. But, after 10 years it shows no signs of cracking and has served my needs perfectly.
My new project was a bit more ambitious. Instead of gaining all of the needed height in one tall wall, I chose to use a taller front wall and a shorter 8-inch wall in the back to gain the needed elevation. Because I have almost no topsoil on my hillside, I guard what little I can scrape together in a rather possessive and protective way. So, for this project I chose to use the tight, sticky clay subsoil for the soil component of the mix.
The ratio of Portland cement to soil depends on the kind of soil you have. If you have a high sand content, the ratio can be as high as 16 parts soil to 1 part Portland cement. If using clay, more cementing agent will be necessary. I finally settled on a ratio of 6:1 clay to Portland cement. Much of the strength of concrete construction comes from the stones (“aggregate” material). I only used the existing clay and Portland cement in the mix, so my wall lacks the strength of conventional concrete construction. Luckily, all it has to do is stand up to foot traffic, because it is impossible to reach this area by anything heavier.
Construction Of The Wall
Fill the form with a layer of about 6 inches of the moist mud blend.
Wooden forms were constructed, but to compensate for the lower strength of mudcrete, I made the forms 12 inches wide instead of the 6-inch thickness normally used. No reinforcing iron (rebar) was used in the wall.
To mix the mudcrete, a shallow trench was dug and the clay soil set aside. A portion of the clay was shoveled back into the trench and the dry Portland cement added. I attempted to stay fairly close to the 6:1 ratio. (When I finished the project and calculated the volume of the wall and compared the number of bags of Portland moved by wheelbarrow up the hill, it turned out to be a 9:1 ratio.) Next, using a tiller the ingredients were mixed, adding water as needed to blend the two components. I wanted the final mix to be about the consistency of moist soil.
Six inches of the mudcrete mix was shoveled into the forms and packed with a manual plate packer. Those 6 inches of loose mudcrete were packed to about 4 inches. If the mix was too wet, packing became a problem. If too dry, it was hard to get a thorough blending of the two components. I completed the pour for each wall in a day’s time, an important step – at least in my estimation – so that there was not a distinct layer between one day’s pour and the next.
Blending Into The Landscape
After 36 hours, the forms were carefully removed and the surface worked with conventional concrete tools to give a smooth surface. I completed the project in late November. We had a significant freeze a week after completion of the project, so some of my surface finishing popped off as curing was not yet complete. But, no matter, that just made it look that much more like a rock. The wall now looks like it has been around for a long time and blends nicely into the garden scene.
Mudcrete has other possible uses in the garden. Be aware that mudcrete is not as strong as conventional concrete construction, so if you are unable to live with the possibility of cracks, avoid this technique.
One area where this material could be used is in drainage areas that are prone to erosion. By tilling the site, mixing in Portland cement and then compacting it, a drainage ditch could be created that would have a nice earth tone and still provide protection against erosion. Similarly, a walkway could be constructed through a garden that would look like an earthen trail, but avoiding the problem of tracking mud into the house. Other uses can be envisioned where you need a small retaining wall but don’t like the look of the ugly, stacked-concrete block walls.
(From State-by-State Gardening Nov/Dec 2005. Photos by Gerald Klingaman, Ph.D.)