The following information was taken from the latest issue of Mother Earth News written by Al Nichols of Penticton, British Colombia. Although that which is presented is a very labor intensive method of constructing raised beds it is also a very inexpensive way to produce multiple beds for ones expanding garden. I have personally toured homes in the foothills of Northern California that were constructed by this rammed earth method; I fell in love with the aesthetics of being surrounding with earth, even walking on rammed earth floors was very pleasant. Some of you may know that there are a few of these types of homes in our own area.
Still, there is more than one way to create a rammed earth grow bed. You may remember that last year I was given a brand new in the crate earth brick machine (it also presses blocks and pavers) manufactured by 'Open Source Ecology'. It has the same capacity of utilizing the earth to produce walls and floors but in units much easier for us old dogs to handle. This type of brick needs no mortar at all to achieve adhesion, when stacked they will adhere to one another quite easily by just dampening the top of the last course of brick. It could especially be helpful in forming new raised grow beds for members of Neighborhood Dirt, but it needs to be assembled; it's only time, right! For more information on assembly, google 'Open Source Ecology'.
Rammed earth garden beds are made by compressing a damp mixture of approximately 70 percent sand, 30 percent clay, and a small amount of cement into an externally supported form or mold, creating a solid, earthen wall after the frames are removed. One of the benefits of rammed earth is that its high thermal mass absorbs heat during the day and releases it at night. My technique for building rammed earth garden beds makes use of local raw materials, is resistant to temperature changes and is remarkably cost-effective.
Construct rammed earth gardens beds by using two frames; a box inside of a box with a three and a half inch space between the walls or boxes. Pack this space with the earth mixture, then remove the frames, leaving earthen walls that are ready to hold your garden soil.
3- sheets of 4-by-8 foot plywood
4- pipe clamps, each 60 inches in length
1- bag of portland cement
1- hand tamper (or optional power tamper)
A mix of local sand and clay
Start by constructing the framework. Cut all three of the 4-by-8 foot plywood sheets in half lengthwise, resulting in six pieces of 2-by-8 foot plywood. Set two of the newly cut pieces aside. Cut the remaining plywood sheets into the following sizes: two at 24-by-48 inches, two at 24-by-41 inches, and two at 24-by-89 inches. Save the leftover pieces for a future project.
The external, larger box uses the two panels that are 2-by-8 feet and the two panels that are 2-by-4 feet, held together with the pipe clamps. The internal form uses the two 24-by 41 inch sheets and the two 24-by-89 inch sheets braced with the twelve 2-by-4s. After the two frames are assembled, you will have two big boxes sitting on the ground, with a 3 1/2 inch space between their walls.
Mix the sand and clay with 8 percent Portland cement and very little water. Layer the mixture between the framed walls, 8 inches deep at a time, and then tamp it down tightly. Repeat this process until the rammed earth is 3 inches from the top of the form. At this point, add a layer of concrete to the top of the forms to finish the process. The following day, strip away the wooden form and then fill the rammed earth garden bed with soil and top it off with mulch.