Building raised beds can be expensive and complicated, especially if you are using lumber or bricks or other construction materials. I decided to use corrugated tin and electrical conduit stakes because they were economical and didn't take up too much space in the width of the garden bins. I found the effort and time invested in the infrastructure of building raised beds has paid off many times over in the later maintenance and increased yield these bins provide. It IS worth the effort, but they are not instant or built without some energy expended.
I had already decided on the circular bins based on the watering design but if I were to do it over again I'd cut the tin across the corrugation rather than try to bend the tin around the angles with the ridges resisting the angles. I figured out how to make new free standing panel lengths from some salvaged tin from a shed that came down in a snow storm. Here's a set of photos and descriptions of how I built some new bins using the modified methods. They work out nicely.
You will need the following to build your bins:
Corrugated tin, either salvaged as seen below or new sheets ranging in length from 8' to 12' long and on average 3-4' wide
Old garden hose (enough lineal feet to top the circumference of the tin)
A nibbler or other tin/metal cutting device. I do NOT recommend you cut the tin with a sawzall or skill saw.
Pliers and or wire cutters
Leather Gloves and ear protection while cutting the tin
Construction/plumbing crayon or other marking tool
Straight edge or piece of wood
Sawhorses or other risers to lay the tin on while cutting
Drill and bit for drilling holes in the tin for connections
Wire to make connections
Shovel and yard tools for moving the soil and debris.
It can be tricky to work the nibbler over some of the ridges in the tin, especially if you're using salvaged tin like I did that has some bends and mangled areas. I found that rocking and jiggling the nibbler a bit till it worked its way over the edges and ridges helped, as well as rolling the tin to the front edge of the sawhorse stands so that the already cut parts bent down and caused less resistance to the cutting blades. My nibbler allows the head to pivot around but I found that awkward to hold with my small hands and still get the pressure needed to work through the tin.
When I've determined where I want to build the bed I rake it clean and cut out any grass that may have grown into it and remove the rocks or other impediments. If the soil is good I lay a plastic tarp or old sheet down and dig out as much soil as I may want to use to top the layers that will be inside the bin. I dig a trench about four to six inches deep where the tin panels will sit and make sure that the trench is as level as possible to help the tin stay straight when it's installed in the trench. Then using a rope or hose I lay it inside the trench to determine the circumference and then measure that with a tape measure to determine how long my panels need to be to make the loop.
To connect the panels together I lay them flat on the ground with one overlapping the other by at least one ridge. I put a section of 2 x 4 underneath as a surface for the drill to press against as I drill the holes. With an electric or cordless drill and 3/16th or 5/32 bit I drill two holes about 2 inches apart at the top and bottom, through the two layers of tin. Then I cut some wire and thread it through the holes like a large staple and twist the wires together on the inside of the panels where it won't catch on a leg from the outside.
Once I have five or six panels connected I laid the sections into the trench standing up and eyeball measure to see if they'll fit. For some I had to adjust the shape and size of the trench so that the sections could overlap properly at the final connection. If the panel is too long you can overlap them more ridges until it fits, like adjusting a hose clamp. Once I have a pretty good idea that it will fit the space I've designed I lay the connected sections back on the ground and drill the last holes for the final connections.
There are many ways of layering a raised bed. Using the concept of the Lasagna Gardening http://www.motherearthnews.com/Organic-Gardening/1999-04-01/Lasagna-Gardening.aspx system I've simplified some of the steps and found that it works fairly well. I start off with cardboard boxes flattened out and then laid out in the bottom of the circle or shape of the raised bed. You can use layers of newspaper wetted down as well. The goal is to block light from getting to the soil so that weeds from below won't grow up through the bins. Then I pile loads and loads of large debris and leaves, chunky stalks and whatever organic material I can collect to fill the bin with. I pile it well above the top of the bin and then stomp it down until the debris is packed fairly tight into the space.
Once I've tromped the debris into the space and re-adjusted the sides as level and even as possible then I put in a thick layer of grass clippings, hay, straw or other organic debris to create a matted layer that will keep the soil from sifting down into the debris layer. Depending on how much soil you have for the top layer you can use the grass and straw layer to build up the volume to within about six inches of the top of the bin. It will settle some as the debris below starts to deteriorate.
When the grass and straw layer are settled and somewhat leveled out then I start to top the whole thing with the soil I've collected, either from the aisles between bins or topsoil from other places. If you dig the topsoil between the aisles make sure not to compromise the soil holding the tin in place or dig so low that it washes out and causes the tin to deform . If you have some bagged soil and compost then you can mix that in on the top. You will need four to six inches of soil on the top of the whole lasagna pile to give the seeds and plants enough soil to hold the moisture against the roots. As the moisture filters down into the debris below it will help break it down. As the plants grow the water will evaporate up to the roots and keep them moist and loose.
When I've filled the bins to within two inches of the top and leveled them off I drill holes about 1 1/5" down from the top edge and spaced about every 8" or so along the rim. Then I cut the old hose down the length, cutting off the nozzles and end pieces. It works best to cut the hose with a 2 x 4 underneath to help lay it out and provide a hard surface underneath. I used a carpet blade but was careful to keep from cutting through both sides.
Then I pry the hose apart and slide it over the top of the tin and open it wider to accommodate the ridges. If I have a tight corner I sometimes cut a small V into the hose on the inside of the curve to help it ease around those spaces. When I've got hose covering the entire surface I wire it down with 6" sections of cut wire and twist it tight and tuck the ends down to the inside of the bin to keep from catching on on hands or legs.
I've had a few spots in my more complex shaped bins that have bowed out, mostly because I failed to dig the trench evenly and level before I settled the tin into the trough. I used some EMT or rebar stakes to support the sides where there was a weak wall and if necessary drill more holes and wire the wall to the stakes. Otherwise I have been able to go without very many stakes at all in these bins and they stand on their own.
When I've got the top protected with the hose sections then I'm ready to plant my seeds, bulbs, and plants. I wet the soil a bit before hand to hold it in place and give it some weight and then plan my planting with the taller plants in the back or center of the bin and cascading down to lower plants in the front edges. Then I just lightly water until the soil is well moistened and then let nature do the rest. Here are three bins around my Honey Locust trees that will be my flower beds. The bin with the least curves seemed to fare better than the others in holding it's shape. There were no roots under that one to contend with so it helped in getting the trench level.
And finally, as the flowers and plants grow they'll cascade over the edges of the bins to help soften the shape and make them look more natural in the setting. Best of all, they'll be easy to water, weed, and change as I try new plantings each season.