Most people do not attempt to build all their garden beds at once, but do it one bed at a time, that is they build a bed then fill it and plant it. Then maybe they build another one in a few months time and get that one going. This makes the work load much less stressful and spreads the costs.
When you are ready to start building the walls you will need,
A set of Moulds
A concrete mixer
A few shovels, some trowels, a rammer or a couple of lengths of 4×2 to use as rammers, a couple of buckets for water.
Bags of cement, roughly 4 for every 7m of bed length depending on how strong you want to make your concrete.
PAP7 or GAP7 or Bedding Scoria or River Gravel and Sand or Rotten Rock, roughly 1 to 1.5 cubic metres for every 7m of bed.
PAP7 is the metal left over after after all the other larger grades of metal have been sieved out and it has an aggregate size of 7mm and below, plus it can include alot of crusher dust which means you might not have to add any sand.
PAP7 can be from volcanic rock such as Basalt or Granite, or it can be of sedimentary rock such as Greywacke. I recommend sourcing Paramagnetic Basalt if possible as this will add minerals and paramagnetic energy to your garden, but I have built many fine walls using Greywacke although I usually don`t use this to customise soil for the garden as it is not paramagnetic and does not contain minerals.
My original beds were built with Bedding Scoria, also called SAP7, and this is good too, but will probably need the addition of Builders Sand as it is quite course. Scoria really is a form of Basalt with a lot of air bubbles in it. I was using 4 shovel fulls of scoria to 1 shovel of sand and making the cement content about 1:10. This worked fine but now I would recommend a higher cement content like 1:8 as the whole idea of this garden is permanence and we want the walls to last a very long time and over that time period the initial investment in cement amounts to nothing. But my original idea was to make the beds as cheap as possible and cement was the most expensive of all the building materials. Also producing cement is very energy intensive and produces alot of CO2 which is bad for the atmosphere. In fact for every ton of cement, a ton of CO2 goes into the air. But now I believe we can compensate for using cement by building up tremendously high carbon and organic matter levels in our raised bed gardens. One very beneficial way of doing that, is to incorporate Biochar in the beds. In the future the energy produced when making Biochar from the Wood Gases could be used to produce the cement.
So constructing the wall is much like building a Sand Castle on the beach, you do not wait for the concrete to set in the mould you move the mould onto the next section the moment you have finished ramming it in and smoothing it off with the trowel. You don`t need to put up and tear down boxing, you can keep on moving the mould along all day long !
The concrete in the walls, free stands like a sand castle and if somebody kicks it , it will collapse like a sand castle. But after a couple of hours it starts to set and hardens, and by the next day it is hard enough for you to start filling up your bed if you want.
The most important thing to get right when making the concrete, that is presuming you have remembered to put the cement in, is to get the water content just right. Because if it is too sloppy mix when you move the mould the wall will slump or fall apart. You have to experiment with a few mixes to work it out, as it will vary according to the moisture content in you metal or the proportions of cement you are using.
After removing the moulds always remember to smooth any sharp edges and round off the wall as much as possble as this greatly improves the appearance of the walls and makes them much more comfortable to work on. The goal is to try and finish a whole bed in one day as you don`t want to come back the next day and try to connect wet concrete to dry concrete as the bonding will be weaker, but it is not a disaster if you have to do this, just make sure you thoroughly wet the dry concrete first, to help get a better bond. It is very easy for a couple of people to finish a 7m bed in 5 hours if everything was set up the day before. They could even build a 15 to 20 metre bed in one day if they worked hard, but they would be very tired at the end of the day !
Make sure you properly pack the concrete into the corners of the mould to get a good connection with the ends of the walls, as this point is the weakest link in the whole wall. Always make sure the concrete is well tapped in with the rammer, but don`t get too obsessed with this , it only needs to be firmed, not bashed too hard, which will cause the mould to distort from the pressure.
The making of the side walls is quite straight forward compared to doing the end walls and there is about 3 wheelbarrow loads of concrete for every move of the Wall Mould.
Keep checking with the tape measure that the inside wall spacing at the “TOP” of the wall is 1.4m . Make sure the two walls are on the same level and if one wall is in a depression you might have to jack the wall mould up slightly with some small peices of wood to get it level with the other one. I don`t like building walls across a contour where one wall is lower than another, this means that you won`t be able to get the soil in the bed level. If you have to go across a contour then terrace it, otherwise if the slope is gentle, build the beds down the slope.
You can make the beds as long as you want, but for the home garden I recommend 7m as a good length to walk around and 3 or 4 of these beds would make a good size Kitchen Garden where you can grow salad greens, herbs, carrots, beans, broccoli, a few tomato plants. In other words stuff you want to harvest fresh from the garden or needs frequent attention or picking. Permaculturalists call it Zone 1.
For a Community Garden I would make longer beds of about 20m, but even beds longer than that could be warranted and the longest bed I have ever made is at Pawarenga and is 23m long.
By having longer beds you significantly reduce the work load by having less end walls to build, also less area in paths means more growing area, and when installing drip irrigation you have much less fittings to buy and less work to do setting it up. So in summary larger and longer beds reduce work and lower costs.
In Cuba some Organoponicos are as big as 1 hectare and employ 10 people. They produce tonnes of vegetables just using hand tools. I would not like to see any area bigger than that covered in concrete walled beds and I feel one acre would be a nice size Community Garden, and if more space is required, then you look for another site in your locality.