If you are installing a rainwater tank, a ring wall could be a good way to elevate the tank. Elevating the tank makes it easier to get the water out - just make sure the water can get into the tank. It can also be necessary if you are installing a rainwater tank in a challenging location. First, make a template of the rainwater tank base.
I have used 500 mm deep limestone blocks to be extra sure the blocks won't sink into the sand under the weight of a full tank. This will depend on various factors including the soil geology, the height from ground level to top of ring wall and the thickness of the blocks. Your local government or the tank manufacturer may be able to help with what size blocks to use.
Make sure there is nothing breakable/unstable within the active footprint of the rainwater tank base. The active footprint increases in size with depth - at a 45˚ angle. So for example if you are connecting the overflow pipe to a pit, and that pit is 500 mm deep, it will need to be 500 mm away from the tank.
Start digging to the required depth.
Use a spirit level, with a straightedge like a wooden beam if it's not long enough, to measure the hole depth - shown with 1 in the image. Make sure this beam is the same as the diameter of the rainwater tank - nearby paths can have a slope. (My beam only extended across the hole, which caused me great annoyance later on!)
Also make a beam (shown behind the two red bricks) at least as long as the tank diameter to the correct level along its whole length. Shown with 2 in the image.
The 'active footprint' also applies to any nearby buildings, also 45˚. Avoid digging into it if you can, if you can't avoid, minimise the extent and be mindful you will need to get the ring wall blocks in quickly to minimise the risk of collapse. Avoid column footings especially.
Put a straightedge like a star picket at the required depth. This will be needed for screeding to the right level.
Using the spirit level, hammer in at least two pegs at the base level of the ring wall - one each side of the straightedge. The more pegs, the easier it will be to screed.
Start screeding the base using the peg and the partly buried straightedge. I used a wooden beam.
Using the template, make sure the hole is big enough.
Time to start putting some blocks in! Wear steel cap shoes and safety first. Get help if required. Use the beam to make sure it's at the right level and the spirit level to make sure it's plumb.
If too high, use a trench digger, trowel or similar to undermine and wiggle it until it's at the right level. If too low, lever it up higher and hammer sand under it with a wooden beam or similar. You may need a combination of techniques depending on how much sand is behind the block, including lifting it. Keep checking position of the blocks with the template.
Backfill with builder's sand. You can purchase it or you may be able to find some.
First, fill to the surrounding ground level.
Either mortar the gaps between the blocks or use garden edging to cover the gaps. I went with garden edging.
Wet the soil, either with a hose or try directing rainwater onto it. Compact it by jumping on it. Finish filling and make it mounded slightly higher than the ring wall just in case. Add some sand on top of the blocks to help with levelling.
Next is placing the rainwater tank. Then enjoy using sky water 😃