I made a 3 x 3 metre by 5 mtr deep soak away in 9 inch walled hole of rough mortared brickbats .
They came in a 20 tonne tipper lorry full of rubbish/ rejected bricks for £ 55 , I sorted them by hand to get anything resembling a whole brick .
Stacked them handy for building the walls
Found someone with a decent JCB & back act to dig the sloping sided hole .
The broken stuff went in the bottom of the hole to make about 2 foot of rubble , broken concrete & other farm inert rubbish all set on top of about 20 empty dumpy bags from a local farmer .
When reasonably flat I then mixed a foot of concrete poured & poured it in over some old chain link fencing .. using some cut lengthwise blue barrels bolted together & staked to the solid ground away from the lip of the hole to make the delivery chute. Used a long pole with a " T" bar securely screwed on the end to roughly level & tamp the wet concrete , then left it alone for a month by which time the hole was about 1.5 mtr deep in water .. that concrete will have set like steel.
Meanwhile I got four eight foot length of 30 mm hollow box tube , marked in pencil first Then nicked them on both sides at every bricks width & 10 mm with a thin blade in the angle grinder to make a brick profile gauge
/guide & drilled several holes that would correspond with middle of each mortar joint so that when I'd carefully built the corners square to a height of about a metre , it was just a case of putting the profile up on plugs & screws once they had set at about day five .
Then string a line round the whole tank and build up till the top over a week end. Letting it dry for a day or three I back filled the hole on the outsides so I could work at an easy height over stages of every 2 feet up and move the profiles up the corners again.
Once walling was about four feet high I put up a simple platform of wood on top of 310 litre blue barrels in middle of the tank & boarded it out to the sides so I had an internal platform to play on . It made bringing in more bricks on the pneumatic tyred sack barrow much easier , the same with wheel barrows of mortar . It only took six evenings to build to the finished height of a foot below the mean ground level.
As I built the wall ,in one corner where the access manhole was going I set in wall steps every six courses alternating as a left then a right one 400 mm apart , so that if needed someone could climb in or out the tank when it was finished .
I'd already made provisions to trench in a 40 mtr long black 120 mm perforated ground drain to a lower point on my land to take away the surplus water in winter & brought it in through the wall at about 1000 mm deep , the rainwater filling pipework was also set in place at the same level in the input trench. Both were set on a foot bed of pea grit , covered with the same to another foot then I laid on lots of strips of cut up dumpy bags to help keep the soak away run clean for 30years or so , then finally back filled the run with earth &topped it with the original turf
Around the rim of the square tank I laid a pre-used 3/4 thick plywood roof over it using old floor joists at 400 centers on new galv wall hanger plates , made an outer screwed on frame with a 9 inch lip which overlapped the wall by about four inches , set in a manhole cover on some glued down polystyrene blocks & hole cut in an extraction float level indicator tube made from some foul water pipe and a collar with a top & bottom spigot /pipe . Laid down two 6 mm dia 100 x 150 square cut to shape rusty weld mesh reinforcer mats . Set these mats on some cut length wise through the middle three hole brick standoffs , wired them together in a decent overlap and then poured something like 30 barrow loads of nice and sloppy home made concrete on it , got my lass to help me tamp it flat.
Over the next few evenings i set up all the rainwater piping then filled & turfed the trenches
After two weeks I lifted the manhole cover and used the drill & jigsaw to remove the ply under the manhole , was quite surprised to see the tank only about 2 mtrs deep in water .
A weeks or later it was full to the soak away point . I set up a float controlled submersible pump , bringing the outlet & power cable up the float rube used the water collected and seeped in water for all the farm equipment washing every week end didn't realise ti at the time that I would eventually be using about half a tank each w/e .
I'm so glad I took the time and money to do the tank and soak away , for we had a big scare from the water board when it was announced on mobile loud speakers that clostridium bacteria had been found present in the local water supply & we were not to drink any boiled or not, treated or not , as it's a very difficult bacteria to kill off once it's in the supply , We had to wait till the mains pipes had been flushed , our water pipes were also flushed & tested then a few days later we were given the all clear .
Mast farmers were tanking in water from elsewhere to give to their stock , I just used the rainwater out my tank & pumped it up into the big header tanks in the sheds via a simple hang over the side ballcock on long hose pipe arrangement after tying off the normal plumbed in ballcocks to stop the waterboards contaminated water coming in .
I nearly forgot ...... I used two campers chlorine purifier tablets a day on each of the sheds header tanks , to knock any nasties in the header tanks on the head by dissolving the tablets n a bucket of warm water and tipping that in the header tanks each evening just before I started the submersible pump & filled them up from the big tank rain water tank.
Once the grass had grown in the soil which was finally finished just above level with the top of the tank lid I put some 900 mm raised beds made of heavy block work on the tank cap in an L shape , on looking at it , it looked as though I'd made a slab of concrete and put a fancy flower bed around two edges of it.