A little job I did for some Kin about 10 years ago. What we did was a cast in situ tank the walls were 4m x24m x.3m,the tank was 5m wide to center of the wall.It was capped with precast concrete slats and is used to store cow faeces.
So basicly you made a septic tank or cow poo, nice! Over here across the pond/lake/ocean whatever you want to call it, we just dig a hole in the ground, and let it (cow poo) sit there.......
We used to do that here but our friendly personages of the local authority no longer allow that as we are no distance from a water source ie a river,as a nipper when we were increasing the national herd this river was so polluted that for most of the summer it had a rank pungent smell,the water was a dark green and the sight of mature rainbow trout floating down it I can still remember.In the last couple of years they introduced more rules and now a farmer has to have enough storage for nearly six months. I think I'll leave it at that....,at that depth we were in rock but only had to break out for less than than 12.5% of the tank,the JCB had excavated the rest of it unaided.