Looks like a fair amount of work Norm to fix this tank from floating..
"we want to put in a couple of hundred meters of trenches and fill them with scoria."
I have scoria under my drive here with a slotted drainage pipe in the scoria then it connects to the storm water drain but I also have 3 Drain boxes and a drainage channel in front of the roller door.
The drainage boxes and the drainage channel work a lot better than the scoria with the slotted pipe.
The only place the water would build up here is where the scoria is ,as the water doesn't get away quick enough so in that spot I have an open concrete drain channel now that takes the water to a drain box.
If you have a lot of water run off then you could put some drain channels around the outside of the water tank and if there is a lot of water run off from next door you may need to divert the water into a drain ,you can use a drainage channel or those concrete half channels that take the water to a drainage pit.
I wouldn't have thought you would need to pump the water, most times you just dig the trench so the water flows down hill but if that's not possible I guess you need to pump the water.
The other way to have a tank out of the way is if your house isn't on a concrete slab, a rubber bladder tank can be used under the house .
Once you get the concrete poured over and around the tank hopefully that will be enough to weigh the tank down Norm.
