Filling in septic tank with concrete

[Jimbo]

Hi,

My property has a disused septic tank behind the garage. The cover is badly rusted and the concrete slab which sits above the tank has a crack which I think is a result of settling.

For safety's sake and to stop the slab settling/cracking any more (I want to build a large lean-to shed at the back of the garage and want a stable floor), I want to fill the tank in with concrete.

Are there any clever ways of finding out how much concrete will be needed to fill the tank? I can't budget for the job until I know the volume.

Thanks
J

 

[ColJack]

dig it out..

you've no way of knowing what nasties are under there and putting a shed on top that might trap any gases etc would be a bad idea..

 

[Jimbo]

But surely if I fill it with concrete that will expel any gases? It hasn't been used in 15 years anyway, so any gas should have escaped by now, especially with the rust holes in the manhole cover!

 

[foxhole]

Fill with dry sand and top off last 1-2 feet with concrete.