shed tank stand footings questions - tie to shed slab?

G'Day,



hoping someone here knows how to work these questions out.....



Background-

I am planning on a tankstand to support a 12KL rainwater tank - the footprint is in the 3mx3m area. It will be located in an internal corner of the shed - Office behind it, main shed beside it.

The tank stand will be paved underneath and the space used for storage.



Questions:

How do I calculate the size of my footings? The soil is clay/gravel, and the water table is 1.5m below NGL. The shed has to be erected before the tank stand, but I can pour in footings/slab as part of the shed pour if it helps.



Should the tank stand footings/slab be connected to the shed slab? - the shed slab will be edge thickened, but then minimum 100mm thick on the office side, and 150mm thick on the workshop side. - the slab under the tank would be min 100mm thick for storage purposes, more if it was needed to form the tank stand footing.



The slab for the tank stand will be about 75mm lower than the shed floor so the shed retains its waterproofing, and the tank slab will have a slight slope to it for drainage.



Thanks,

Des