We live in Utah with rocky soil. We will put 6" of sand and then 6" of topsoil on top of the tank. Will this be sufficient for my lawn and plants in the beds to grow?
We are installing a large tank underground. It will be under lawn, under beds filled with shrubs. How much soil on top of tank for plants to grow?
Fine for lawn, though the shallow soil can make issues with accessing deeper water, but lawn in Utah probably implies irrigation anyway, so likely not a large issue. Common to have a brown patch outlining the shallowly-buried tank when under water stress in unirrigated situations, where the rest of the lawn can go deep for water and that over the buried item cannot.
A bit shallow for many shrubs, though there are shallow-rooted shrubs such as blueberries (but that's also a plant that requires special soil conditions or active amendments to keep it happy with sufficiently acidic soils.) Smaller shrubs will generally handle shallow soils better than larger shrubs will.