Interesting that you should point that out. I did a calculation the other night but wasn't brave enough to post it in case my math was off.
One cubic meter of sand covers 10 square meters to a depth of 10cm.
If the top layer is 30cm then 3 cubic meters is needed per 10 meter squared. So roughly (1800 length/10) *20 width)) x 3 = 10,800 cubic meters.
A cubic meter of sand weighs 1682kg.
So 10,800 cubic meters x 1682 kg = 18,165,600 kg or 18,165 tonnes.
I'm sure @barryb will correct me if I'm wrong.