Converting tonnes of contaminated soil to cubic meters
The density varies greatly depending on whether it is dry or wet, and organic-rich topsoil vs sand and clay. If dry, perhaps 1.0 - 1.6 t/m³, if wet, 1.6 - 2.0 t/m³.
If you need any precision, you probably need to weigh a sample of known volume. For rough estimate, pick somewhere in the ranges above, as appropriate, and divide the number of metric tons by that density to get cubic meters.
Dry compacted in-situ clay will vary dependant on mineral composition, however, generally it is about 1750 lbs per cubic yard. Converting cubic yards @ (1750lbs/cyd) X 1.30795 = 2,414 lbs per cubic metre clay.