site - won spoil from an excavation which is a predominantly non - granular cohesive material can be converted into a material suitable for immediate use as a backfill material by a process of mechanically mixing the spoil with between 0 and 30 % by weight or volume of added granular material and between 1 and 10 % by weight or volume of a powder material comprising 1 - 5 % lime or with the additon of 1 - 5 % cement and / or 1 - 5 % pulverised fly ash . the process works well with clay soils and the range of suitable spoil can be extended by adding cement to the powder material . the proportions in the mixture are chosen in dependence on the clay and moisture content of the spoil . a machine which can be set to mix the ingredients and water in the correct proportions on site is also described .