Abstract: 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.