This invention provides a method of preparing a foundation structure including the steps of forming foundation trenches in the ground; and heating the ground defining the trenches sufficiently to irreversibly change the properties of the ground defining the trenches. In the case where the ground consists primarily of clay, it is heated to a temperature above a level where the ground is baked into a brick-like consistency, wherein the structural water of crystallisation is removed and any carbonaceous material and Fe is oxidised. If the ground consists primarily of sand, it is heated to a temperature above its vitrification point wherein mullite (AI6Si2O13) begins to form. The method includes the further step of pre-treating the ground by adding an additive such as potash or slaked lime to the soil, prior to the step of heating.