The ground as permanent formwork
Slabs for roads, buildings etc may be cast directly on the ground. For buildings, a membrane, such as polythene, will be laid on the ground first to prevent the bottom layer of the concrete from being contaminated and to act as a damp-proof layer.
Similarly the soffits (and perhaps the sides) of ground beams and foundations may be cast directly against the ground. (See also Structural elements/foundations/trench fill foundations.) In bored piles the ground acts as the formwork for the whole of the structure (see also Structural elements/foundations/pile foundations.)
When using the ground as formwork two factors need special attention; the sides of concrete elements difficult to excavate to the required dimensions, and the outer layer of concrete may be contaminated with the soil. In such cases care should be taken in determining load-carrying capacity of the finished structure.