The natural surface of ground is not always level and mostly to such an extent that there may be a considerable slope either across or along or both across and along the site of most buildings.
Where the ground floor is to be at or just above ground level at the highest point, it is necessary to import some dry fill material such as broken brick or concrete hardcore to raise the level of the over site concrete and floor. This fill will be placed, spread and consolidated up to the external wall once it has been built.
To economize and expend less in excavation and foundation walling on sloping sites where the subsoil, such as gravel and sand, is compact it is practice to use a stepped foundation, which contrasts diagrammatically the reduction in excavation, and foundation walling of a level and a stepped foundation.
A footing in which the desired width is secured by a series of steps in about the proportion of one unit of horizontal dimension to two units of vertical dimension A foundation constructed in a series of steps that approximate the slope of the bearing stratum.
The purpose is to avoid horizontal force vectors that might cause sliding. On sloping sites an initial decision to be made is whether the ground floor is to be above ground at the highest point or partly sunk below ground.
The foundation is stepped up the slope to minimize excavation and walling below ground. The foundation is stepped so that each step is no higher than the thickness of the concrete foundation and the foundation at the higher level overlaps the lower foundation by at least 300 mm.
The load bearing walls are raised and the foundation trenches around the walls back-filled with selected soil from the excavation. The concrete over site and solid ground floor may be cast on granular fill no more than 600 mm deep or cast or placed as a suspended reinforced concrete slab. The drains shown at the back of the trench fill are laid to collect and drain water to the sides of the building.
source: iamcivilengineer.com
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