Site Classification and Footings for Residential Construction

About this course.

The ground matters because a house does not float: every load from the roof, walls, floors, fittings and occupants is ultimately carried down through the footings and into the soil. If that soil is stable and strong, the footings stay put and the building stays square. But much of Australia's ground is reactive — clay-rich soils swell as they take up moisture and shrink as they dry — and this movement can be large enough to lift, drop or distort footings unevenly across a site. When different parts of a footing system move by different amounts, the building above cracks, racks and jams. Footings usually fail not by being crushed, but by moving.

To understand this, it helps to picture the load path. Gravity loads accumulate downward through the structure — roof to walls, walls to floor and footings, footings to soil. The footing's job is to spread those concentrated loads over enough ground area that the soil beneath is not overstressed, and to tie the building together so it behaves as one unit. Two soil-related things can go wrong. The soil can fail to carry the load, settling as it compresses (a bearing or settlement problem), or the soil can change volume on its own as its moisture content changes, pushing the footing up or letting it drop (a reactivity or movement problem). In Australian residential construction, reactive-soil movement is the dominant concern across large parts of the country, and it is the main reason site classification exists.