We worked on a residential development in the Hawkesbury Sandstone belt where the top 3 metres of profile were completely unsaturated, yet seasonal moisture changes triggered differential movement in the slab. That project taught us that ignoring the unsaturated zone — the layer above the water table where capillary forces still hold water — can lead to expensive remedial works later. In Sydney, where the annual rainfall hovers around 1,200 mm but summer dry spells are intense, the upper soil mantle undergoes repeated cycles of wetting and drying. We use tensiometers and filter paper tests to measure matric suction, then combine that data with cimentaciones-superficiales design checks to see whether the footing will ride the seasonal heave.
Unsaturated soils in Sydney can lose up to 80 % of their shear strength when suction drops below 50 kPa after prolonged rainfall.