In most terrestrial near-surface situations the soil is unsaturated. Why is it then that soil mechanics theory developed for saturated soil is being used to interpret soil test results and for the design of footings and earthworks in unsaturated soil? A possible reason is that some of the theory and testing are too complex and time consuming for practical use. This is to be regretted because the attempt to use theory developed for saturated soil for situations where the soil is unsaturated leads to confusion and conservative design. This paper outlines recent findings about those factors that determine the shear strength of unsaturated Keswick clay which occurs in the Adelaide Metropolitan area. It transpires that it is not difficult to determine these factors and hence shear strength. This paper also indicates how to use this information for the design of footings and earthworks.
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