The rapid development of computing power and of numerical modelling software over the past forty years has made sophisticated analysis of geotechnical problems accessible to most practising engineers. Typically computer packages now offer a wide range of constitutive models, which the design engineer needs to choose between, and then obtain parameters for. For those structures designed to be far from failure, for example supporting urban excavations, strains in the ground are small. A sound knowledge of stiffness parameters at small strain is essential if realistic predictions of the ground movements that may affect adjacent buildings or underlying infrastructure are to be made. This lecture reviews what is now known about the complex stiffness behaviour of soil and weak rocks in the context of elasticity, arguably the simplest of constitutive behaviour. Drawing on experience gained through field observation and numerical modelling, the case is made for the routine use of non- linear anisotropic stiffness. The determination of the parameters required is then explored, and the usefulness of advanced triaxial testing, and dynamic laboratory and field testing examined.
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