Over the past 30 years or so the fundamental non-linear stress-strain behaviour of soils and soft rocks has been established by laboratory and in situ tests leading to development of new theoretical models for soil behaviour and complex numerical analyses. But for much day-to-day practice, ground engineers do not need such complex and detailed methods and they commonly employ simple routine calculations. These use either a factor of safety, or load factor, so that movements are limited, or a single stiffness modulus in a simple elastic analysis. The choice of an appropriate load factor or stiffness modulus depends to a great extent on the characteristics of the non-linear stress-strain behaviour of the soil.
Soil parameters which can be measured simply and reliably are the strength and the strain at failure which can be related to initiation of shear bands or slip planes and the stiffness at very small strain which can be determined from measurement of the velocity of shear waves. These parameters lead to a simple concept of degree of non-linearity which guides choice of load factor or stiffness for routine design.
The lecture explored how new knowledge of non-linear soil stiffness and development of relatively simple soil tests can lead to logical choices of load factor and stiffness modulus for use in routine analysis of common geotechnical constructions.
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