Non-linear soil stiffness in routine design

Soil stress-strain behaviour is highly non-linear and this has an important influence on the selection of design parameters for simple routine geotechnical calculations. Non-linear behaviour can be characterized by rigidity and degree of non-linearity and these can be determined from measurements of very small strain-stiffness, peak strength and failure strain. Very small strain-stiffness can be found from measurements of shear wave velocity in situ or in laboratory tests. Peak strength and failure strain can be measured in routine laboratory tests but are strongly influenced by initiation of shear bands. The important non-linear stiffness parameters for soil are related to its composition and to its current state. Back-analyses of the load settlement behaviour of full-scale and model foundations demonstrate the influence of non-linear soil behaviour. Variations of stiffness with settlement calculated from full-scale and model foundations agree well with non-linear soil stiffnesses based on rigidity and degree of non-linearity. These results suggest a simple method for routine design which takes account of soil non-linear stiffness.