Paper

 Atkinson, J. H. (2000). Geotechnique 50, No. 5, 487±508

Non-linear soil stiffness in routine design
J. H . AT K I N S O N Ã Soil stress-strain behaviour is highly non-linear and this has an important in¯uence on the selection of design parameters for simple routine geotechnical calculations. Non-linear behaviour can be characterized by rigidity and degree of nonlinearity and these can bedetermined 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 in¯uenced by initiation of shear bands. The important non-linear stiffness parameters for soil arerelated to its composition and to its current state. Back-analyses of the load settlement behaviour of fullscale and model foundations demonstrate the in¯uence 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 simplemethod for routine design which takes account of soil non-linear stiffness.
KEYWORDS: Design footings/foundations; in situ testing; laboratory tests; settlement stiffness

 à Le comportement contrainte-deformation du sol est extreme  ment non lineaire et ce fait in¯uence considerablement le choix Á  des parametres de design pour les simples calculs geotechni à ques de routine. Lecomportement non lineaire peut etre       caracterise par la rigidite et le degre de non linearite, deux Á à   Á parametres qui peuvent etre determines en mesurant une tres     faible deformation-rigidite, la resistance maximale et la deforÁ Á  mation a la rupture. On trouve une tres faible deformation   rigidite en mesurant la velocite de l'onde de cisaillement in situ  ou dans lesessais en laboratoire. La resistance maximale et la  Á à  deformation a la rupture peuvent etre mesurees par des essais  de routine en laboratoire mais sont fortement in¯uencees par Á l'initiation de bandes de cisaillement. Les parametres impor   Á tants de rigidite non lineaire pour le sol sont lies a sa composiÁ   tion et a son etat actuel. Des retro-analyses du comportement de tassement pourdes fondations grandeur nature et des  maquettes montrent l'in¯uence du comportement non lineaire  d'un sol. Les variations de rigidite en fonction du tassement,  Á  calculees d'apres des fondations reelles et des maquettes, Á    correspondent bien a la rigidite non lineaire du sol basee sur     Á la raideur et le degre de non linearite. Ces resultats suggerent    une methode simplepour les etudes de routine, methode qui   tient compte de la rigidite non lineaire du sol.

In the Autumn of 1969 when I started research at Imperial College on soil stiffness I had three textbooks. These were: Soil mechanics in engineering practice (Terzaghi & Peck, 1948), The measurement of soil properties in the triaxial test (Bishop & Henkel, 1957) and Critical state soil mechanics(Scho®eld & Wroth, 1968). As a young research student it was dif®cult to understand that these three books were all dealing with soils in ground engineering. Two important themes of my work have been to try to clarify the principal issues covered in these three books and to research soil strength and stiffness. It is now well known that the stress±strain behaviour of soil is highly non-linear and soilstiffness may decay with strain by orders of magnitude. This means that for a geotechnical structure such as a foundation, retaining wall or tunnel, soil stiffness varies both with position and with loading. Many aspects of non-linear soil stiffness are now well understood. They have been incorporated into numerical models and have been used with success in geotechnical design. Many of these...