Rate effects on elastic and inelastic stress-strain behaviours of geomaterials observed in experiments

Fumio Tatsuoka
Department of Civil Engineering
Tokyo University of Science

Essentially rate-dependent and irreversible stress-strain behaviour(i.e., elastic behaviour) of geomaterial, including granular materials, can be observed only at small strain levels, say less than 0.001 %. It is shown that the strain range in which the stress-strain behaviour is essentially elastic decreases with a decrease in the strain rate, while the secant stiffness approaches an upper bound (i.e., the elastic modulus) with an increase in the strain rate. This complicated trend can be modelled by a linear three-component model. The compliance matrix for elastic deformation, which may be inherently anisotropic and becomes more anisotropic as the stress state becomes more anisotropic, is described.  

<>As the stress state approaches the peak state and then the residual state, the rate effects on the stress-inelastic strain behaviour (i.e., viscosity) becomes significant and very complicated, while the effects of particle properties (i.e., particle shape, particle strength and stiffness, grading, inter-particle bonding ....) and strain level are systematic. The viscosity evaluated by stepwise changing the strain rate and performing sustained loading during otherwise monotonic loading (ML) at a constant strain rate and also by performing ML tests at different constant strain rates showed that there are four basic viscosity types, Isotach, Combined, TESRA and Positive & Negative. The Isotach type is the most classical one and, in the case of ML, the current viscous stress component is a function of instantaneous irreversible strain and its rate. So, the strength during ML at constant strain rate increases with an increase in the strain rate. With the TESRA type, the viscous stress increment that has developed at a given moment decays with an increase in the irreversible strain during subsequent ML. So, the strength during ML at constant strain rate is rather independent of constant strain rate. The Combined type is in between the Isotach and TESRA types. The P &N type is the latest finding and most peculiar: i.e., the strength during ML at constant strain rate decreases with an increase in the strain rate, while the viscous stress suddenly increases upon a strep change in the strain rate, followed by fast decay of the viscous stress component during subsequent ML at constant strain rate, as the TESRA type. The viscosity type of a given granular type under given initial conditions tends to change from the Isotach type toward the P&N type as the particle shape becomes less angular, as the grading becomes more uniform, as the inter-particle bonding becomes stronger and as the strain becomes larger. A general mathematical expression that can describe all these four types and transitions with strain among them is proposed. The effects of ageing on the elasto-viscoplastic stress-strain properties observed in experiment are described and simulated by a constitutive model.

[introduction from 2000 Burmister Lecture at Columbia University, will be updated]
Prof. Fumio Tatsuoka is the Professor of Geotechnical Engineering at the Department of Civil Engineering, University of Tokyo.  His major contributions in geotechnical research include, soil dynamics, laboratory stress-strain testing, soil and rock behavior, soil constitutive modeling, geosynthetics, reinforced soil structures, among others.  He is the author of numerous technical papers and monographs, including several state-of-the-art and keynote papers. He was awarded the best paper award for young researchers (1974), the best paper award (1986) and the best technical contribution award (1994) from the Japanese Geotechnical Society; the 1994 IGS award the 1995 best paper award form the Japanese Society for Civil Engineers; the best paper award of Ground Improvement Journal (1997) from ISSMFE, the 1996 and 2000 Hogentogler Award from ASTM and the 1996-1997 Mercer Lectureship from ISSMFE.  He served as the Editor in Chief; Soils and Foundations from 1993 to 1999.  He is currently the editorial board member for Geotechnical Testing Journal, ASTM, Geotextiles and Geomembranes, IGS, Geosynthetics International, IGS and Ground Improvement, ISSMFE Chairman of TC 29 on Laboratory Stress-Strain Testing Method, ISSMGE, and Council member of IGS. Prof. Tatsuoka served as an engineering consultant to numerous governmental committees and construction projects, including the Akashi Strait Bridge Project and Trans-Tokyo Bay Highway Project.