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Instability of earth structures and yielding and flow in soils due to earthquake loadings may be traced quite often to the problem of large displacements directly relatable to the resultant dynamic load condition occurring in the soil. This study examines the problem of large strain performance of clays under impulsive-type loadings, where the ultimate objective is to assess material behavior with a view to specification of pertinent and significant material parameters.
The formulation of a flow rule relying upon a base value at particular strains and strain-rates and additive strength performance is shown, and further study of the contribution to strength arising from increased strain-rates is examined. At the higher rates of sample straining, the significant increases in strength of the clay soils cannot be totally accounted for by linear extrapolations of rate effects and the contribution from coupled inertia terms. It is shown that these contributions, can be due to nonlinearity in rate effects at the higher end of the strain-rate spectrum, coupled with the formation of a quasi shock front. It appears that the rate of impulsive loading is higher than the ability of the specimen to deform plastically in the region adjacent to the loading front. Hence, a moving shock front is created and significant increases in strength are realized because of the change in material properties in the presence of this moving shock front.
In the problem of loading arising from impulsive mechanisms, instability in yield surfaces can be realized, and hence the problem of definition of a critical time-dependent yield surface as a function of strain-rate becomes apparent. This concept is also examined and discussed in this paper in relation to the previously described material characteristics.
clay soils, earthquake, dynamic loads, compression tests, soil mechanics, soil dynamics, earth (soil), tests, evaluation
Professor of Civil Engineering and Applied Mechanics and Director, Soil Mechanics Laboratory, McGill University, P.Q.
Assistant professor, McGill University, Montreal, P.Q.