Cyclic stress strain response of compacted gravel

The positive effect of artificial densification on the stress strain performance of granular materials is acknowledged from very ancient ages but its important has strongly increased after the development of powerful machineries for the construction of big earth and rockfill dams. It is however pointed out that the large exploitation of gravely soil is rarely supported by a thorough analytical assessment of the compaction effects on the constitutive relationships of materials, as would be desirable considering the massive dimension of these works and the complexity of typical loading conditions. The present research is aimed to fill this gap by means of a detailed experimental investigation and a theoretical analysis on the stress-strain response of dense gravels under monotonic and cyclic loading. The experimental campaign consists of a large number of triaxial tests performed at different initial mean effective stresses, following different stress paths and sequences, on artificially reconstituted large dimension samples compacted at different initial densities. The great care placed in the accuracy of laboratory instrumentation enables a high repeatability of experimental results, necessary to have a clear focus of non-linearity in the limited strain range of the pre-failure response of gravel. Based on curve fitting method the ingredients of an elasto-plastic constitutive model have been defined to predict the response of gravel under monotonic and cyclic loading. Elastic stiffness is simulated with a model derived from literature which assumes a dependency on soil density together with inherent and stress induced anisotropy. Plastic strain development from different initial stress and volume states of gravel is simulated by a Critical State, multiple yielding constitutive model. Hardening and flow rules for this latter have been obtained by modifying previously existing laws in order to better reproduce the observations under monotonic compression, extension and cyclic loading. Validation of the proposed model is finally provided by comparing simulations and experimental results in a variety of testing conditions.