Hypoplastic constitutive modelling of the wetting induced creep of rockfill materials

Abstract

Discrete element simulations of one-dimensional compression of breakable granular assemblies were performed to investigate the capability of the exponential compression equation suggested by Bauer. The relationship between the so-called solid hardness and the particle strength was studied so as to provide a physical background for the introduction of a time-dependent solid hardness. A hyperbolic flow rule, describing the relationship between the inclination of the strain path and the stress ratio during wetting, was proposed based on typical triaxial wetting experiments on two different rockfill materials. The flow rule was then extended and incorporated into the transformed stress based hypoplastic model to capture the direction of creep strains. Meanwhile, a new density factor was introduced to the extended model to take into account the dependence of the magnitude of creep strains on the packing density. The stiffness tensor given by the extended model was discussed and the flowchart for the integration of the constitutive equation was designed. The extended model was then embedded into a finite element program and used to simulate the triaxial compression and wetting experiments performed on the aforementioned rockfill materials. Good agreement between the model predictions and the measured results lends sufficient credibility to the extended model in reproducing the stress-stain behaviour under loading and the creep behaviour during wetting. The extended model and the finite element program were also used to investigate the deformation behaviour of an earth-rock dam at the end of construction and during first impounding. The familiar phenomena such as the wetting induced settlement of the upstream shell and the movement of the dam crest towards the upstream were successfully captured by the numerical model, which confirms the feasibility of applying the extended model to dam engineering in the future.