Hypoplasticity for sand enhanced for modelling cyclic and fabric effects

A recently published hypoplastic constitutive model is presented that combines novel and already established advanced constitutive theories to realistically capture the mechanical behaviour of sands under monotonic and cyclic loading. The proposed model, published in [1] and further called HP+GIS+ACST, is based on the hypoplastic formulation after von Wolffersdorff (HP) [2] and motivated by Yang et al. [3]. It integrates two major theoretical extensions: (A) the Generalized Intergranular Strain (GIS) concept [4] for capturing cyclic and strain accumulation effects, and (B) the Anisotropic Critical State Theory (ACST) [5] to account for an evolving anisotropic fabric and its impact on the conventional measurable macroscopic soil behaviour. The model uses six internal state variables, including the effective stress, the void ratio, a fabric tensor, the intergranular strain tensor, and two scalar variables memorizing the cyclic and dilative preloading.

To validate the model’s predictive capabilities, element simulations are performed and compared against experimental results from cyclic and monotonic tests on Karlsruhe fine sand (KFS) [6, 7] and Fraser River sand [8]. ... mehrTo demonstrate the prediction quality of the HP+GIS+ACST, additional comparisons with the widely used hypoplasticity after von Wolffersdorff [2] with the Intergranular Strain extension [9] (HP+IS) are presented. The novel HP+GIS+ACST resolves key limitations of earlier hypoplastic models. Figure 1 shows, as an example, the eliminated issue of the so-called overshooting of the Asymptotic State Boundary Surface (ASBS) due to an un- and reloading using a drained triaxial test. Additionally, as shown in Figure 2, the HP+GIS+ACST reproduces cyclic soil liquefaction in undrained triaxial experiments in loose and dense sands. Simulations with the widely used HP+IS model show qualitatively and quantitatively inaccurate curves.

This contribution illustrates how integrating established theoretical concepts into the hypoplastic framework can substantially improve our ability to model realistic soil behavior under challenging loading conditions. The resulting HP+GIS+ACST represents a next step in the evolution of hypoplastic constitutive models. Note that implementations (umat.for and udsm.dll) are available from the author.