AVISA: anisotropic visco-ISA model and its performance at cyclic loading

Abstract

In this work, a constitutive model able to capture the strain rate dependency, small strain effects and the inherent anisotropy is proposed considering the influence of the overconsolidation ratio (OCR). Small strain effects are captured by using an extended ISA plasticity formulation (Fuentes and Triantafyllidis in Int J Numer Anal Methods Geomech 39(11):1235-1254, 2015). The strain rate dependency is reproduced by incorporating a third strain rate mechanism (in addition to the elastic and hypoplastic strain rate). A loading surface has been incorporated to define a three-dimensional (3D) overconsolidation ratio and to account for its effects on the simulations. Experimental investigations using Kaolin Clay and Lower Rhine Clay with horizontal bedding plane have shown that under undrained cycles of small strain amplitudes (\10 À4), the effective stress path in the p-q space is significantly inclined towards the left upper corner of the p À q plane. Consequently, a transversely (hypo)elastic stiffness has been successfully formulated to capture this behaviour. The performance of the model has been inspected by simulating the database of approximately 50 cyclic undrained triaxial (CUT) tests on low-plasticity Kaolin Clay (Wichtmann and Triantafyllidis) considering different deviatoric stress amplitudes, initial stress ratios, displacement rate, overconsolidation ratio and cutting direction. Furthermore, 4 CUT tests conducted on high-plasticity Lower Rhine Clay were simulated, whereby the influence of the displacement rate, as well as the deviatoric stress amplitude, has been analysed. The simulations showed a good congruence with the experimental observations.

A simple hypoplastic model with loading surface accounting for viscous and fabric effects of clays

“…In the isotropic axis, the simple relation OCR=p ei /p is adopted. For deviatoric stress states, a similar schema to the one proposed in previous works, 28,30,31…”

“…The behaviour of clays exposed to cyclic loading remains also to be developed in future work. Basic principles of this model are, however, given in previous works 28,37 based on the ISA plasticity.…”

“…Up to now, a relatively thorough understanding was implemented in constitutive models for time-independent fine-grained soils. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Some of these works are restricted to either normally consolidated 22 or overconsolidated 21 fine-grained soils. Some models need various parameter sets for different overconsolidation ratios, which for the users is very confusing.…”

“…Some models need various parameter sets for different overconsolidation ratios, which for the users is very confusing. In order to capture the strain-rate dependency typical for plastic clays, different models [27][28][29][30][31][32][33][34][35][36][37] have been developed. The majority of recent works in this field have opted for the viscoplastic strains to replace the original plastic strains in order to describe the time and strain rate effects observed in experiments with soft clays.…”

“…Up to now, a relatively thorough understanding was implemented in constitutive models for time‐independent fine‐grained soils 14‐28 . Some of these works are restricted to either normally consolidated 22 or overconsolidated 21 fine‐grained soils.…”