Ta-Ger constitutive model for clays under monotonic and cyclic loading

Abstract

This paper presents Ta-Ger constitutive model for clays that has been implemented in the finite difference codes FLAC and FLAC3D. The model is based on a theoretical framework in multiaxial space combining perfect plasticity with Bouc-Wen-type smooth hysteresis through the formulation of an explicit elastoplastic matrix, which is able to provide a continuous function between input (displacement, strain etc) and output (force, stresses etc). This plasticity framework was originally proposed by Tasiopoulou and Gerolymos (2016a) and was used to develop Ta-Ger constitutive model for sands (Tasiopoulou and Gerolymos, 2016a,b; Tasiopoulou et al., 2019, 2021). In this paper, the same framework is used to develop Ta-Ger constitutive model for soils with clay-like behavior under monotonic and cyclic loading. The fundamental capabilities of capturing clay-like behaviour are illustrated through element-level comparisons of the calibrated model with monotonic and cyclic laboratory tests conducted on near-normally and overconsolidated clays from Izmit Bay, Türkiye. Key aspects of clay response captured by the model include: i) monotonic hardening, ii) strain-dependent hysteretic responses at different shear strain levels in terms of realistic stress-strain loops, secant shear modulus and strength, iii) strain-dependent shear modulus degradation and hysteretic damping, iv) strain-dependent cyclic degradation as a function of the number of loading cycles, v) post-peak shear strength decrease to a residual value with increasing shear strain, vi) ratcheting caused by asymmetric loading. The paper also presents system-level validation against centrifuge tests simulating the seismic wave propagation through soft, normally consolidated clays encountered in the San Francisco Bay under strong shaking inducing significant nonlinearity.