49.
Apart from time-driven creep or relaxation, most viscoplastic models (without plastic and viscous strain separation) generate
no or a very limited accumulation of strain or stress due to cyclic loading. Such pseudo-relaxation (or pseudo-creep) is either
absent or dwindles too fast with increasing OCR. For example, the accumulation of the pore water pressure and eventual liquefaction
due to cyclic loading cannot be adequately reproduced. The proposed combination of a viscous model and a hypoplastic model
can circumvent this problem. The novel visco-hypoplasticity model presented in the paper is based on an anisotropic preconsolidation
surface. It can distinguish between the undrained strength upon triaxial vertical loading and horizontal loading. The strain-induced
anisotropy is described using a second-order structure tensor. The implicit time integration with the consistent Jacobian
matrix is presented. For the tensorial manipulation including numerous Fréchet derivatives, a special package has been developed
within the algebra program
MATHEMATICA (registered trade mark of Wolfram Research Inc.). The results can be conveniently coded using a special
FORTRAN 90 module for tensorial operations. Simulations of element tests from biaxial apparatus and FE calculations are also shown.
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