# A finite-strain hyperviscoplastic model and undrained triaxial tests of peat

## TL;DR

This paper develops a finite-strain hyperviscoplastic model for peat, incorporating rate-dependent and hysteresis effects, and validates it against undrained triaxial tests at multiple strain rates.

## Contribution

It introduces a thermodynamically consistent hyperviscoplastic model for peat and demonstrates its effectiveness through experimental validation under undrained conditions.

## Key findings

- Model accurately simulates undrained triaxial tests at various strain rates.
- Experimental data supports the model's ability to capture hysteresis and rate effects.
- Model parameters related to solid matrix properties were successfully fitted.

## Abstract

This paper presents a finite-strain hyperviscoplastic constitutive model within a thermodynamically consistent framework for peat which was categorised as a material with both rate-dependent and thermodynamic equilibrium hysteresis based on the data reported in the literature. The model was implemented numerically using implicit time integration and verified against analytical solutions under simplified conditions. Experimental studies on the undrained relaxation and loading-unloading-reloading behaviour of an undisturbed fibrous peat were carried out to define the thermodynamic equilibrium state during deviatoric loading as a prerequisite for further modelling, to fit particularly those model parameters related to solid matrix properties, and to validate the proposed model under undrained conditions. This validation performed by comparison to experimental results showed that the hyperviscoplastic model could simulate undrained triaxial compression tests carried out at five different strain rates with loading/unloading relaxation steps.