Taking typical soft rock-muddy siltstone as research objective

the unloading triaxial rheological tests by constant axial stress and unloading lateral stress by steps are performed under different stress levels. According to experimental results

the axial and lateral rheological deformations are very significant and have strong aeolotropy. The axial rheological deformation is larger at higher confining pressures; while the development of lateral deformation is more rapid than that of the axial direction with the decrease of confining pressure. At failure confining pressure

the lateral deformation of sample is much more larger than that of the axial direction. After detailed research and analysis of the unloading rheological test results

the linear rheological Burgers model can preferably present attenuation and steady rheological stages of rheology curves at different confining pressures; but it can not describe nonlinear accelerative rheological stage at failure confining pressure. Based on the above analyses

a nonlinear damage rheological model is proposed. By taking the least square embedded in Levenberg-Marquardt algorithm

the test data curves are fitted to obtain the nonlinear rheological parameters of soft rock. By comparing with the test data

the fitting curves are well fitted with test curves. It is shown that the rheological model can preferably present unloading rheological curve characteristics of soft rock at various stages.