The unloading triaxial rheological tests by keeping σ1 and unloading lateral stress or by keeping σ1-σ3 and unloading laterals stress by steps were performed in laboratory with diabase samples taken from Dagangshan hydropower station dam foundation. The rheological deformation characters by the two different stress path were analyzed. The yield approach index (YAI) was introduced to evaluate the close degree from the yield surface of sample in triaxial stress state in the unloading confining pressure rheological process and used as the standard in distinguishing the steady-state creep and tertiary creep in test curves. The results show that in the two modes, the lateral deformation shows both lateral dilasions in process of unloading confining pressure, but the axial deformation characters are different. The sample shows compression in axial deformation when tested with constant σ1 while when tested with constant σ1-σ3 a tiny elongation and gradual transformation to axial compression. With the unloading of confining pressure, the deformation is transferred from elasticity to plasticity, and thus linear elastic theory is no longer applicable. The Burgers model is improved to have variable parameters which take the nonlinear deterioration of viscous coefficient in the tertiary creep stage into consideration. The improved model with variable parameter performs better in describing the transient creep, steady-state creep and tertiary creep in the fitting test.%以大岗山水电站坝基的辉绿岩为研究对象,进行σ1恒定卸围压和σ1-σ3恒定卸围压流变试验,分析2种不同应力路径下辉绿岩的流变变形特征.引入屈服接近度指标来评价卸围压流变过程中不同三向应力状态下岩样临近屈服破坏的程度,并以此作为试验曲线中等速蠕变和加速蠕变的区分标准.最后对Burgers模型进行参数变异处理.研究结果表明:在2种方式下和在卸围压过程中,岩样的横向变形都表现为侧向膨胀,但轴向变形规律并不相同:σ1恒定时轴向变形一直轴向压缩,而σ1-σ3恒定时先是有微小增大再逐渐转变为轴向压缩.这说明随着围压的不断卸载,岩体变形逐渐由弹性向塑性转变并最终破裂,线弹性理论已不再适用.加速蠕变阶段的串联黏滞系数出现了非线性交化,以此来描述加速蠕变.参数变异的Burgers模型可以描述岩体的减速、等速和加速蠕变阶段,拟合结果与试验结果吻合较好.
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