考虑先期卸荷静偏应力的花岗岩残积土动力特性研究

Dynamic characteristics of granite residual soil considering pre-unloading static deviatoric stress

先期静偏应力对土体的动力特性具有重要影响,以往的研究多侧重于加载引起的静偏应力,考虑先期卸荷作用影响的土体动力性质试验研究还很少见。围绕先期卸荷条件下原状花岗岩残积土的动力响应特征,开展了一系列包含先期卸荷阶段的动态三轴试验;基于土体强度包络线、试样固结及卸荷后应力状态三者之间的位置关系定义了卸荷度,用于表征实践中受诸多因素影响的土体应力释放程度,卸荷度反映了卸荷土体距离破坏状态的相对远近;根据试验结果分析了动应力幅值和卸荷度对土体累积塑性变形和动强度的影响规律。结果表明:随着动应力幅值的增大,轴向塑性变形累积模式加速向“破坏型”发展;先期卸荷作用助长了花岗岩残积土的轴向累积塑性变形,间接表现为临界动应力幅值随卸荷度的增大而减小,且此种助长效应随卸荷度的增大呈非线性(加速)增强趋势;相同循环周次下,花岗岩残积土的动强度随卸荷度的增大而减小。在此基础上,以临界动应力幅值为归一化因子,发现不同卸荷度下土体的动强度曲线可归一化为唯一的曲线,据此建立了可考虑先期卸荷程度影响的花岗岩残积土动强度表达式。

Pre static deviatoric stress has an important influence on dynamic characteristics of soil.Previous studies mostly focus on static deviatoric stress caused by loading,and test studies on dynamic properties of soil considering effects of pre-unloading are still rare.Here,for dynamic response characteristics of undisturbed granite residual soil under pre-unloading,a series of dynamic tri-axial tests containing the pre-unloading stage were conducted.Based on positional relations among soil body strength envelope,sample consolidation and stress state after unloading,the unloading degree was defined to characterize soil body stress release degree affected by many factors in practice.The unloading degree could reflect the relative distance of unloaded soil from its failure state.According to the test results,effects of dynamic stress amplitude and unloading degree on soil body cumulative plastic deformation and dynamic strength were analyzed.The results showed that with increase in dynamic stress amplitude,the cumulative mode of axial plastic deformation accelerates to“failure type”;pre-unloading action encourages axial cumulative plastic deformation of granite residual soil to indirectly reveal critical dynamic stress amplitude decreases with increase in unloading degree,and this fuelling effect speeds up nonlinearly with increase in unloading degree;under the same number of cycles,the dynamic strength of granite residual soil decreases with increase in unloading degree.Finally,taking the critical dynamic stress amplitude as the normalization factor,it was shown that the soil body dynamic strength curve under different unloading degrees can be normalized to a unique curve.Accordingly,the granite residual soil dynamic strength expression being able to consider effects of pre-unloading degree was established.