[目的]为研究各种加卸荷组合条件下土体的力学特性,对杭州淤泥质黏土开展了室内试验研究.[方法]首先通过三轴剪切试验和归一化分析法,研究竖向和水平向同时卸荷复杂工况下土体的强度与变形特性;然后分析土体的应力-应变特性及应力路径;最后考虑预剪应力及卸荷后再剪切的应力条件,对原始的Dun-can-Chang 模型的应力-应变曲线和初始剪切模量表达式进行修正,得到了修正的Duncan-Chang模型及淤泥质黏土的模型参数.[结果]不同卸荷路径下的偏应力与轴向应变均呈双曲线关系,峰值抗剪强度随卸荷应力比的增大而增大;不同土样的应力路径均从K.固结线的同一点出发,在卸荷与再剪切过程中经历不同的p-q变化,最终均到达Kf线发生剪切破坏,试验数据拟合的Kf线线性相关性较高,根据拟合的Kf线及Mohr-Coulomb破坏条件,可以不用绘制摩尔圆直接得到土的黏聚力和内摩擦角.[结论]本研究结果对深入分析复杂卸荷条件下土体的强度和变形特性及多个地下工程相互交叠或近邻时的设计与施工具有参考价值.
[Objective]To investigate the mechanical behavior of soil under different loading-unloading combination conditions,laboratory tests on Hangzhou very soft clay were conducted.[Method]Firstly,the strength and deformation characteristics of soil under complex vertical and horizontal unloading conditions were studied by triaxial shear tests and normalization methods;then,the stress-strain characteristics and stress paths of soil were analyzed;finally,considering the preshearing stress and post-unloading reshearing stress conditions,the expressions of stress-strain curve and initial shear modulus in the original Duncan-Chang model were revised,obtaining the modified Duncan-Chang model and the model parameters of very soft clay.[Result]It shows that the deviatoric stress and axial strain curves are hyperbolic under different unloading paths,with the peak shear strength increasing with the unloading stress ratio;the stress paths of different soil samples start from the same point of K0 consolidation line,and experience different p-q changes in the process of unloading and reshearing,and finally reach Kf line where shear failure occurs;the Kf line fitted by the test data has good linear correlation,and according to the fitted Kf line and Mohr-Coulomb failure condition,cohesion and internal friction angle of the soil can be obtained directly without drawing Mohr's circles.[Conclusion]This study can provide a reference to analyze the strength and deformation characteristics of soil under complex unloading conditions,and design and construction of multi-crossing and neighboring underground works.