双向耦合循环剪切条件下饱和砂土体应变发展规律试验研究

砂土的剪胀与循环应力路径密切相关。针对饱和南京细砂,利用空心圆柱扭剪仪(HCA)进行了一系列均等固结条件下轴向-扭转耦合循环剪切排水试验,研究了复杂应力路径下饱和砂土的剪胀性及其体应变量化方法。研究表明:双向耦合循环剪切条件下饱和砂土的剪胀由一个完全可逆的循环体应变分量和一个不可逆的累积体应变分量构成,循环应力路径对累积体应变发展规律影响显著;以等效循环应力比ESR作为表征复杂应力路径下动应力大小的指标,饱和砂土累积体应变与ESR值具有事实上的唯一性关系,累积体应变随ESR的增加而线性累积;通过引入参数ESR,提出了双向耦合剪切条件下饱和砂土累积体应变规准化方法。验证性试验表明新的体应变增量模型的预测值与试验结果的吻合度较高,而基于循环直剪试验结果建立的Byrne模型对双向耦合剪切条件下饱和南京细砂的体应变预测偏小。

Experimental investigations on volumetric strain behavior of saturated sands under bi-directional cyclic loadings

The dilatancy of sands highly depends on the cyclic stress path. A series of axial-torsional coupling shear tests are performed on the Nanjing fine sand under isotropically consolidated condition by using the hollow cyclic apparatus (HCA). The dilatant behavior of saturated sands is investigated under complex stress paths, using the correspondent mathematical model. The results are summarized as follows: the volumetric strain of sands is composed of a completely reversible component and an irreversible component. The cyclic stress path has significant effects on the development of the volumetric strain. The equivalent cyclic stress ratio (ESR), which is defined as the ratio of the mean value of the maximum shear stress in a loading cycle to the initial effective confining pressure, can be used as an index to quantitatively characterize the cyclic stress paths of the soil samples under bi-directional shear loadings. The volumetric strain increment may be uniquely correlated to the applied ESR, which accumulates linearly with the increase of ESR. By introducing ESR, a normalized incremental model for volumetric strain of the saturated sands under bi-directional shear loadings is proposed. Retrospective simulation of a laboratory test using the proposed model shows good agreement, calibrating the reliability of the model. However, the Byrne model based on the data of direct shear tests significantly underestimates the volumetric strain accumulation of the Nanjing fine sand under the axial-torsional coupling shear loadings.