粉土界面恒刚度循环剪切试验研究

竖向循环荷载作用下桩土界面的作用机理是研究桩土摩擦疲劳的关键。针对循环荷载作用下桩-粉土界面的剪切性能,使用改进的剪切试验装置在恒刚度条件下进行循环剪切试验,研究循环次数、累积位移和法向刚度对其摩擦疲劳性能、循环后单调剪切性能的影响。试验结果表明,粉土在循环剪切过程中,法向应力和剪应力在初始10个循环内随循环数增加快速衰减,随着循环进行,逐渐趋于稳定;单次循环内在剪切位移方向变化时,土体呈现表现出剪缩-剪胀-剪缩交替现象,总体变形呈现剪缩的趋势;循环荷载作用下,粉土界面的法向应力和剪应力随法向刚度增大衰减速率增大,达到稳定的累积循环位移越小;粉土循环后的单调剪切、法向应力恢复的单调剪切的剪应力比小于首次单调剪切试验值,且法向应力恢复的循环后剪切试验的剪胀程度较小,表明循环剪切过程中界面处粉土颗粒棱角破碎,颗粒变得光滑。在对试验数据分析的基础上,提出了与累积位移、法向刚度和初始应力相关的无量纲累积位移,建立了法向应力和界面摩擦角随累积位移的衰减方程。

Experimental studies on cyclic shear behavior of steel-silt interface under constant normal stiffness

The friction mechanism of pile-soil interface under vertical cyclic loading is the key to study the friction fatigue of piles. In order to study the cyclic shear behavior of pile-silt interface, cyclic shear tests are carried out under constant normal stiffness (CNS) using a modified shear test device. The effects of cycle numbers, cumulative displacement and normal stiffness on the friction fatigue performance and post-cyclic monotonic shear behavior are studied. The results show that the normal and shear stresses decrease rapidly in the initial 10 cycles and gradually become stable with the cycle. The soil exhibits shearing contraction, dilatation and contraction alternating phenomenon with the shear displacement direction changing in a single cycle, and the overall deformation shows a tendency of shearing contraction. The higher normal stiffness results in higher reduction rate of the normal and shear stresses in the shear interface, and the cumulative cyclic displacement for cyclic stability is smaller. The shear stress ratios from the post-cyclic monotonic and post-cyclic monotonic shear tests with the same initial normal stress are lower than those from the initial monotonic shear tests, and the post-cyclic monotonic shears from the same initial normal stress tests have low shear dilation degree, which indicates that the soil sample experiences particle crushing and soil particles become smooth due to the cyclic shearing effect. Based on the analysis of experimental data, the dimensionless cumulative displacement related to the cumulative displacement, normal stiffness and initial normal stress is proposed. The attenuation equation for the normal stress and interface friction angle with cumulative displacement is established.