动荷载作用下兰州地铁黄土的动力变形特性研究

Study of the Dynamic Deformation Properties of Loess Under a Dynamic Load in the Lanzhou Metro

摘要文章通过对取自兰州市轨道交通1号线的黄土施加不同优势频率的地铁循环荷载进行动三轴试验，并通过与1Hz等幅正弦循环荷载作用下的动三轴试验结果进行对比，分析了地铁动荷载作用下兰州黄土的动应力一动应变关系、动剪切模量比和阻尼比特征，研究了地铁循环荷载作用下兰州黄土的累积变形特性。结果表明：振动频率对黄土动力特性和累积塑性变形的影响显著，随着振动频率增加，达到一定变形所需的动应力明显增大；频率越小，动剪切模量比随动剪应变的增大衰减较快，动剪应变相同时Gd/Gdmax最小；振动频率为2．5Hz时黄土在相同动应变下的动应力和Gd／Gdmax均最大。不同振动频率的循环荷载作用下黄土的累积塑性变形均经历急剧变形、缓慢变形和稳定三个阶段，但在缓慢变形和稳定阶段，达到一定变形所需的振动次数随振动频率的增大而增大。频率范围为0．4-0．6Hz的地铁振动荷载对兰州黄土变形的影响最为显著。

Dynamic triaxial tests of loess samples from the Lanzhou Subway Line 1 were performed under subway- included dynamic loadings with different dominant frequencies and a constant cyclic dynamic loading with the fre- quency of 1 Hz. With the comparison of test results, we analyze the dynamic stress-strain relation, dynamic shear modulus ratio and damping ratio characteristics of the loess under the effects of dynamic load and also study the ac- cumulated deformation properties of loess under cyclic loading. The results show that vibration frequency has an ob- vious influence on the dynamic behaviors and cumulative plastic deformation of the loess--with an increasing vibra- tion frequency, the dynamic stress required for achieving a certain stage of deformation increases significantly. With a smaller vibration frequency, the dynamic shear modulus ratio decreases more rapidly with an increase of the dy- namic strain; with the same dynamic shear strain, the Gd/Gdmax is smallest while at a vibration frequency of 2.5 Hz, and it reaches maximum dynamic stress and Gd/Gdmax under the same dynamic strains. Moreover, the cumulative plas- tic deformation of the loess under dynamic loadings with different vibration frequencies could be split into three stag- es, which are： the rapidly-deforming stage, the slowly-deforming stage, and the fundamentally-stable stage. Howev- er, in the slowly-deforming stage and the fundamentally-stable stage, vibration times required to achieve a certain deformation will increase with the increase of the vibration frequency. Dynamic loadings from a subway with a vibra- tion frequency between 0.4 Hz and 0.6 Hz have the most serious influence on the deformation of loess in Lanzhou.