循环加载下碎石层细观孔隙结构演化分析

列车荷载作用下路基翻浆冒泥的实质是路基土体细颗粒迁移,细颗粒迁移会改变路基上覆碎石层孔隙结构,影响上覆碎石层力学及水力学性质。为此,采用自主研发的路基翻浆冒泥试验模型进行试验,结合X射线计算机断层（computed tomography,CT）扫描技术,分析循环荷载作用下路基土体细颗粒迁移对上覆碎石层细观孔隙结构的影响。结果表明:循环荷载作用下,碎石层孔隙的压缩作用及路基土体细颗粒迁移至碎石层的堵塞作用,不仅改变碎石层面孔隙率的分布特性,还减小碎石层孔隙率及孔隙连通度；孔隙尺寸分布特性不受加载影响,加载前后,碎石孔隙均呈现小孔隙多、大孔隙少的特点,孔隙分布特征曲线满足对数正态分布,但随着循环荷载的施加,碎石层内孔隙总数量减小；孔隙的形状参数（细长比和扁平度）均呈正态分布,细长比随循环荷载作用次数无明显变化规律,而加载后碎石层孔隙扁平度的分布更加均匀；碎石层孔隙具有明显的分形特性,加载前孔隙三维分形维数为2.40~2.50,随着循环荷载的施加,孔隙三维分形维数逐渐减小。

Evolution analysis of microscopic pore structure in gravel layer under cyclic loading

The essence of subgrade mud pumping under train load is the migration of fine particles in subgrade soil. The migration of fine particles will change the pore structure of overlying gravel, thus affect the mechanical and hydraulic properties of gravel layer. To study the influence of fine particle migration from subgrade soil on the microscopic pore structure of gravel layer under cyclic loading, a series of tests are carried out by a self-developed subgrade mud pumping test model, combined with X-ray computed tomography (CT) scanning technology. It has been shown that under cyclic loading, compression of the gravel pores and clogging of migrated fine particles not only change the distribution characteristics of gravel plane porosity, but also decrease the porosity and porosity connectivity. Moreover, the characteristics of the pore size distribution are not affected by loading. The pores of the gravel layer show features of more small pores and fewer large pores, and the characteristic curves of the pore distribution satisfy log-normal distribution before and after loading. However, with the application of cyclic load, the total number of pores decreases. The distribution of pore shape parameters (slenderness ratio and flatness) shows a normal distribution characteristic. The slenderness ratio has not changed significantly with the number of cycles, while the distribution of flatness is more uniform after loading. The pores of the gravel layer have a distinct fractal character. In addition, the three-dimensional fractal dimension of the pores is distributed between 2.40 and 2.50 before loading. With the application of cyclic loading, the three-dimensional fractal dimension gradually decreases.