波动水力条件下土体内侵蚀特性的透明土试验研究

Experimental study on internal erosion behaviors under fluctuating hydraulic condition using transparent soil

在渗流作用下土中细粒随着水流在粗粒骨架间运移的现象被称为内侵蚀。以往对于内侵蚀的研究主要集中在恒定的水力梯度下,缺乏对于波动水力条件下内侵蚀发展特征和细观机制的认识。设计了透明土渗流试验装置,开展了恒定和波动水力条件下不同内部稳定性土体的渗流试验,以探究波动水力条件下内侵蚀的发展特征。宏观试验现象表明,对于内部稳定性较差型土,当水力梯度达到临界水力梯度后,波动水力条件下的渗透系数比恒定水力条件下呈现出更快的增长趋势,说明水力波动会加剧细颗粒的迁移和流失。为了进一步揭示波动水力条件加剧土体内侵蚀发展过程的细观机制,利用一种基于平面激光扫描的二维截面图像开发的土体细观组构三维重建方法,建立了粗粒骨架、内部孔隙通道和细颗粒的三维可视化数字模型。通过观察细粒在孔隙通道中的分布情况,发现恒定水力条件下,细粒在运移至狭窄的孔隙喉道处时会发生堵塞堆积现象;而波动水力条件下的水流扰动会破坏这种暂稳态的堵塞堆积结构,继而重启细颗粒的迁移过程。

Internal erosion is a phenomenon that fine particles migrate through the channels within coarse matrix under seepage flow.Previous studies mainly focused on the internal erosion under steady hydraulic gradient,while the behavior and mesoscopic mechanism of internal erosion under fluctuating hydraulic condition were paid little attention.In this study,a setup of transparent soil seepage test was developed.Seepage tests of two kinds of soils with different internal stabilities were conducted under steady and fluctuating hydraulic conditions respectively to investigate the internal erosion behaviors under fluctuating hydraulic condition.The macroscopic experimental phenomenon showed that,when the hydraulic gradient exceeded the critical hydraulic gradient,the increase of the hydraulic conductivity under fluctuating hydraulic condition was faster than that under steady hydraulic condition for internally unstable soil,manifesting that hydraulic fluctuation aggravated the migration of fines.In order to further reveal the mesoscopic mechanism,a three-dimensional reconstruction method was employed to rebuild the mesoscopic fabric of soil based on the two-dimensional cross-sectional images obtained by the planar laser scanning;and a three-dimensional visualization digital model including coarse matrix,inter-granular pore channels and fine particles was established.By observing the distribution of fine particles in pore channels,it was found that fine particles might clog and accumulate at narrow pore throats under steady seepage.While the perturbation caused by hydraulic fluctuation could break these weak stable structures of clog and accumulation,and then restart the migration process of fine particles.