孔隙率对砂土渗透稳定性影响的内部可视化研究

Internal visualization study of the effect of porosity on the internal stability of granular soil

砂土的渗透稳定性判别对堤坝灾害防治意义重大。忽视孔隙率对土体渗透稳定性的影响,可能会加大灾害风险。本文借助透明土和平面激光诱导荧光技术,以相同级配、不同相对密实度的材料为试验对象,开展了渗透破坏试验,分析了孔隙率对渗透稳定性的影响。结果表明:从宏观水力角度看,密实和松散两组试样的临界水力梯度实测值接近太沙基临界水力梯度,表现为渗透稳定材料。从内部可视化分析上看,密实试样整体隆起,属于渗透稳定材料;而松散试样的细粒明显迁移至试样顶部,粗粒骨架基本未发生移动,属于典型的管涌破坏现象,故应被判定为渗透不稳定材料。这说明通过传统水力现象评估的渗透稳定性存在偏差,试样的渗透稳定性受孔隙率影响,应在渗透稳定性判别准则中加以考虑。之后,提出了基于灰度累积分布函数的修正图像分割方法,该方法通过图像离散化与相对灰度转换避免了区域灰度差异及整体灰度波动的影响,实现了对试验全程试样内部结构变化的定量分析,印证了试样的渗透稳定性判别结果。最后,总结了国内外7种渗透稳定性判别准则,并根据试验结果对其进行了对比分析,结果显示多数未考虑孔隙率影响的准则偏不安全,考虑了孔隙率影响的准则又过于保守,KOVáCS提出的准则从机理层面同时考虑了孔隙率和颗粒粒径的影响,更具应用潜力。

The internal stability determination of granular soil is important for the prevention of dyke and levee incidents.Ignoring the effect of porosity on internal stability of soil may increase disaster risk.This paper used transparent soil and PLIF(Planar Laser Induced Fluorescence)technology to conduct seepage failure soil column tests on materials with the same particle size distribution but different porosities.From a macroscopic hydraulic perspective,the measured critical gradients were close to the Terzaghi critical gradient,indicating that the two groups of specimens behaved like internal-stable soils.However,the internal images of the two groups of specimens show different failure modes.The denser one heaved and thus it is indeed an internal-stable material.As for the looser one,its fine particles migrated to the top,and the structure of the coarse particles remained stable.It is a typical piping phenomenon and thus the looser specimens should be determined as internal-unstable.It means that there is a deviation in the internal stability assessment based on traditional hydraulic phenomena.The porosity affects the internal stability of granular soil and thus should be considered in the internal stability criterion.Subsequently,this paper proposes a modified image segmentation method based on the cumulative gray-level curve.The regional deviation and fluctuation of images grayscale are avoided by the image discretization and the relative grayscale transformation.Using this method,a quantitative analysis of the global internal behaviors throughout the entire process was conducted to validate the internal stability conditions of the samples.At last,seven existing internal stability criteria were summarized in this paper.A comparative analysis was carried out according to the experimental results.It is found that most of the criteria that do not consider the effect of porosity are unsafe,while those considering the porosity effect are too conservative.The criterion proposed by KOVACS incorporates the effects of porosity and particle size showing the most promising potential.