Particle transport within water-saturated porous media: Effect of pore size on retention kinetics and size selection

The transport and filtration behaviour of fine particles (silt) in columns packed with sand was investigated under saturated conditions by using step-input injections. Three samples of different particle size distributions (coarse medium, fine medium and a mixture of both) were used in order to highlight the influence of the pore size distribution on particle retention and size selection of recovered particles. The main parameters of particle transport and deposition were derived from the adjustment of the experimental breakthrough curves by an analytical model. The higher particle retention occurs in the mixture medium, owing to its large pore size distribution, and the filtration coefficient decreases with increasing flow velocity. Particle size distribution of recovered particles shows a thorough size selection: (i) the first recovered particles are the coarser ones; (ii) the size of the recovered particles increases with increasing flow velocity and enlarger pore distribution of the medium.     

脱湿状态下南京下蜀土的土水-力学特性

用经改造的GDS非饱和土三轴仪测得下蜀土在不同净围压下的变形特性和持水特性,并以van Genuchten模型对试验数据的拟合结果为基础,分析了净围压对下蜀土吸力应力特征曲线的影响,全面系统地研究了脱湿状态下南京下蜀土的土水-力学特性。研究表明,下蜀土的变形特性受净围压与基质吸力影响,基质吸力越高,下蜀土的压缩性越低,可以用二元指数函数本构模型分析孔隙比与应力状态变量间的关系。净围压使下蜀土持水能力增强,净围压与进气值呈幂函数关系。土的压缩性越高,净围压对其持水特性的影响越显著;随着净围压的增大,下蜀土孔隙比趋同,净围压对持水特性的影响逐渐减弱。外部荷载对下蜀土吸力应力特征曲线的影响是分段的,在边界效应区范围内,净围压增大时,吸力应力特征曲线间的差异逐渐减小;在过渡区范围内,吸力应力随状态变量变化的快慢程度不受外部荷载影响。根据双电层理论分析了高吸力时吸力应力的物理意义与计算方法,提出以界限有效饱和度作为吸力应力函数的分界点。     

Identification of the behavior of sandy soil to static liquefaction and microtomography

In this paper, undrained triaxial compression tests were performed on specimens obtained in Oued Rass sand (Chlef, Algeria). The objective of this study is to investigate the effects of specimen deposition methods, the initial density, and the confining pressure on the undrained behavior of this sand. The test results show that the initial confining pressure and the relative density affected, in a significant manner, the resistance to liquefaction. However, it increases with confining pressure and relative density. The results also show that the specimens prepared by dry deposition method have a greater resistance to liquefaction than those prepared by moist tamping. A nondestructive comparative analysis of the deposits by X-ray microtomography carried out at the granular scale made it possible to characterize more precisely the difference of the obtained structures for the two deposition modes.     

干湿循环下高庙子钙基膨润土持水和变形特性

膨润土因具有湿胀、干缩特性,其水力-力学性质易受干湿循环的影响。采用饱和盐溶液蒸汽平衡法研究了干湿循环对压实高庙子钙基膨润土持水特性的影响。对相同条件下制作的试样进行0～6次干湿循环,选取其中的原试样（0次）、循环3次和6次试样,进行蒸汽平衡法试验,测定其土-水特征曲线。同时,在0～6次干湿循环过程中,对每一次脱湿后试样表面采集图像,用数字图像处理技术提取了收缩及裂隙开展区域,分析得到收缩率与裂隙率。试验结果表明：随着干湿循环次数的增加（0→3次时）,土-水特征曲线向下平移、持水性下降、孔隙比增大、平均骨架应力减小,试样烘干时收缩率和裂隙率增加明显;但随着循环次数的继续增加（3→6次时）,持水性、收缩率和裂隙率趋于稳定。     

A laboratory study of the initial structure and the overconsolidation effects on the undrained monotonic behavior of sandy soil from Chlef region in northern Algeria

The effects of initial state of the samples and the overconsolidation ratio (OCR) on the behavior of Chlef sand were studied in this article. For this purpose, the results of two series of undrained monotonic triaxial compression tests on medium dense sand are presented. In the first test series, the influence of the specimen's fabric and confining pressure has been studied. The tests were conducted at initial confining pressure of 50, 100, and 200 kPa. Two methods of sample preparation, which included the dry funnel pluviation and the wet deposition, were utilized. All the samples were subjected to a monotonic loading after consolidation phase. The results of the tests demonstrate that an increase in initial confining pressure leads to an increase in the resistance to liquefaction. The results also show that the samples prepared with the dry funnel pluviation method exhibit a greater resistance to liquefaction than those prepared with the wet deposition method. In the second series of tests, the overconsolidation influence on the resistance to the sand liquefaction has been realized on samples at an effective stress of 100 kPa for OCR varying between 1 and 8. The tests show that the increase of OCR improves the stiffness of sand and accelerates the appearance of dilatancy.     

珊瑚砂三维孔隙微观特性研究

孔隙是多孔介质内渗流的发生场所,与介质渗透性存在必然的联系。珊瑚砂因其特殊的物质来源和形成过程,较陆源砂具有截然不同的孔隙特性。通过一系列微观研究手段,从本质上揭示了珊瑚砂特殊孔隙性质的原因。研究发现,从孔隙形状、孔喉尺寸和整体连通性3个角度描述孔隙性质较为合理。其中,孔隙形状用形状因子度量,孔喉尺寸包括孔隙半径和喉道半径,珊瑚砂多孔介质整体连通性利用配位数进行描述。而影响孔隙形状、孔喉尺寸和整体连通性的主导因素包括颗粒形状和颗粒表面粗糙度两方面。其中颗粒形状主要影响孔隙形状、喉道尺寸、孔喉尺寸离散性和介质内部连通性的均匀分布情况。颗粒表面粗糙度主要影响孔隙形状、孔隙形状离散性、孔隙尺寸和介质整体连通性。     

南京粉细砂空间孔隙结构表征单元体确定

确定统一的表征单元体(REV)尺寸是研究土体细观孔隙结构时首先需要解决的问题。使用6.5μm分辨率同步辐射显微CT扫描南京粉细砂试样,从土样三维重构模型的5个代表性部位提取5组立方体孔隙REV,对每个REV采用最大球算法分析,建立孔隙网络模型,从中提取孔隙率、单位体积孔隙数、孔隙平均体积、孔隙最小体积、孔隙最大半径、孔隙最小半径、孔隙平均半径、孔隙截面积平均形状因子等8个孔隙结构参数,建立其与REV尺寸间的相关性。利用假设检验T检验和F检验,最终确定样品孔隙结构参数的统一REV边长尺寸为400像素,即2.60mm。该方法可用于砂土、粉土等颗粒土体细观孔隙结构分析。     

Mesoscale experimental study on chemical composition,pore size distribution,and permeability of tailings

To study the seepage characteristics of tailing sand, an indoor sand column test was performed with three kinds of tailing sand obtained from Henan and Shaanxi provinces, China. River sand, quartz sand, and glass beads were also employed for comparison. Using the nuclear magnetic resonance, the researchers studied pore size distribution and permeability under different hydraulic gradients. The chemical composition and particle morphology were analyzed by energy-dispersive spectrometry and X-ray diffraction. The results were as follows. (1) The three kinds of tailings sand are small in diameter, mainly in the range from 0.15 to 0.5 mm. Their diameter distribution is more concentrated than river sand. The surface of tailing sand is rough. As for chemical composition, tailing sand contains many kinds of metal elements. (2) With hydraulic gradient increasing, the micropores in the tailings samples become fewer, the larger pores grow in number, and the peak of the pore size distribution changes to the left; blocking is more obvious. (3) The small pores gradually decrease during the test. Under the influence of groundwater pressure, the smaller gravel could migrate and block tiny pores. The structure of particle arrangement becomes denser. Therefore, it would prevent the seepage liquid from passing through the medium. The permeability coefficient K decreases.     

Test method and application for microstructures of undisturbed silty sand and sandy silt

The physical and mechanical properties of soil are determined by microstructure. Sandy silt, silty sand and other weak cohesive materials have low strength in the dry state, limiting the apply scope of scanning electron microscope and mercury porosimeter in such material. The experiment proposes the use of solid–liquid–gas three-phase transformation principle of menthol to strengthen the soil particle, include related sampling device and experimental procedures, to obtain parameters of the microscope image and porosity characteristics. Scanning electron microscopy show the clear photograph of particle form, spatial position, contact mode, plane pore characteristics and so on for undisturbed silty sand and sandy silt in Shanghai area. Freeze–thaw action can change the micro-pore parameters of soil. The mercury intrusion porosimetry test demonstrates that the pore volume, accumulated pore area, the most probable pore size and appearance probability of frozen-thawed silty sand increase with the decrease of freezing temperature. The pore size shows a normal distribution. Besides, pore size distribution of frozen-thawed sandy silt shows the interval distribution. Increasing the freezing temperature, the pore volume and accumulated pore area for sandy silt increase, while bigger value and smaller probability of the most probable pore diameter are obtained.     

Evaluation of Static Liquefaction Characteristics of Saturated Loose Sand Through the Mean Grain Size and Extreme Grain Sizes

Liquefaction of soils is a natural phenomenon associated with a dramatic loss of the soil shear strength in undrained conditions due to a development of excess pore water pressure. It usually causes extensive damages to buildings and infrastructures during earthquakes. Thus, it is important to evaluate extent of influential parameters on the liquefaction phenomenon of soils in order to clearly understand the different mechanisms leading to its triggering. The soil gradation is one of the most important parameters affecting the liquefaction phenomenon. In this context, a series of undrained compression triaxial tests were carried out on eighteen natural loose (Dr = 25%) sandy samples containing low plastic fines content of 2% (I_p = 5%) considering different extreme sizes (1.6 mm ≤ D_max ≤ 4 mm and 0.001 mm ≤ D_min ≤ 0.63 mm) and two mean grain size ranges (0.25 mm ≤ D₅₀ ≤ 1.0 mm) and (1.0 mm ≤ D₅₀ ≤ 2.5 mm). The initial confining pressure for all tests was kept constant (P′_c = 100 kPa). The obtained test results indicate that the mean grain size (D₅₀) and extreme grain sizes (D_max and D_min) have a significant influence on the undrained shear strength (known as liquefaction resistance) and appear as pertinent factors for the prediction of the undrained shear strength for the soil gradation under study. The undrained shear strength and the excess pore water pressure can be correlated to the extreme grain sizes (D_max and D_min) and the mean grain size (D₅₀) of tested wet deposited samples.     

基于不同分形模型的冻融黄土孔隙特征研究

为了得到冻融作用对黄土孔径分布的影响规律, 以重塑黄土为研究对象, 利用压汞法测试经历不同冻融循环次数后黄土样品的孔隙特征, 采用3种分形模型对冻融作用后的黄土微观孔隙结构进行定量表征和对比研究。结果表明： 未经冻融作用的黄土孔隙分布曲线呈单峰分布, 经历冻融作用的黄土孔隙分布曲线呈双峰甚至多峰分布。冻融作用对黄土中孔径分布在0.1 ~ 10 μm范围内的孔隙影响较大。前10次冻融作用使黄土孔隙率增加, 特别是经历6次冻融作用后, 与未经历冻融作用的黄土相比孔隙率增大约18.8％。随着冻融作用的继续, 黄土孔隙率减小且趋于稳定。经历不同冻融循环次数后的黄土孔隙分布均具有良好的统计分形特性。基于热力学模型和毛细管压力曲线法表征黄土孔隙结构时, 黄土孔隙呈现显著的分形特性, 可在整个孔径尺度范围内给出唯一且合理的分形维数。基于Menger海绵模型表征的经历冻融作用后黄土孔隙分形特征呈现多尺度分形, 在不同的尺度范围内, 有不同的分形维数。结合分形理论可知冻融作用改变了黄土孔隙均匀性及复杂程度。     

基于数字图像处理技术的砂土颗粒级配分析研究

砂土颗粒级配是影响砂土工程性质的重要参数之一,传统的颗分试验方法由于试验原理不同而具有不同的适用范 围。为更加方便、准确地获取砂土颗粒级配,提出了一种基于数字图像处理技术来获取SEM照片中的砂颗粒粒径参数及级 配特征的新方法。将准备好的砂土样品打磨成薄片放在扫描电镜下拍照,然后利用Photoshop软件对照片进行拼接,并采用 自主研发的SMAS数字图像分析系统对拼接好的照片进行定量分析处理,获取样品的微观结构参数,将获得的“土颗粒等 效直径”和“土颗粒面积”分别等效为实际土颗粒粒径和土颗粒质量,进而换算得到砂土的颗粒级配曲线;然后将通过数 字图像技术获得的砂土颗粒级配曲线与通过传统筛分法和激光粒度分析法得到的颗粒级配曲线进行对比,验证了提出的数 字图像处理方法的有效性和可靠性,同时分析了两种方法之间存在的差异及其原因,并对数字图像处理方法和传统颗分方 法各自的优缺点进行了讨论。     

基于数字图像处理技术的砂土颗粒级配分析研究

砂土颗粒级配是影响砂土工程性质的重要参数之一，传统的颗分试验方法由于试验原理不同而具有不同的适用范 围。为更加方便、准确地获取砂土颗粒级配，提出了一种基于数字图像处理技术来获取SEM照片中的砂颗粒粒径参数及级 配特征的新方法。将准备好的砂土样品打磨成薄片放在扫描电镜下拍照，然后利用Photoshop软件对照片进行拼接，并采用 自主研发的SMAS数字图像分析系统对拼接好的照片进行定量分析处理，获取样品的微观结构参数，将获得的“土颗粒等 效直径”和“土颗粒面积”分别等效为实际土颗粒粒径和土颗粒质量，进而换算得到砂土的颗粒级配曲线；然后将通过数 字图像技术获得的砂土颗粒级配曲线与通过传统筛分法和激光粒度分析法得到的颗粒级配曲线进行对比，验证了提出的数 字图像处理方法的有效性和可靠性，同时分析了两种方法之间存在的差异及其原因，并对数字图像处理方法和传统颗分方 法各自的优缺点进行了讨论。     

击实黄土孔隙结构对土水特征的影响分析

土水特征曲线是非饱和土的基本土物理-力学关系,即将含水率这一物理参数转化为土粒间力的作用,土水特征曲线受土的结构控制。为了探讨击实黄土孔隙结构对土水特征曲线的影响,本文在3种不同的初始含水率（小于最优含水率8%、最优含水率17%和大于最优含水率19%）下制备不同结构的击实黄土试样,分别用压汞试验测其孔隙分布曲线,用滤纸法测其土水特征曲线,并用扫面电镜获得其微观结构图像。对以上测试结果的分析表明,3种击实土样的孔隙分布曲线在相应的大孔径范围内相差较大,在小孔径范围内趋于一致;土水特征曲线在低吸力区差异较大,小于最优含水率的击实黄土土水特征曲线最陡;在高吸力区,3种击实土样的土水特征曲线趋于一致,这与孔隙分布特征一致。对比孔隙密度分布曲线与土水特征区曲线发现,土的土水特征受孔隙分布的控制,孔隙密度越大,土水特征曲线的斜率越陡。SEM图像也显示出3种击实土样的结构特点,小于最优含水率的土样有较多架空孔隙,优势孔径最大;高于最优含水率的土样,大孔隙减少,小孔隙增多,优势孔径最小。而最优含水率的击实黄土的孔隙分布较均匀,优势孔径覆盖范围大。     

Experimental study on characteristics of pore water conversion during methane hydrates formation in unsaturated sand

Understanding the pore water conversion characteristics during hydrate formation in porous media is important to study the accumulation mechanism of marine gas hydrate. In this study, low-field NMR was used to study the pore water conversion characteristics during methane hydrate formation in unsaturated sand samples. Results show that the signal intensity of T₂ distribution isn’t affected by sediment type and pore pressure, but is affected by temperature. The increase in the pressure of hydrogen-containing gas can cause the increase in the signal intensity of T₂ distribution. The heterogeneity of pore structure is aggravated due to the hydrate formation in porous media. The water conversion rate fluctuates during the hydrate formation. The sand size affects the water conversion ratio and rate by affecting the specific surface of sand in unsaturated porous media. For the fine sand sample, the large specific surface causes a large gas-water contact area resulting in a higher water conversion rate, but causes a large water-sand contact area resulting in a low water conversion ratio (C_w=96.2%). The clay can reduce the water conversion rate and ratio, especially montmorillonite (C_w=95.8%). The crystal layer of montmorillonite affects the pore water conversion characteristics by hindering the conversion of interlayer water.©2022 China Geology Editorial Office.     

An approach to characterisation of multi-scale pore geometry and correlation with moisture storage and transport coefficients in cement-stabilised soils

An experimental approach to the characterisation of the complex, multi-scale pore geometry in cement-stabilised soils is presented, in which the pore size distribution inclusively spans at least six orders of magnitude from ~3 nm up to >3 mm. These most likely result from the combined effects of granular inter-particle packing, clay/cement clothing and bridging effects, cement hydration and clay/cement pozzolanic reactions, and alteration of larger pore geometries as a result of solid mass mobilisation and transport following capillary wetting/drying regimes. Experimental data are presented and were obtained through a combination of X-ray computed tomography, mercury intrusion porosimetry and N₂ physisorption supported by ‘wet mode’ environmental scanning electron microscopy. Data strongly suggest that macropore/capillary pore size distribution, mean pore size, sorptivity and transport coefficients are a function of particle size distribution (when compaction energy is constant). Mesopore size distribution, which dominates hygric sorption/desorption behaviour, occurs within the clay/cement matrix and also appears to be strongly influenced by the particle size distribution of the granular phase. All other factors being equal, manipulation of granular particle size distribution can be used to engineer the hygric (vapour) and capillary (liquid) potentials and also the fluid transport coefficients of these materials.     

Heating–freezing effects on the pore size distribution of a kaolinite clay

This study aims to quantify the effect of heating and freezing temperatures on the pore size distribution of saturated clays. Three kaolinite clay specimens were subjected to different temperatures: 20, 70, and ? 10 °C. Upon achieving the desired temperature for each specimen, the specimens were flash frozen in liquid nitrogen to preserve their microstructure. Each specimen was, then, freeze-dried for 24 h after which consecutive two-dimensional (2-D) SEM images were taken using a dual focused ion beam/scanning electron microscope. The produced 2-D images of each specimen were used to reconstruct three-dimensional tomographies of the specimens, which were analyzed to determine the pore size distribution at each temperature. Compared to the specimen at room temperature, the pores in the specimen subjected to ? 10 °C were larger; this is believed to be due to the formation of ice lenses inside the pores upon freezing and potential merging between initial pores to form larger pores. On the other hand, the heated specimen showed an increase in the volume of the smaller pores and a decrease in the volume of the larger pores compared to the specimen at room temperature. This opposite behavior between the small and large pores in the heated specimen is justified considering (1) the easier flow of water out of the larger pores compared to that in the smaller pores and (2) the anisotropic nature of the thermal expansion of the clay particles.     

A comparative study of suction stress between sand and silt under unsaturated conditions

The purpose of this study is to estimate and compare suction stress between sand and silt sampled from the coast of Korea. The water content and matric suction of sand (Joomunjin) and silt (Saemangeum) were first examined using an automated soil–water characteristic curve (SWCC) apparatus based on the axis translation technique. SWCCs were then estimated from the test results using the van Genuchten (1980) model. At equal matric suction, the corresponding water content of silt was higher than that of sand. Moreover, the saturated water content and air-entry value (AEV) of silt were larger than those of sand. Using the fitting SWCC parameters, suction stress characteristic curves (SSCCs) were estimated according to the method proposed by Lu and Likos (2006). The SSCC behavior for sand and silt was different and significantly depended on the material properties, particularly pore size and pore size distribution. For sand, the suction stress exhibited rapid variation with changes in matric suction, but for silt, the suction stress approached a constant value as the matric suction increased. In addition, when the matric suction was smaller than the AEV of soil, the suction stress was equal to the magnitude of the matric suction. In contrast, when the matric suction exceeded the AEV of soil, suction stress had a nonlinear shape with respect to the matric suction.     

Undrained Cyclic Pore Pressure Response of Sand–Silt Mixtures: Effect of Nonplastic Fines and Other Parameters

Literature regarding the pore pressure generation characteristics and in turn the cyclic resistance behaviour of silty sand deposits is confusing. In an attempt to clarify the effect of nonplastic fines on undrained cyclic pore pressure response of sand–silt mixtures, an experimental programme utilising around 289 stress-controlled cyclic triaxial tests on specimens of size 50 mm diameter and 100 mm height was carried out at a frequency of 0.1 Hz. Specimens were prepared to various measures of density through constant gross void ratio approach, constant relative density approach, constant sand skeleton void ratio approach, and constant interfine void ratio approach to study the effect of nonplastic fines on pore pressure response of sand–silt mixtures. The effect of relative density, confining pressure as well as the frequency and magnitude of cyclic loading was also studied. It was observed that the pore pressure response is greatly influenced by the limiting silt content and the relative density of a specimen corresponding to any approach. The influence of other parameters such as relative density, confining pressure and magnitude of cyclic loading was as usual but an increase in frequency of cyclic loading was seen to generate excess pore pressure at a higher rate indicating an impact load type of behaviour at higher frequency. Utilising the entire test results over a wide range of parameters a new pore pressure band for sand–silt mixtures in line with Lee and Albaisa (1974) has been proposed. Similarly another pore pressure band corresponding to 10th cycle of loading as suggested by Dobry (1985) and up to a shear strain of around 25% has been proposed. These two bands can readily be used by researchers and field engineers to readily assess the pore pressure response of sand–silt mixtures.