Determination of shape parameters of sands: a deep learning approach

Acta Geotechnica - The shape parameters of sand particles can be determined by either a Krumbein-Sloss chart or mathematical computation based on the particle image. However, these approaches are...     

Correlation between grain shape and critical state characteristics of uniformly graded sands: A 3D DEM study

Acta Geotechnica - The multi-scale characteristics of particle morphology, including the overall form, local roundness and surface roughness, affect the critical state behavior of sands and should...     

Provenance bias between detrital zircons from sandstones and river sands:A quantification approach using 3-D grain shape,composition and age

Preservation bias may significantly impact the application of detrital zircon geochronology in reconstructing Earth surface processes.Here we compare detrital zircons from the actively eroding Murchison River channel in Western Australia with Ordovician fluvial sediments that have drained similar source rocks along the western margin of the West Australian Craton.In addition to standard analysis of detrital zircon age spectra we apply multivariate statistics to test the relation between 3-D grain shape,U-content and U-Pb ages,with the objective to quantify differences between both sample groups and track preservation along the transport pathway of the Murchison River.Our results show that zircon grains in modern river sands display an upstream trend toward larger surface areas,volume equivalent diameters and grain widths,as well as toward higher U-contents and lower apparent grain densities.3-D grain shape,size and age spectra of Murchison River zircons evolve consistently downstream,but even at the river outlet remain distinct from the Ordovician samples,as a less mature representation of source.We interpret Ordovician river zircons to represent a significantly depleted subset from which up to 22% of the zircon population may have been lost compared to the actively transported detrital load.This discrepancy between the characteristics of detrital zircons in modern active rivers and ancient fluvial Ordovician sandstones demonstrates a bias that could be relevant for other source-sink detrital transport systems throughout Earth history.     

The fractal shape of sutured quartz grain boundaries: application as a geothermometer

Along grain boundaries of quartz from metamorphic and igneous rocks complex interfingering (sutures) may occur. Under the light microscope the lengths of the sutures range from approximately 10–1000 m. The sutured grain boundaries are statistically self-similar over one to two orders of magnitude. They represent fractals. Their mathematical counterpart are Koch curves which are developed after two to four iterations. The fractal (Hausdorff-Besicovitch) dimensions D of sutured quartz grain boundaries from different grades of metamorphism are between ca. 1.05 and 1.30. The D-value decreases with increasing temperature during formation of the sutures. On a statistical basis, D can be used as a measure of this temperature and, therefore, as a deformation-related thermometer.     

砂土中盾构隧道开挖面失稳土体三维形状分析

合理确定盾构隧道开挖面前方失稳土体的形状是开挖面极限支护压力计算及开挖面失稳风险评估的基础和难点。建立模拟盾构隧道开挖面失稳过程的数值模型,利用Handy拱效应理论,依据Mohr-Coulomb破坏准则,得出失稳土体破坏位置和形状的计算方法。研究结果表明,水平方向由于隧道左右两端的支承作用产生水平压力拱,使失稳土体存在一个极限边界,极限边界内垂直方向的土体移动产生悬链线形的最小主应力拱,失稳土体形状类似贝壳形。以南京地铁3号线浦珠路站－滨江路站区间盾构隧道工程作为算例,理论分析和数值模拟结果吻合较好。     

Experimental study of the grading characteristic effect on the liquefaction resistance of various graded sands and gravelly sands

The liquefaction vulnerability of fine- to coarse-graded saturated sand and gravelly sand is evaluated by stress control cyclic triaxial laboratory tests. Cyclic triaxial test on reconstituted specimens is carried out with relative density of 45 %. Cyclic triaxial tests performed by sinusoidal wave form with 1-Hz frequency and with the effective consolidation pressure equal to 100 kPa. Cyclic stress ratio which caused initial liquefaction in 15 cycles recognized as the liquefaction resistance in cyclic triaxial test of present study. The result had been used to find a relationship between liquefaction resistance and the grading characteristics (e.g., coefficient of curvature and coefficient of uniformity) of various graded sands and gravelly sands. Then, it is realized that a relationship between liquefaction resistance and any of the sizes (i.e., D ₁₀, D ₃₀, D ₅₀, D ₆₀) appears to be more practical.     

Capillary collapse of loose pyroclastic unsaturated sands characterized at grain scale

The reduction in volume for unsaturated soils wetted at constant total stress is indicated as capillary collapse. Several studies conducted on standard laboratory specimens (macro-scale) outlined the role of initial void ratio, confining pressure and matric suction on collapse onset. Conversely, few observations were made at grain scale, although an important influence of soil structure has been supposed since years. This paper investigated the collapse of coarse and fine sands derived from a pyroclastic soil of Southern Italy. The X-ray computed tomography was used to identify the mechanisms acting at grain scale and to measure the local variations of soil structure. The experimental procedure consisted in preparing remoulded unsaturated specimens and reducing the matric suction until the collapse occurred under self-weight. At different stages of the process, the sample was imaged by X-ray tomography. The experimental results provided original insight into: (1) transformation of soil structure during the wetting tests; (2) variation of porosity, water content and degree of saturation for the whole specimen; and (3) local variations of those variables in several representative sub-volumes. It is worth noting that collapse of coarse sand specimen occurred before saturation. This was also emphasized by the presence of macro-voids at collapse.     

Discrete element modeling of crushable sands considering realistic particle shape effect

This study proposed a novel approach for generating crushable agglomerates with realistic particle shapes in discrete element modeling (DEM). The morphologies of sand particles were obtained by X-ray micro-computed tomography scanning and image processing. Based on the particle surface reconstructed by spherical harmonic analysis, the crushable agglomerates with realistic particle shapes can be generated in DEM simulations. The results of single particle crushing tests showed that particle shapes significantly influence the fracture patterns and crushing strengths of sand particles. Furthermore, two one-dimensional compression tests were conducted to investigate the particle shape effect on micro- and macro-mechanical behaviors of crushable sands.     

Particle shape: a review and new methods of characterization and classification

Shape is a fundamental property of all objects, including sedimentary particles, but it remains one of the most difficult to characterize and quantify for all but the simplest of shapes. Despite a large literature on the subject, there remains widespread confusion regarding the meaning and relative value of different measures of particle shape. This paper re‐examines the basic concepts of particle shape and suggests a number of new and modified methods which are widely applicable to a range of sedimentological problems; it is shown that the most important aspects of particle form are represented by the I/L ratio (elongation ratio) and S/I ratio (flatness ratio). A combination of these two ratios can be used to classify particles in terms of 25 form classes. A method of obtaining a quantitative measure of particle roundness using simple image analysis software is described, and a new visual roundness comparator is presented. It is recommended that measurements of both roundness and circularity (a proxy measure of sphericity) are made on grain images in three orthogonal orientations and average values calculated for each particle. A further shape property, irregularity, is defined and a classification scheme proposed for use in describing and comparing irregular or branching sedimentary particles such as chert and coral.     

Packing states of ideal reservoir sands: Insights from simulation of porosity reduction by grain rearrangement

The question being tackled in this study is to which extent grain rearrangement contributes to porosity reduction in very well sorted quartzose sands (ideal reservoir sands). A numerical model, RAMPAGE (an acronym of random packing generator), has been developed to address this long-standing problem. RAMPAGE represents a synthesis of various algorithms designed to simulate packing of equal-sized spheres, which have been used to represent ideal solids, liquids, and gases, as well as natural porous media. The results of RAMPAGE simulations compare favourably to theoretical and experimental data from various disciplines and allow delineation of the field of gravitationally stable random packing of equal-sized spheres in the 2-D state space of porosity (P) versus mean coordination number (N). Three end-member packing states have been identified: random loose packing (RLP: P = 45.4%, N = 5.2), random close packing (RCP: P = 36.3%, N = 7.0), and bridged random close packing (Bridged RCP: P = 39.5%, N = 5.2). Unlike previously proposed models, RAMPAGE can simulate the transition from RLP to any other point in the stability field. The RLP state is fully consistent with wet-packed porosities of synthetic sands with lognormal mass-size distributions reported in the literature. The much higher in-situ porosity values reported for modern (air-packed) sands are unlikely to be preserved at depth on geological time scales. Data on the relation between intergranular volume and burial depth indicate that the observed intergranular volume reduction in the upper ~ 800 m of the sediment column corresponds to the evolution of RLP to RCP, and is thus fully explained by non-destructive grain rearrangement.     

颗粒级配和结构对粉砂力学性质的影响

分析了几组原状、重塑和配制土样的静三轴试验结果,认为颗粒级配和结构性是影响粉砂应力-应变关系和强度特性的两个主要因素。当颗粒级配不同时,结构性原状粉砂土样和不具有结构性的配制土的强度均不同。相同颗粒级配的原状和重塑粉砂的强度也不同。而颗粒级配稍有不同的一种配制粉砂与原状粉砂在应力-应变关系和强度特性方面相接近。此种配制粉砂可替代原状粉砂做为模型试验用土。     

泥沙颗粒形状对非均匀沙起动条件的影响

根据非均匀沙颗粒的不均匀性,引入颗粒形状系数,通过分析泥沙颗粒形状系数和相对暴露度的影响,建立了等效粒径表达式;并通过泥沙起动受力分析,推导得到泥沙起动临界条件表达式.实测资料验证表明,计算值与实测值符合较好.     

Effect of particle shape of glass beads on the strength and deformation of cemented sands

Acta Geotechnica - Few studies have focused on the influence of particle shape on the mechanical properties of cemented sand. To address this lack of information, this study investigated the...     

碎石颗粒形状测量与评定的初步研究

颗粒形状是影响碎石料密实特性及力学、渗流特性的因素之一。选取粒径为2～5 mm和5～10 mm的两组灰岩碎石颗粒样本作为研究对象,采用影像测量仪和特制夹具,获取不同旋转角度下的颗粒轮廓影像;使用图形处理软件获得颗粒几何尺寸测值;计算获得各旋转角度下常用颗粒形状评定参数值,运用其平均值进行统计分析,避免了依据单一角度测值评定伴随的人为因素影响。结果表明,灰岩碎石颗粒与标准圆有较大差异, 且粒径大者差异性更明显;两组样本颗粒形状参数均服从偏态分布;长宽比、扁平度和球形度能够更敏感地反映颗粒偏离球形颗粒的程度,而长宽比和球形度便于获取,因而更具优势。     

颗粒形状对碎石料孔隙特性影响研究

颗粒形状是影响碎石料孔隙特性的主要因素之一,进而影响碎石料的渗透特性和力学行为。选取粒径为2~5 mm和5~10 mm的灰岩碎石颗粒填筑试样,同时与相同级配和孔隙率的玻璃球试样进行对比研究,采用CT扫描技术和图形处理软件获取多孔介质孔隙边界轮廓,选取0.5 mm开度阈值分割孔隙;每组试样选取3张扫描断面图像,取定量参数的平均值为研究对象,对孔隙特性进行定量分析。结果表明：玻璃球多孔介质孔隙形状的复杂性甚于碎石料;玻璃球多孔介质孔隙系统的连通度大于同粒组碎石料。     

The effect of crystallization time on plagioclase grain shape in dolerites

The average aspect ratio of plagioclase, measured in thin section, varies systematically through dolerite sills, with a symmetrical “M”-shaped profile observed in sills thinner than ~200 m. Thicker sills show the same marginal reversal at the base, but average aspect ratios appear to continue increasing towards the top, creating an “S”-shaped profile. A distorted “M”-shaped profile is visible in the stratigraphic variation of plagioclase average aspect ratios in the prehistoric Makaopuhi lava lake, with the centre of the “M” in the olivine-enriched horizon close to the base of the lake. Higher overall values of average aspect ratio are observed in thin sills compared to thicker sills, demonstrating that the plagioclase in more slowly crystallized bodies is more equant than that in more rapidly crystallized bodies. There is a strong correlation between the plagioclase average aspect ratio in the central parts of the sills and the crystallization time calculated using a simple one-dimensional thermal model assuming conductive cooling. The cause of the marginal reversals in average aspect ratio is not well understood, but may result from early grain impingement in the developing crystal mushy layer.     

Excess pore pressure and grain crushing of sands by means of undrained and naturally drained ring-shear tests

An investigation of excess pore-pressure generation of a weathered granitic sand, taken from the source area of a typical landslide caused as a result of liquefaction, and a fine silica sand was conducted, in which grain crushing within the shear zone of ring-shear test specimens was examined as the key phenomenon of rapid long-runout motion of landslides. In order to investigate and explain the low average apparent-friction angle, mobilized in a liquidized landslide, speed-controlled ring-shear tests were conducted under undrained conditions on weathered granitic-sand specimens, formed under a wide range of initial void ratios. It was revealed that very small steady-state shear resistances were obtained irrespective of the initial void ratios, which can explain the low average apparent-friction angle. In addition, two series of ring-shear tests on weathered-granitic and fine-silica sands were conducted under naturally drained conditions by keeping the upper drain valve of the shear box open during the tests. The first series of tests was performed under differing total normal stresses, but at the same shear speed, and the second series was conducted at differing shear speeds, but under the same total normal stress. In order to investigate and analyze excess pore-pressure generation and dissipation within the shear zone that is associated with grain crushing, permeability analyses were conducted by passing water through the sample box of the ring-shear test apparatus before and after shearing. In addition, grain-size distribution analyses of samples taken from the shear zone after shearing were carried out. For the weathered granitic-sand samples, a significant change in bulk permeability and large amount of grain crushing were observed. In these tests on the above soil, a considerable reduction of shear resistance, which increased proportionally to the total normal stress and shear speed, were obtained. It was observed that due to grain crushing, finer grains that lowered the permeability of the soil in the shear zone, were formed. It is likely that the decrease in permeability facilitated the generation of high excess pore pressures by reducing the pore-pressure dissipation rate from the shear zone; thus, flow behaviour was exhibited even under naturally drained conditions.     

颗粒形状对粗粒土孔隙特征和渗透性的影响

粗粒土的颗粒级配、形状和密实度都是影响其孔隙结构,进而影响其渗透特性的重要因素。但长期以来,对颗粒形状影响的关注度较少,其主要原因在于不容易定量描述颗粒形状及其影响的孔隙结构特征。选取已获取长宽比和圆形度的不规则形状碎石、规则形状的球和八面体颗粒分别装填试样,针对颗粒级配和颗粒形状的差异,开展孔隙结构特征和渗透性的对比研究。通过CT扫描试样内部结构图像,重构试样的三维孔隙结构并计算孔隙比表面积。通过渗透试验测得试样的渗透系数。结果表明：试样孔隙比表面积是表达孔隙结构特征的有效参数,在相同级配和孔隙率的条件下,试样孔隙比表面积随颗粒圆形度和趋近球形程度的增加而减小;相同级配和孔隙率的条件下,渗透系数随孔隙比表面积的减小而增大,随颗粒圆形度增大而增大;球形颗粒试样的渗透性最强,试样颗粒越偏离球形,孔隙系统中水流阻力越大,试样渗透性越弱。     

Quantitative characterization of grain shape: Implications for textural maturity analysis and discrimination between depositional environments

Grain shape plays an important role in textural analysis of sedimentary grains. Textural analysis helps to determine the formation, transportation and deposition processes of sedimentary rocks. However, there is a lack of standardized methodology for quantitative characterization of grain shapes. The utility of fully automated image analysis for grain shape measurement is assessed in this paper. This research aimed to identify the most useful shape parameters for textural characterization of populations of grains and determine the relative importance of the parameters. A key aspect of this study is to determine whether, in a particular sedimentary environment, textural maturity of the samples can be ranked based on their grain shape data. Furthermore, discrimination of sedimentary depositional environments is explored on the basis of grain shape. In this study, 20 loose sediment samples from four known depositional environments (beach, aeolian, glacial and fluvial) were analysed using newly implemented automatic image analysis methods. For each sample, a set of 11 shape parameters were calculated for 200 grains. The data demonstrate a progression in textural maturity in terms of roundness, angularity, irregularity, fractal dimension, convexity, solidity and rectangularity. Furthermore, statistical analysis provides strong support for significant differences between samples grouped by environment and generates a ranking consistent with trends in maturity. Based on novel application of machine learning algorithms, angularity and fractal dimension are found to be the two most important parameters in texturally classifying a grain. The results of this study indicate that textural maturity is readily categorized using automated grain shape parameter analysis. However, it is not possible to absolutely discriminate between different depositional environments on the basis of shape parameters alone. This work opens up the possibility of detailed studies of the relationship between textural maturity and sedimentary environment, which may be more complicated than previously considered.