Characterization of microstructural and physical properties changes in biocemented sand using 3D X-ray microtomography

An experimental study has been performed to investigate the effect of the biocalcification process on the microstructural and the physical properties of biocemented Fontainebleau sand samples. The microstructural properties (porosity, volume fraction of calcite, total specific surface area, specific surface area of calcite, etc.) and the physical properties (permeability, effective diffusion) of the biocemented samples were computed for the first time from 3D images with a high-resolution images obtained by X-ray synchrotron microtomography. The evolution of all these properties with respect to the volume fraction of calcite is analysed and compared with success to experimental data, when it is possible. In general, our results point out that all the properties are strongly affected by the biocalcification process. Finally, all these numerical results from 3D images and experimental data were compared to numerical values or analytical estimates computed on idealized microstructures constituted of periodic overlapping and random non-overlapping arrangements of coated spheres. These comparisons show that these simple microstructures are sufficient to capture and to predict the main evolution of both microstructural and physical properties of biocemented sands for the whole range of volume fraction of calcite investigated.     

Improvement of uniformity of biocemented sand column using CH3COOH-buffered one-phase-low-pH injection method

Acta Geotechnica - Biocement based on a Microbially Induced Calcite Precipitation (MICP) process has emerged to be a promising alternative to cement for soil improvement. A two-phase injection...     

Influence of the microstructural properties of biocemented sand on its mechanical behavior

The mechanical efficiency of the biocementation process is directly related to the microstructural properties of the biocemented sand, such as the volume fraction of calcite, its distribution within the pore space, coordination number, contact surface area, and types of contact. In the present work, some of these microscopic properties are computed, from 3D images obtained by X-ray tomography of biocemented sand. These properties are then used as an input in current analytical models to estimate the elastic properties (Young and shear moduli) and the strength properties (Coulomb cohesion). For the elastic properties, the analytical estimates (contact cement theory model) are compared with classical bounds, self-consistent estimate and numerical results obtained by direct computation (FEM computation) on the same 3D images. Concerning the cohesion, an analytical model initially developed to estimate the cohesion due to suction in unsaturated soils is modified to evaluate the macroscopic cohesion of biocemented sands. Such analytical model is calibrated on experimental data obtained from triaxial tests performed on the same biocemented sand. In overall, the presented results point out the important role of some microstructural parameters, notably those related to the contact, on such effective parameters.     

基于多视角二维图像的钙质砂颗粒三维重构方法

以多视角二维图像为基础进行复杂形状颗粒三维重构。以具有复杂形状的钙质砂颗粒为样本,针对块状、条状以及树枝状等典型复杂形状颗粒进行重构,并以凸度、圆形度以及长宽比等形状指标为依托对其最终重构精度进行表征。在重构过程中,以颗粒初始投影面为基础绕轴转动获取一系列颗粒二维投影图像,并提取其边界坐标值,而后利用三维点云与所获取的二维图像轮廓坐标进行定位匹配,并删除位于二维图像轮廓外部点集,最终得到使得所有点均位于所获取的系列二维图像轮廓内。在此基础上,进一步对点云实体化处理并构建得到三维实体。通过对150个不同形状颗粒展开重构,统计发现90%以上颗粒的重构误差在10%以内,其中,误差范围最大的为树枝状颗粒,最大误差为10.84%,最小误差则小于1.00%。该方法原理简单,可有效地构建得到精度较高的复杂形状颗粒三维模型。     

Tectonic evolution of the Karakoram metamorphic complex (NW Himalayas) reflected in the 3D structures of spiral garnets: Insights from X-ray computed micro-tomography

Spiral garnet porphyroblasts are known to record lengthy periods of deformation and metamorphism by preserving single or multiple FIAs (Foliation Intersection Axis) formed normal to tectonic shortening directions. Thanks to technological advances in X-ray computed micro-tomography (XCMT), FIAs can now be readily determined in relatively large samples in contrast to previous methods that require the preparation of a set of radial vertical and horizontal thin sections of samples. XCMT scanning not only alleviates tedious thin section based procedures but also illuminates the complete internal architecture of a rock sample allowing three-dimensional (3D) quantitative shape analysis of an individual porphyroblast as well as precise measurement of FIAs. We applied the technique to a sample from the Hunza Valley in the Karakoram metamorphic complex (KMC), NW Himalayas, containing numerous garnet porphyroblasts with spiral-shaped inclusion trails. The XCMT imaging reveals an E–W trending FIA within the sample, which is consistent with orthogonal N–S collision of the India-Kohistan Island Arc with Asia. Garnet long axes (X_GT) have variable plunges that define a broad sub-vertical maximum and a small sub-horizontal maximum. The X_GT principle maxima lie at N-090 and N-120. Smaller maxima lie at N-020 and N-340. Geometric relationships between X_GT axes and FIA orientation in the sample suggest that porphyroblast shapes are controlled by the geometry of the lens-shaped microlithons in which they tend to nucleate and grow. The orientation of inclusion trails and matrix foliations in the sample are correlated with three discrete tectono-metamorphic events that respectively produced andalusite, sillimanite and kyanite in the KMC. Late staurolite growth in the sample reveals how the rocks extruded to the surface via a significant role of roll-on tectonics, which can be correlated with the Central Himalayas.     

Microscopic investigation on bonding fracture of biocemented sand from novel in situ brazil splitting tests

Acta Geotechnica - The failure mechanism of biocemented sand has been largely speculated based on scanning electron microscopy images of biocemented sand after mechanical tests. However, some...     

珊瑚砂微生物固化体三轴压缩试验及损伤本构模型研究

利用微生物诱导碳酸钙沉积（MICP）技术固化南海某岛礁的陆域吹填珊瑚砂,对珊瑚砂微生物固化体进行了三轴压缩试验,基于损伤力学理论建立了珊瑚砂微生物固化体的损伤本构模型。结果表明,利用MICP技术固化珊瑚砂效果好,强度高;固化体的三轴压缩应力–应变曲线可分为近似线弹性阶段、屈服阶段与延性流动阶段。将固化体划分为匀质微元进行损伤演化分析,根据连续介质损伤力学的有效应力理论与应变等效假说,定义了损伤变量,假定固化体强度服从双参数的Weibull分布及Druker-Prager准则,建立了损伤本构模型。模型参数包括固化体力学参数和Weibull分布参数,由三轴试验和线性回归法确定,并用试验资料初步验证了模型的合理性。     

Drained instability analysis of sand under biaxial loading using a 3D material model

The diffused and localized instabilities in sand under drained biaxial loading have been analyzed here following a plane strain bifurcation framework, where the rate independent material is defined using a generalized 3D non-associative constitutive model. This study is focused on how various instability modes emerge with respect to initial density, confining pressure, and the applied boundary conditions. Results from large deformation framework have been compared with those from small deformation approximation and the later is noticed to fail in capturing the emergence of diffused modes and predicts delayed onset of localization. The theoretical predictions compares well with existing experimental observations.     

Multi-scale characterisation of coastal sand aquifer media for contaminant transport using X-ray computed tomography

To be able to predict contaminant transport in groundwater, an accurate conceptual and physical understanding of aquifer properties at multiple scales is required. In this study, physical and hydraulic properties of a coastal sand aquifer were derived using micro and macro X-ray computed tomography (XCT) techniques. Qualitative and quantitative data improved conceptualisation of the aquifer structure at micro and macro scale. At the macro scale (50-mm diameter by 1,500-mm long core) XCT images of undisturbed drill core identified coarse grained laminae (not obvious to the naked eye) of increased porosity and permeability, variations in mineral assemblage and particulate organic matter distribution within the core. Micro-XCT analysis (16-mm diameter cores) of the three main aquifer layers provided quantitative micro-scale data on permeability, porosity, grain, pore and throat size distribution statistics, and grain sphericity. Ratios of mean pore to grain diameter were ~0.65–0.75 and ratios of mean throat to mean grain diameter were ~0.2. Multiple permeability and porosity values were derived from micro domains (~4.35 mm³) within each micro-XCT core. Permeability values varied between and within micro-XCT core samples reflecting the heterogeneity at the millimetre core scale in these sediments. Sphericity values were similar for all three layers (average ~0.4) which reflected SEM observations of the semi-spherical nature of the dominant quartz and feldspar grains. The results of this study indicate that models based on the assumptions of homogeneity in depositional structure below centimetre scale may not suitably address factors affecting flow and transport of contaminants.     

砂土颗粒三维形态的定量表征方法

颗粒形态是影响砂土力学特性的重要因素,特别是影响砂土在低应力状态下的抗剪强度、剪胀效应和临界状态行为,以及高应力状态下的颗粒破碎行为。因此,准确地重构砂粒的三维形态,并量化计算其形态表征参数是研究砂粒形态效应的前提工作。借助于高精度的CT扫描技术和图像处理技术,获得近海石英砂和风化花岗岩残积砂这两类砂土颗粒的三维形态信息。采用球谐函数序列实现两种砂颗粒三维形态的准确重构,并通过球谐函数分析计算砂土颗粒的体积来验证该方法的有效性。基于球谐重构的三维砂粒表面,提出了实用性的方法来计算砂粒的表面积、表面曲率和三维尺寸等,进而计算砂粒的三维球度、圆度和伸长率等形态表征参数。结果表明,当球谐函数阶达到15时,其重构的砂粒基本形状和表面纹理均与真实砂粒非常接近;近海石英砂在水流搬运和磨蚀的作用下颗粒形态较为规则和圆滑,球度和圆度较大,而风化花岗岩残积砂则在物理风化和剥蚀作用下颗粒形态较为复杂和粗糙,球度和圆度较小;而这两种地质作用对砂土颗粒的伸长率则没有明显的影响。     

Modifying the mechanical properties of sand by using different hydrophobic conditions

Acta Geotechnica - Constructing a hydrophobic barrier structure is an effective means in reducing rainwater infiltration and maintaining the stability of earthen structures. The hydraulic...     

Influence of a permeable sand layer on the mechanism of backward erosion piping using 3D pipe depth measurements

Backward erosion piping (BEP) poses a threat to the stability of water-retaining structures. This can lead to severe erosion and collapse of embankments. A novel economically appealing measure against BEP is the coarse sand barrier (CSB). The CSB is a trench filled with coarse sand that is placed below the blanket layer on the landward side of the embankment, which prevents the pipe from developing upstream when it encounters the CSB. Inclusion of a CSB creates a vertically layered sand, which is the situation that can also exist in practice but is different from traditional BEP tests with one homogeneous sand. This paper presents new observations and measurements in medium-scale laboratory tests. 3D measurements of the pipe depth and dimensions are presented and analysed. This analysis indicates how the pipe dimensions evolve during the piping process and shows the erosion mechanism for BEP in vertically layered sands. The findings demonstrate the significance of three-dimensional study of the pipe rather than two dimensions. The pipe depth, width and depth-to-width ratios at the pipe tip in critical erosion stages are measured and presented. In the presented tests, two different erosion behaviours (stepwise pipe progression until failure and straight failure) are found and analysed with respect to possible influential parameters. Higher head drops and flow rates are found in tests with straight failure at the stage before progression. A linear relationship between the hydraulic conductivity contrast (k_c) and the critical head drops (h_c) is found and observations are used to investigate deviations from the line.

     

Experimental evaluation of the anisotropic critical state theory for sand using 3D fabric evolution data of triaxial experiments

Acta Geotechnica - The critical state theory has been widely implemented in many models to predict the constitutive behavior of clayey soil. However, the notion of a unique critical state locus for...     

New point-to-face contact algorithm for 3-D contact problems using the augmented Lagrangian method in 3-D DDA

This paper presents a new point-to-face contact algorithm for contacts between two polyhedrons with planar boundaries. A new discrete numerical method called three-dimensional discontinuous deformation analysis (3-D DDA) is used and formulations of normal contact submatrices based on the proposed algorithm are derived. The presented algorithm is a simple and efficient method and it can be easily coded into a computer program. This approach does not need to use an iterative algorithm in each time step to obtain the contact plane, unlike the ‘Common-Plane’ method applied in the existing 3-D DDA. In the present 3-D DDA method, block contact constraints are enforced using the penalty method. This approach is quite simple, but may lead to inaccuracies that may be large for small values of the penalty number. The penalty method also creates block contact overlap, which violates the physical constraints of the problem. These limitations are overcome by using the augmented Lagrangian method that is used for normal contacts in this research. This point-to-face contact model has been programmed and some illustrative examples are provided to demonstrate the new contact rule between two blocks. A comparison between results obtained by using the augmented Lagrangian method and the penalty method is presented as well.     

Numerical study of the 3D failure envelope of a single pile in sand

The paper presents a comprehensive study of the failure envelope (or capacity diagram) of a single elastic pile in sand. The behavior of a pile subjected to different load combinations is simulated using a large number of finite element numerical calculations. The sand is modeled using a constitutive law based on hypoplasticity. In order to find the failure envelope in the three-dimensional space (i.e. horizontal force H, bending moment M and vertical force V), the radial displacement method and swipe tests are numerically performed. It is found that with increasing vertical load the horizontal bearing capacity of the pile decreases. Furthermore, the presence of bending moment on the pile head significantly influences the horizontal bearing capacity and the capacity diagram in the H–M plane manifests an inclined elliptical shape. An analytical equation providing good agreement with the 3D numerical results is finally proposed. The formula is useful for design purposes and the development of simplified modeling numerical strategies such as macro-element.     

Simulating the membrane contact patterns of triaxial sand specimens

One of the common sources of error in triaxial tests is the penetration of the latex membrane into the peripheral voids of the specimen as confining pressure is applied. Existing analytical solutions developed to predict the magnitude of this membrane penetration have all assumed that the peripheral sand particles in contact with the surrounding membrane are arranged so that the centroids of the sand–membrane contact areas form a square pattern of side length D₅₀ on a plane parallel to the undeflected membrane. Measurements made on digital images of sand–membrane contact patterns for triaxial specimens of varying characteristics have shown this assumption to be inadequate. The analysis presented herein shows how actual contact pattern can be reasonably replicated by simulated patterns. These simulated patterns are generated by adding randomly selected distances to the centroid co‐ordinates of contact areas initially placed in a systematic grid pattern. The results indicate that it is possible to systematically model sand–membrane contact patterns as a function of triaxial specimen characteristics such as relative density and preparation method. The ability to simulate the real patterns eliminates the reliance of existing analytical solutions on a single assumed square contact pattern and hence can serve as the basis for improving the accuracy of future solutions to account for the effects of membrane penetration. Copyright © 2000 John Wiley & Sons, Ltd.     

透明砂土与天然砂土动力特性对比

人工合成透明土材料在支撑岩土工程可视化模型试验技术领域得到了良好的应用。然而,关于该新型材料的动力特性研究仍相对缺少,一定程度上影响了其在动力相关模型试验中的推广应用。基于共振柱、动扭剪试验仪器,开展人工合成透明砂土的动变形与动强度特性试验研究,并与天然砂土及福建标准砂的相关动变形与动强度特性进行对比分析;透明砂土由折射率一致的熔融石英砂和混合油配制而成,混合油由15号白矿物油和正12烷按比例混合而成。试验测定并分析了透明砂土的动剪切模量-应变、阻尼比-应变、动剪切模量-阻尼比等关系曲线,以及孔压、动强度特征。试验结果表明,由熔融石英砂与混合油配制而成的透明砂土的动变形和动强度特性与天然砂土相似,可以模拟天然砂土材料开展动力相关模型试验研究。     

Equivalent continuum strain calculations based on 3D particle kinematic measurements of sand

Constitutive modeling of granular materials has been a subject of extensive research for many years. While the calculation of the Cauchy stress tensor using the discrete element method has been well established in the literature, the formulation and interpretation of the strain tensor are not as well documented. According to Bagi, 1 researchers mostly adopt well‐known continuum or discrete microstructural approaches to calculate strains within granular materials. However, neither of the 2 approaches can fully capture the behavior of granular materials. They are considered complementary to each other where each has its own strengths and limitations in solving granular‐mechanics problems. Zhang and Regueiro 2 proposed an equivalent continuum approach to calculating finite strain measures at the local level in granular materials subjected to large deformations. They used three‐dimensional discrete element method results to compare the proposed strains measures. This paper presents an experimental application of the Zhang and Regueiro 2 approach using three‐dimensional synchrotron microcomputed tomography images of a sheared Ottawa sand specimen. Invariant Eulerian finite strain measures were calculated for representative element volumes within the specimen. The spatial maps of Eulerian octahedral shear and volumetric strain were used to identify zones of intense shearing within the specimen and compared well with maps of incremental particle translation and rotation for the same specimen. The local Eulerian volumetric strain was compared to the global volumetric strains, which also can be considered as an averaging of all local Eulerian volumetric strains.     

基于CT的数字岩心三维建模

为研究储层微观孔隙结构和建立微观渗流模型,本文通过对东营凹陷沙三中亚段H152井区的典型低渗透储层岩样进行CT（computed tomography）扫描及图像处理,建立了微观尺度的数字岩心模型;继而经过对该模型进行分析和计算,提取了储层孔喉网络模型,在三维空间上直观、清晰地显示了不同尺度的孔隙及喉道的形态、大小和分布;最后通过对孔隙结构特征、孔隙度、渗透率和压降等动静态参数的分析和计算,建立了储层岩样的微观渗流模型。根据算法对比和参数分析结果认为：与传统中值滤波相比,非局部均值滤波算法可在相似性比较的基础上进行滤波处理,从而提高模型的准确性;基于CT的数字岩心建模可为地质研究提供可靠的数字模型;根据近似等压面假设的微观渗流数值模拟分析了流体渗流特征,为揭示低渗透储层流体渗流规律提供了一种新的途径。