Effects of dry-wet cycles on three-dimensional pore structure and permeability characteristics of granite residual soil using X-ray micro computed tomography

Due to seasonal climate alterations, the microstructure and permeability of granite residual soil are easily affected by multiple dry-wet cycles. The X-ray micro computed tomography (micro-CT) acted as a non-destructive tool for characterizing the microstructure of soil samples exposed to a range of damage levels induced by dry-wet cycles. Subsequently, the variations of pore distribution and permeability due to dry-wet cycling effects were revealed based on three-dimensional (3D) pore distribution analysis and seepage simulations. According to the results, granite residual soils could be separated into four different components, namely, pores, clay, quartz, and hematite, from micro-CT images. The reconstructed 3D pore models dynamically demonstrated the expanding and connecting patterns of pore structures during dry-wet cycles. The values of porosity and connectivity are positively correlated with the number of dry-wet cycles, which were expressed by exponential and linear functions, respectively. The pore volume probability distribution curves of granite residual soil coincide with the χ² distribution curve, which verifies the effectiveness of the assumption of χ² distribution probability. The pore volume distribution curves suggest that the pores in soils were divided into four types based on their volumes, i.e. micropores, mesopores, macropores, and cracks. From a quantitative and visual perspective, considerable small pores are gradually transformed into cracks with a large volume and a high connectivity. Under the action of dry-wet cycles, the number of seepage flow streamlines which contribute to water permeation in seepage simulation increases distinctly, as well as the permeability and hydraulic conductivity. The calculated hydraulic conductivity is comparable with measured ones with an acceptable error margin in general, verifying the accuracy of seepage simulations based on micro-CT results.     

复杂地下水环境下喷膜防水材料损伤的CT试验

利用X射线CT技术对复杂地下水环境下喷膜防水材料的损伤情况进行了研究,探讨了此方法在喷膜防水材料失效性判断中的有效性.以NaC1,Na2SO4,NaOH及两两混合溶液为介质,对喷膜防水材料进行浸泡腐蚀试验后再CT扫描,利用CT数和CT方差的变化来实现其失效性的评价.结果表明:上述3种单溶液中NaOH溶液对材料的损伤最大,NaCl溶液最小;溶液两两混合后对材料的损伤影响相比单溶液有所变化,但并非都增大;在两两混合溶液中,其中一种介质种类的改变会导致另一种介质的浓度对材料损伤的敏感度发生变化.由此可知,CT数和CT方差可以作为喷膜防水材料损伤评价方法的有效指标.     

Evolution of cracks in the shear bands of granite residual soil

The evolution of shear bands and cracks plays an important role in landslides. However, there is no systematic method for classification of the cracks, which can be used to analyze the evolution of cracks in shear bands. In this study, X-ray computed tomography (CT) is used to observe the behavior of granite residual soil during a triaxial shear process. Based on the digital volume correlation (DVC) method, a crack classification method is established according to the connectivity characteristics of cracks before and after loading. Cracks are then divided into six classes: obsolete, brand-new, isolated, split, combined, and compound. With evolution of the shear bands, a large number of brand-new cracks accelerate the damages of materials at the mesoscale, resulting in a sharp decrease in strength. The volume of brand-new cracks increases rapidly with increasing axial strain, and their volume is greater than 50% when the strain reaches 12%, while the volume of compound cracks decreases from 54% to 21%. As cracks are the weakest areas in a material, brand-new cracks accelerate the development of shear bands. Finally, the coupling effect of shear bands and cracks destroys the soil strength.     

Isotropic compression behavior of granular assembly with non-spherical particles by X-ray micro-computed tomography and discrete element modeling

The particle morphological properties, such as sphericity, concavity and convexity, of a granular assembly significantly affect its macroscopic and microscopic compressive behaviors under isotropic loading condition. However, limited studies on investigating the microscopic behavior of the granular assembly with real particle shapes under isotropic compression were reported. In this study, X-ray computed tomography(μCT) and discrete element modeling(DEM) were utilized to investigate isotropic compression behavior of the granular assembly with regard to the particle morphological properties,such as particle sphericity, concavity and interparticle frictions. The μCT was first used to extract the particle morphological parameters and then the DEM was utilized to numerically investigate the influences of the particle morphological properties on the isotropic compression behavior. The image reconstruction from μCT images indicated that the presented particle quantification algorithm was robust, and the presented microscopic analysis via the DEM simulation demonstrated that the particle surface concavity significantly affected the isotropic compression behavior. The observations of the particle connectivity and local void ratio distribution also provided insights into the granular assembly under isotropic compression. Results found that the particle concavity and interparticle friction influenced the most of the isotropic compression behavior of the granular assemblies.     

泡沫混凝土孔结构的表征及其对性能的影响

采用压汞法(MIP)、计算机断层成像(X-CT)技术及真空饱水吸水率法测试了泡沫混凝土的孔结构.对由X-CT技术获得的二维切片图,采用Image-Pro Plus软件进行图像分析,实现了对泡沫混凝土宏观孔隙率、孔径分布及孔形状因子等的表征.研究了密度等级和粉煤灰掺量对泡沫混凝土孔结构、抗压强度及吸水率等的影响.结果表明:泡沫混凝土的平均孔径和孔形状因子随其密度等级的降低而增加,掺入适量粉煤灰能够改善泡沫混凝土孔结构和力学性能.     

基于CT技术的混凝土三维有限元模型构建

提出了一种基于CT数字图像技术的混凝土材料三维有限元模型构建的新方法.首先,采用医用CT扫描系统,对混凝土试样进行逐层扫描,生成材料的内部细观结构图像,利用CT图像的三维重构技术,将得到的剖面细观图像逐层叠加,形成三维细观结构图像;然后,将该图像映射到有限元程序中进行三维网格剖分,建立真实状态下的混凝土细观数值分析模型;在此基础上,对钢纤维混凝土进行了劈裂试验.结果表明,这种基于CT技术的数字图像三维有限元模型构建方法可真实地反应混凝土材料的内部结构与材料的固有力学特性,其数值分析结果与物理试验结果吻合,是数值分析领域中的一种新的尝试与创新.     

基于遗传算法的停车诱导系统(PGIS)信息显示设施定位研究

针对停车诱导信息系统(PGIS)中可变停车诱导信息显示板的定位优化问题,以诱导信息量最大为优化的目标,分析并建立了优化模型,考虑了可变信息显示板总数约束、单交叉口上信息板个数约束、所有车流均有至少一次被诱导的条件约束及可变信息板设置的疏密程度约束;探讨了用遗传算法进行模型优化求解的方法。以一个仿真示例对模型构造及求解的过程进行了阐释,运用该模型思路及模型求解方法可确定信息板在道路网络中的最佳位置。当可利用的数据资料不足时,介绍了一种简便方法来确定信息板设置的合理位置。对停车诱导系统中的可变信息板定位问题进行定量分析并建立优化模型是确定信息板在路网中最佳位置的有效方法,运用该方法可科学的指导停车诱导系统的方案设计。     

基于CT图像的混凝土单轴压缩裂隙与应变场分析

利用X射线计算机层析术(CT)对混凝土试件单轴压缩过程进行原位扫描,获得了不同荷载下试件的CT图像.通过对CT图像进行分析,获得了灰度均值及其均方差、孔隙率和计盒维数随荷载变化的规律.结合数字体散斑法(DVSP)获得了试件内部三维位移场与应变场.结果表明:试件内部不同尺度裂隙的发展导致灰度均值及其均方差、孔隙率和计盒维数发生相应的变化,灰度均值及其均方差可以揭示小于CT尺度裂隙的发展规律,孔隙率和计盒维数更适用于描述大于CT尺度裂隙的特征.试件内部的位移场与应变场能够直观地揭示出小于CT尺度微裂隙引起的应变局部化区域的产生及发展过程,为混凝土细观破坏机理研究提供了新方法.     

加速溶蚀条件下铸铁管内衬水泥砂浆的孔结构演变规律

设计了一种用离心工艺制作的内衬水泥砂浆铸铁管试件,采用流动的6mol/L氯化铵溶液对水泥砂浆内衬进行加速溶蚀试验,并利用压汞(MIP)法和X射线断层扫描(X-CT)方法分析了水泥砂浆内衬孔结构的变化.结果表明:加速溶蚀过程中水泥砂浆内衬的孔结构变化明显,溶蚀150d时水泥砂浆内衬中的净浆层孔隙率(体积分数,下同)从溶蚀前的9.57%增加至35.49%,最可几孔径从17.11nm增加到26.30nm,并有少量危害孔(单孔体积大于4×10~(-6) mm~3)出现;溶蚀450d时水泥砂浆内衬中的净浆层孔隙率达到61.54%,并出现大量危害孔;水泥砂浆内衬中的砂浆层孔隙率和危害孔数量随溶蚀时间的增加而增加.     

在外力作用下土体内部裂隙发育过程的CT研究

介绍了笔者设计制造的一台适用于CT连续观察的装置 ,并用这一装置对由多种材料组成的土体模型进行了垂直荷载、直剪和水平荷载试验 ,同时用CT对土体模型的不同剖面进行了连续拍摄以观察内部裂隙的发展过程。试验表明 ,CT可以用来研究一般情况下无法观察到的土体内部结构的变化过程 ,是岩土工程分析方面一个强有力的无损伤和无侵入的观察手段     

Gas transport parameters of differently compacted granulated bentonite mixtures (GBMs) under air-dried conditions

Granulated bentonite mixtures (GBMs) have been regarded as effective buffer materials in the deep geological disposal of radioactive waste due to their operational advantages, such as ease of transportation and in-situ placement/backfilling. Many studies have been done to characterize the hydraulic and thermal properties of GBMs as well as their swelling properties. Only limited studies, however, have investigated their gas transport properties, even though these properties affect their compactness during in-situ placement/backfilling and subsequent gas diffusion and advection in the buffer zone. The aim of this study is to understand the gas transport parameters, i.e., air permeability (k_a) and gas diffusivity (D_p/D_o), of tested samples compacted at different dry densities (DDs) under air-dried conditions, linking them with the measured density distribution characteristics determined by microfocus X-ray computed tomography (MFXCT) analysis. Two types of GBMs were used in this study: 1) FE-GBM (prepared from National Standard® bentonite, Wyoming, USA): this material was used in the Full-scale Emplacement (FE) experiment at the Mont Terri rock laboratory, Switzerland) and 2) OK-GBM (prepared from a bentonite, originating from Japan, with the trade name of OK bentonite, Kunimine Industries). The tested samples were firstly packed in a 100-cm³ acrylic core with different DDs, ranging from loose to dense (1.09 to 1.75 g/cm³), and scanned by MFXCT. The weighting factors, w_f (fine fraction; lower density) and w_c (coarse fraction; higher density) (w_f + w_c = 1), were determined after the peak separation of the measured CT brightness histograms from the reconstructed three-dimensional multiplanar reconstruction (MPR) images of the MFXCT analysis. The measured k_a and D_p/D_o were highly dependent on the DDs, the k_a (?) values fitted well with a power law model, and the D_p/D_o (?) was predicted accurately by several previously proposed models. For both FE-GBM and OK-GBM, there were good linear relationships between the gas transport parameters and w_c × DD, implying that the weight of the coarse fraction controlled k_a and D_p/D_o. Moreover, the Kozeny-Carman model, incorporating the measured volumetric surfaces from the MFXCT analysis, was able to predict the k_a values well.