Microstructure Of Clay Research Articles

Research on structural performance evolution of clay based on distribution force model

PurposeThe purpose of this paper is to explore the variation law between the clay microstructure and macro external force by using soil scanning electron microscope (SEM) images.Design/methodology/approachFirst, SEM images of clay were pre-processed by MATLAB, and quantitative statistical parameters such as directional probability entropy, fractal dimension and shape factor are extracted. Second, the distribution force model was proposed, considering that the microscopic parameters of soil particles were independent of each other, and the distribution coefficient was determined according to the analytic hierarchy process (AHP). Then, the fitted formula of quantitative statistical parameters based on the distribution force model was obtained by taking the macroscopic distribution force as independent variable and the microscopic parameters of soil particles as dependent variable. Finally, the correctness of corresponding fitting formula was verified.FindingsThe results showed that the change of external consolidation pressure has great influence on the directional probability entropy and fractal dimension, while the shape factor reflecting the regular degree of soil particle shape is less sensitive to the consolidation pressure. The fitting formula has high accuracy, and mostly the R value can reach more than 0.9. All the data have passed the test, which proves that the distribution force model proposed in this paper is rational.Originality/valueThe model can be used to connect the macroscopic stress of soil with the micro-structure deformation of soil particles through mathematical formula, which can provide reference for engineering practice.

Remediation Trend of Engineering Behavior of Wastewater Contaminated Clay Using Iron Nano Oxide: Experimental Studies

Soil polluted with urban wastewater due to defect of wastewater disposal and leakage from wastewater channels is a common type of pollution in urban areas which in addition to environmental damage, has significant effects on soil engineering parameters. In present study, effects of municipal wastewater on mechanical behavior of soil and clay microstructure was studied, and then effects of iron oxide nanoparticles on remediation trend of contaminated soil was investigated. To achieve this, unconfined compressive strength (UCS), collapse and SEM analysis tests were performed on contaminated samples containing 20%, 60% and 100% wastewater at 1, 3 and 5 months and also on contaminated soil remidiated with 0.5-4% Iron nano oxide. Results showed that wastewater reduces shear strength of clay and this decreasing trend increases with increasing percentage and contamination duration. UCS of soil contaminated with 100% wastewater decreased by 49% after 5 months of contamination. Also, wastewater in the soil caused to soil collapse after 5 months . SEM images showed the clay structure became clotted after contamination and soil pores increased compared to natural soil. Improvement phase results showed that by addition of Iron nano oxide to contaminated soil, shear strength significantly increased, and optimal percentage of Iron nano oxide was 3% in which UCS increased by 105.2%. By increasing the percentage of Iron nanoxide, intensity of collapse index of contaminated soil decreases. Best case senario, final strain of soil decreases by 43.4% compared to contaminated soil. Therefore, utilizing Iron nanooxide is recommended to improve engineering behavior of contaminated clay.

Quantitative Analysis of the Microstructures of Deep Silty Clay Subjected to Two Freezing–Thawing Cycles under Subway Vibration Loading

The bypass of subway tunnels in soft soil areas is usually constructed by the artificial freezing method due to the waterproof property and high strength of frozen soil. The thawing settlement and deformation of silty clay under subway vibration loading is becoming a major issue. The Shanghai subway lines cross vertically and horizontally, of which the deepest station reaches 33.1 m. With the rapid development and utilization of underground space, the depth of subway excavation will become increasingly deeper. Under subway vibration loading, the dynamic characteristics and microstructures of deep clay should be investigated. In this paper, a series of scanning electron microscopy (SEM) tests and mercury intrusion porosimetry (MIP) tests were carried out to study the microscopic parameters of deep silty clay subjected to two freezing–thawing cycles under cyclic loading. The freezing–thawing soil produced higher axial strain than that of the unfrozen soil with the same loading. After freeze–thaw cycling, the soil particles show the phenomenon of agglomeration. As the pore diameter increases, the pore shape coefficient decreases gradually from 0.45 to 0.85. Under similar vibration loading, the anisotropy of the frozen–thawed clay increases as the freezing temperature decreases. The fractal dimension obtained by SEM tests is between 1.25 and 1.33. There is an approximate linear relationship between the axial strain and the fractal dimension. In MIP tests, there exists a bottleneck effect in the process of mercury intrusion. The pore volume and pore diameter of the freezing–thawing soil increase with decreasing freezing temperatures and freezing–thawing cycles, respectively. The thermal fractal model is applicable to the fractal characteristics of silty clay. The results can provide a reference for the design and construction of deep-buried subway tunnels in soft soil areas.

Evolution of Unsaturated Shear Strength and Microstructure of a Compacted Silty Clay on Wetting Paths

Abstract Silty clay is a common embankment material whose moisture increases over time due to soil–atmosphere interactions and climate changes. The objective of this study is to investigate the uns...

Influence of Soil Particle Size on the Engineering Properties and Microstructure of a Red Clay

Particle size exerts a significant influence on the mechanical behavior of soil. However, insufficient research has been carried out on red clay formations, which are widespread in some Chinese provinces. Here, using unconfined compressive strength (UCS) tests, wetting–drying (WD) tests, and low-temperature nitrogen adsorption tests, we examined the relationship between the particle size and a number of mechanical and microstructural characteristics of a red clay outcropping at a construction site in China. Our results suggest that, depending on the surface area, porosity, particle size, and dry density, the failure mechanism in UCS tests will be different. That is, as the particle size increases, the failure mode of soil changes from split failure into shear failure. In addition, as the dry density increases, the UCS of the soil sample is significantly improved, and its total porosity and fractal dimension decrease. We also evaluated a dependence on particle size and the number of WD cycles on the distribution and size of cracks in WD tests. We conclude by suggesting that particle size, dry density, and WD behavior should all be taken into account in roadbed designs in red clay formations.

Rock physics of sand-shale mixtures: Classifications, theoretical formulations and study on real dataset

Shale and clay minerals as common constitute in the sandstones can be distributed in various scenarios which in turn can result in different reservoir and elastic properties. A new classification with distinct subclasses for clay distribution has been proposed based on the plausible configuration of shale/clay particles within the sandstone reservoirs. These subclasses give more quantitative insights into laminated, structural and dispersed clay scenarios by taking into account more details of the shale and clay microstructures. Some rock physics-based workflows were proposed and implemented on a real dataset to validate the given strategies to model different clay distribution types. These workflows can be used to generate a template in the elastic domain in order to differentiate various clay distributions. Accordingly, our dataset which was taken from a shaly sandstone in the Persian Gulf, was investigated in more details using the generated template for detecting different clay distribution types as well as observing different elastic and reservoir properties when clay distribution type is changed. We observed that there are four different classes (class I, II, III and IV) of properties with different trends in the crossplot of P-wave velocity versus porosity. Class I includes properties which their P-wave velocity increased when porosity is reduced. In contrast, class II includes properties which show a decreasing trend for P-wave velocity when porosity is reduced. Class III belongs to the properties with two different trends towards structural and dispersed clays. These properties do not follow the same trends for P-wave velocity changes versus porosity similar to classes I and II. However, when the given property decreases or increase by moving toward dispersed clay or structural clay, their P-wave may increase or decrease, respectively. Finally, class IV includes properties which their values do not show any particular trend within the given clay distribution types template.

Scanning Electron Microscopy and clay geomaterials: From sample preparation to fabric orientation quantification

Scanning Electron Microscopy (SEM) has been widely used to investigate the microstructure of clays, and several studies have examined the influence of the particles orientation on the mechanical behaviour of the soil. However, as the effect of the sample preparation that precedes the imaging on the orientation of the soil particles has not been systematically studied yet, the extent to which the clay microstructure is disrupted is not quantified. This work investigated samples having different mineralogy, stiffness, particles orientation and aggregation, and stress history imaging them at different magnifications in their Freeze, Oven and Air dried state using SEM, and wet state using the Environmental SEM (ESEM) and CryoSEM. The particles orientation imaged in the different micrographs was quantified with MiCA (Microstructural Clay Analyser), a Computer Vision based tool. The results shows the greatest effect for the dispersed soft samples, where the detected order increases with decreasing drying speed, regardless of mineralogy. However, this seems to affect the degree of the particles orientation when the CryoSEM technique is employed. Moreover, soft flocculated samples generated at acidic sedimentation environment overall experience a lower degree of reorientation. Finally, the fabric of stiff undisturbed samples is not significantly influenced by the applied method.

Numerical Model about the Influence of Cyclic Load on the Hydraulic Conductivity of Soft Clay

The hydraulic conductivity k is one of the most important engineering properties of soft clay, and it plays an important role during the whole life cycle of underwater tunnel. Therefore, it is necessary to systematically study the response of k to dynamic load. In this study, a series of seepage tests were conducted on remolded Hangzhou soft clay using dynamic triaxial seepage apparatus after cyclic loading. The results show that the hydraulic conductivitys of all samples decrease after cyclic loading, and the maximum drop of the hydraulic conductivity can reach 29.1%. Also, the hydraulic conductivity of soil samples is significantly affected by cyclic stress ratio (CSR). With the increase of CSR, the decrease range of hydraulic conductivity under cyclic loading is between 22.3% and 24.3%. The dynamic load reconstructed the microstructure of clay and made the number of large pores getting decreased and the small pores increased. This resulted in a decrease in the smaller hydraulic conductivity (k) of the specimens. Futhermore, based on the NMR scanning on the microstructure of soil samples under cyclic loading and the microscopic pore channel theory, a prediction model for the k of soft clay is proposed. The validity of the model is verified based on test data. It is believed that the proposed model could provide some theoretical support for the design and long-term performance prediction in practical engineering issues.

Geotechnical properties and microstructure of lignin-treated silty clay in seasonally frozen regions

Geotechnical properties and microstructure of lignin-treated silty clay in seasonally frozen regions

Shear strength and microstructure of silty clay subjected to CaSO4 solution and stability effect on a gypsum dump

Shear strength and microstructure of silty clay subjected to CaSO4 solution and stability effect on a gypsum dump

Evolution of soil-water and shrinkage characteristics of an expansive clay during freeze-thaw and drying-wetting cycles

This paper investigates the evolution of the soil-water characteristic curve (moisture content-suction relationships, SWCC), soil shrinkage characteristic curve (moisture content-void ratio relationships, SSCC) and microstructure of a compacted expansive clay during drying-wetting (DW) cycles and closed-system freeze-thaw (FT) cycles. Experimental results reveal that FT and DW cycles significantly change the clay's macropore system by destructing the compaction-induced macropores but have little impact on the micropore system. FT cycles induce invisible microcracks and DW cycles result in visible macrocracks, both of which (i) significantly reduced the clay's water retention capacity in the low suction range and volumetric shrinkage from saturated condition to oven-dryness, (ii) introduce structural shrinkage stage to the SSCC and (iii) have no impact on the shrinkage and plastic limits of the SSCC. The slope of the SSCC in the normal and residual shrinkage stages reduces after FT cycles due to the development of microcracks but is less affected by DW cycles. The moisture content-void ratio-suction relationships obtained from specimens with different FT and DW histories are likely distributed on a unique surface and this surface was reasonably described by a model proposed in this study using only one set of parameters. Studies in this paper are useful for advancing the understanding of the influences of environmental factors on the hydromechanical behaviour of expansive clays.

Research on Microstructure of Soft Clay under Various Artificial Ground Freezing Conditions Based on NMR

Nuclear magnetic resonance (NMR) can help to collect soft clay pore size data. This paper uses NMR to study the effect of freezing duration, the gap time between two freezing actions and thawing temperature on the pore size distribution (PSD) in soft clay. It was found that most of the freezing and thawing progress was completed during the first two hours. A longer time gap between the first and second freezing cycles leads to greater structural damage. An appropriate thawing temperature results in less subsidence induced by the artificial ground freezing (AGF). A new method to distinguish water types for various freezing conditions is proposed. This freezing method to determine the cutoff value could fill the defect in the previous methods.

Analysis of the Mechanical Properties and Mechanism of Zinc Ion‐Contaminated Red Clay

To study the effects of different concentrations of zinc ions on the mechanical strength, material composition, and microstructure of red clay, a triaxial test, an x‐ray diffraction test, an x‐ray fluorescence spectrometry test, a scanning electron microscopy test, and a mercury intrusion test were carried out on contaminated soil to investigate the mechanisms of zinc ion‐contaminated red clay. The results show that the higher the concentration of zinc ions, the smaller the shear strength and cohesion of the red clay. The internal friction angle is increased first and then decreased. From material composition, zinc ion makes montmorillonite and hemite disappear in red clay. With the increase of zinc ion concentration, quartz semiquantitative increase and kaolinite semiquantitative decrease and the content of SiO2, Fe2O3, and Na2O reduces. Microscopically, the structure of red clay changes from floc structure to granular and aggregate structure after the zinc ions are added, while the contact of the particles is converted to point contact. With the increase of the concentration of zinc ions, the porosity and the fractal dimension of the red clay gradually increase, and the stability of the granular structure is weakened.

Microstructure observations in compacted clays subjected to thermal loading

The response of clayey materials to thermal variation has been the subject of extensive studies, given the wide range of engineering applications which involve subjecting soils to substantial temperature fluctuations. A number of hypotheses have been proposed to explain the volumetric changes induced in the clays as a result of temperature variations. Most associate the observed volumetric changes to re-orientation as well as changes in the clay microstructure, with no microstructural experimental evidences to date. The work presented in this note is a first attempt at studying the evolution of the internal structure of two types of clays, an Illite and a Kaolin, compacted dry of optimum, submerged until saturation, reconsolidated to various vertical effective stresses and then subjected to thermal loading. A series of thermal oedometer, mercury intrusion porosimetry (MIP) and tomography tests were conducted in order to induce, detect, and quantify microstructural alterations within the clay as a consequence of temperature changes. Results of heating and cooling tests on Illite showed a thermal contraction which could be attributed to the deformation/collapse of macro-pores in its dual-porosity structure assemblage. The magnitude of the observed contraction varied with the level of pre-imposed effective vertical stresses. Higher effective vertical stresses resulted in larger shear stresses at the contacts of clay-assemblages, and thus in easier deformation of the macro-pores. The Kaolin samples which presented a unimodal pore size distribution, with a relatively small dominant pore size (0.25 μm), did not exhibit changes in the microstructure which could be captured by the MIP.

Pores Evolution of Soft Clay under Loading/Unloading Process

Loading/unloading tests and field emission scanning electron microscope (FESEM) tests were performed on undisturbed soft clay specimens to study the pore evolution under the loading/unloading process. The results showed that small pores (<0.2 μm) had intrinsic characteristics, and the distribution and the fragmentation fractal dimension of small pores were basically unchanged with pressure, while large pores (>0.6 μm) changed greatly under loading/unloading. The pore-size distribution was mainly influenced by large pores. The microstructure of soft clay before unloading has an influence on the change of the swelling index (Cs) and the pores evolution under unloading. Cs increased as the surface fractal dimension of the pores and the area of large pores decreased, and the fragmentation fractal dimension of the pores increased under the loading process. The variations in fractal dimensions and large pore area increased under unloading. Moreover, the compression index (Cc) changed nonlinearly with the pore evolution under loading. Below 100 kPa, Cc increased slightly with a small increase of the fractal dimensions and large pores area under loading. From 100 kPa to 400 kPa, Cc increased to a peak value of 0.484, and the fractal dimensions and large pore area were the greatest under loading. Above 400 kPa, all of them changed slowly. Based on the evolution of pore fractal characteristics, the loading/unloading process could be divided into three stages: the natural structural stage, the structural adjustment stage, and the new equilibrium stage, which was important to study the loading/unloading properties of soft clay.

Impact of salinity on strength and microstructure of cement-treated Champlain Sea clay

An experimental investigation was conducted to understand the impact of salinity on the microstructure, phase composition, and shear strength of cement-treated Champlain Sea clay. Based on a scanning electronic microscopy (SEM) analysis, Champlain Sea clay exhibits an open structure fabric with flocculated particles. Leaching results in de-flocculation of the aggregates and leads to an increase in inter-aggregate porosity in the clay. Cement mixing transforms a dispersed microstructure into a flocculated state with a large amount of clay-binder aggregates. Cementitious products were formed and reduced both inter-aggregate porosity and intra-aggregate porosity in the cement-treated clay specimens. Sodium chloride salt has a negative effect on the strength development of cement-treated Champlain Sea clay. Under the same cement dosage of 50 kg/m3, the unconfined compressive strength of cement-treated leached clay samples exhibited higher shear strength values than those at natural or a higher salinity level. An optimum salinity level was found to be at 1.33 g/L in this study to achieve the highest shear strength, which was confirmed with the densest microstructure in the SEM and a series of stronger and wider cementitious peaks of cementitious products in the X-ray diffraction analysis.

Mechanical Properties and Microstructure of Lime-Treated Red Clay

This research conducted a triaxial test on lime-treated clay with different contents to analyze the effect of mechanical properties. Scanning electron microscope (SEM) and low-field nuclear magnetic resonance (NMR) analysis were used to analyze the effect. X-ray diffraction (XRD) was used to analyze the chemical changes of calcium oxide on red clay. The results show that with the increase of calcium oxide content, the strength increases, and it can be increased by 282% at 200 kPa confining pressure. The calcium oxide in the red clay reacted with the water in the soil to form calcium hydroxide. The alkaline environment will erode the edge of the soil particles and cause pores to become larger. SEM observed that the addition of calcium oxide caused the agglomeration of soil particles and changed the pore structure of red clay. Low-field NMR showed that calcium oxide had a significant effect on the pore structure of red clay. Calcium oxide increased the total pore volume of the soil sample. Calcium oxide had a substantial influence on the three pore distribution ranges, I (1.1–11 µm), II (15–137 µm), and III (137–512 µm). The porosity fitted with shear strength, it negatively correlated with I and III, and positively correlated with II.

Effects of elevated temperatures on strength and microstructure of mortar containing nano-calcined montmorillonite clay

This study focuses on the residual mechanical strengths and microstructure of nano-calcined montmorillonite clay (NCMC) cement mortars at high temperatures. NCMC was prepared via thermal activation of nano-clay, and the produced mortars with 0%, 1%, 3%, and 5 wt% NCMC replacements were subjected to temperatures of up to 900 °C. The fire-resistance properties of the produced mortars and their role were analysed through thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy.The results revealed that the use of thermally treated NCMC reduced the density and widths of microcracks, improved the mass-loss behaviour, reduced the amount of calcium hydroxide crystals, and strengthened the matrix, which explains their higher residual mechanical strengths.

Pore‐Scale Study of Water Adsorption and Subsequent Methane Transport in Clay in the Presence of Wettability Heterogeneity

AbstractClay minerals are abundant in shale, characterized by a lamellar structure and dimensions smaller than a micron, giving rise to nanometer‐scale pore sizes and large specific surface area. They are commonly associated with water. However, the spatial distribution of the unsaturated water in clay is not very well understood, which significantly affects the subsequent shale gas flow capacity. Wettability heterogeneity in the presence of hydrocarbons further complicates the water distribution in clay. In this study, we use a 3‐D lattice Boltzmann model to study water adsorption and condensation in a reconstructed clay microstructure based on broad‐ion‐beam scanning electron microscopy images. Three wettability conditions are considered including a water‐wet case, a water‐repellent case, and a mixed‐wet case. The spatial distribution of the condensed liquid water phase is visualized, and the connectivity of the remaining open pore spaces is characterized. For the water‐wet case, water preferentially condenses in the small and narrow pore spaces, while the water‐repellent case reports the opposite trend. The mixed‐wet case results in mixed behavior, and connectivity of the pore space is bound by the former two cases. We then study the subsequent methane flow capacity with the presence of water for these three cases, and the calculated methane relative permeability curves show a percolation threshold for the water‐wet case, which is in good agreement with available experimental data in the literature measured on geosynthetic clay liners.

Experimental study about the influence of cyclic load on the hydraulic conductivity of clay

The hydraulic conductivity k, one of the most important engineering properties of soft clay, plays a great role during the whole life cycle of underwater tunnel. Therefore, it is necessary to systematically study the responses of k to the dynamic load under the background of the great development of geotechnical engineering in the world. In this study, a series of seepage tests after cyclic loading were conducted on reconstituted kaolin clay using a modified hollow cylinder apparatus. The influence of cyclic load on the permeability characteristics of soft clay was illustrated in two aspects. The cumulative axial deformation of clay induced by cyclic loading resulted in the smaller hydraulic conductivity of the specimens, and also, the dynamic load reconstructed the microstructure of clay and made the number of large pores getting decreased and the small pores increased. There was a positive correlation between the deformation of soil and the change of hydraulic conductivity, but the reconstruction effect was irregular with the frequency of dynamic load. The measured k values got affected at the beginning, this phenomenon appropriately explains the positive correlation between the number of cycles of dynamic load and the change of hydraulic conductivity.