Soil Microstructure Research Articles

Role of Diatom Microstructure in Determining the Atterberg Limits of Fine-Grained Diatomaceous Soil

The presence of diatoms in diatomaceous soil gives it geotechnical properties that are unusual compared with common clays. The most typical physical property of diatomaceous soil is its abnormally high Atterberg limits compared to fine-grained soil without diatoms. For diatomaceous soil, the Atterberg limits are associated with many factors, such as diatom content, diatom crushing degree, etc. In the study reported here, it was ascertained that more diatoms lead to higher plastic and liquid limits. Once the diatoms are crushed, the plastic and liquid limits decrease. The pore fluid salt concentration barely influenced the Atterberg limits of diatomaceous soil. Additionally, the porous diatom microstructure and trimodal pore size distribution of diatomaceous soil were investigated via scanning electron microscopy, transmission electron microscopy, and mercury intrusion porosimetry. The underlying mechanism of abnormally high liquid and plastic limits of diatomaceous soil is revealed as the water stored in the special diatom microstructure. However, water in diatoms has no contribution to plasticity. Also discussed is the applicability of the current soil classification systems for diatomaceous soil. The findings of this study can help for a better understanding of Atterberg limits of diatomaceous soil and provide suggestions for the classification of diatomaceous soil.

Compressibility, permeability and microstructure of fine-grained soils containing diatom microfossils

Fine-grained soils containing diatom microfossils of biological origin are found worldwide. However, although these soils are acknowledged to have unique physical and mechanical properties, the exact role of diatoms in determining their compression and hydraulic behaviours remains unclear, especially when high effective stress is involved, and the underlying micromechanism is yet to be revealed. This study investigated the compression and permeability of diatomaceous soils prepared by mixing pure diatom and kaolin powders through one-dimensional constant rate of strain compression tests with a maximum vertical stress of 10 MPa. The microstructure of the soils studied was observed by atomic force microscopy, scanning electron microscopy and mercury intrusion porosimetry, and the microstructural evolution during compression was traced. The results indicate that the special hollow structure of diatom particles with internal pores contributes significantly to the high void ratio, compressibility and permeability of diatomaceous soils, and increasing the diatom content improves the pore distribution non-uniformity of the diatom–kaolin mixtures. In addition, the diatom particles have high brittleness and breakage potential, leading to microstructural rearrangement during compression, especially pore-structure adjustment. Lower compressive stress deforms the inter- and intra-aggregate pores, but higher vertical stress damages the diatoms’ hollow structure, which changes the compression and hydraulic parameters in a different manner from the case of conventional clay. This paper enriches the knowledge concerning the multilevel behaviour of fine-grained soils containing diatom microfossils and provides a fundamental dataset.

Study on the solidification property and mechanism of soft soil based on the industrial waste residue

Abstract The solidification property and mechanism of soft soil based on the industrial waste residue was studied systematically. First, the properties of soft soil solidified by single blast furnace slag, phosphogypsum and cement were carried out, and the solidification effect of Portland cement was better compared with blast furnace slag and phosphogypsum. Second, the composite solidification agent formula with excellent solidification performance was developed, and the mass ratio of Portland cement, blast furnace slag and phosphogypsum was 1:2.3:0.8. At this time, compressive strength was up to 8845.1 kPa. Finally, the composition and microstructure of the solidification soft soil was studied by X-ray diffractometer and scanning electron microscope. Calcium hydroxide formed by the hydration of Portland cement and calcium sulfate in phosphogypsum were used as activators for hydration reaction of blast furnace slag, and more hydration products under the synergistic action of Portland cement hydration and blast furnace slag hydration were formed. Hydrated calcium silicate colloid and Ettringite crystal played an important role in skeleton and filling, and the structure of soft soil solidified by composite solidification agent was the most compact.

The evolution of soil microstructure and micromineralogy along a soil age gradient on the Galápagos Islands (Ecuador)

The islands of the Galápagos archipelago are of different ages due to the combination of a geostationary volcanic hotspot and the eastward movement of the Nazca tectonic plate. Taking advantage of this, a soil chronosequence (1.5–1070 ka) has recently been established covering four of the islands. This was used to investigate the temporal development of soil microstructure and micromineralogy using micromorphological methods in combination with synchrotron-based mineralogical and geochemical microanalyses. Along the chronosequence, the microstructure changed from intergrain microaggregates in the young soils to angular blocky microaggregates in the older soils. In the young soils, a high amount of vitric scoria and weatherable minerals occur, but they quickly disappear with age, and micromass increased fast. The micromorphological studies as well as micro-XRD and micro-XRF revealed increasing heterogeneity of soil microstructure and micromineralogy due to weathering and pedogenic processes until intermediate soil age. In the oldest soils, the microstructure became more uniform again, de-mixing pedofeatures like clay coatings have completely disappeared, and kaolinite, hematite and gibbsite dominate the mineralogical composition. Our study shows that the development of soil microstructure mirrors macroscopic soil profile development showing increasing differentiation until intermediate developmental stages and receding heterogeneity in highly weathered soils.

A study on the shear strength characteristics and microscopic mechanism of coal-bearing soil under dry-wet cycles

The slope of coal-bearing strata distributed along the high-speed highway (railway) is affected by the atmospheric dry-wet cycles (DWC), and the collapse occurs many times during the construction of such highways, in Pingxiang city, in the province of Jiangxi. The DWC affect the strength characteristics of the unsaturated coal-bearing soil (CBS). In order to study the shear-strength characteristics of the unsaturated CBS under the DWC, the relationship between the shear strength and matric suction was analyzed by using direct shear test of the unsaturated CBS, the filter paper method of the matrix suction measurement, and the scanning electron microscope test. The internal reasons of shear strength attenuation of unsaturated CBS under DWC are revealed from microscopic perspective. The results show that the DWC at 0 to 4 times, with an increase in the water content, the clay domains expanded unevenly. Further, the clay minerals that served as the cementing junctions and soluble salts were softened and dissolved, and the bonding strength between the basic units and the cohesion of the samples decreased, so that the shear strength of unsaturated CBS samples decreased with an increase in the water content, and increased with the increase of matric suction. Under the influence of the DWC, the CBS samples slaked, the quartz matrix between the fissures slaked, and produce fragments and debris which reduced the size of large particles, and the bonding strength between the basic units was low. Therefore, the matric suction and the shear strength of the unsaturated CBS samples with the same moisture content, under the same normal pressure, decreased gradually with an increase in the number of the DWC. It is feasible to study the strength characteristics of unsaturated CBS by combining the test methods of macroscopic strength, matric suction and microstructure of soil.

Effect of Height-to-Diameter Ratio on the Compression Test Results of Remodeled Loess and Its Mechanism

The confined compression test is the most widely used test to study the compressive deformation of soil in a laboratory, and the height–diameter (H/D) ratio has a significant effect on the confined compression test results. However, there has been limited research on the effect of the H/D ratio on loess compression deformation. In this work, a series of confined compression tests were carried out on the remolded loess samples using a modified oedometer to explore the effect of the H/D ratio on the compression deformation. Scanning electron microscopy (SEM) tests were used to investigate the variation of the microstructure in soils with different H/D ratios along the sample height. The results showed that the axial strain of the remolded loess decreases with the increase in the H/D ratio. The influence of the H/D ratio on the compression deformation is not obvious at lower pressures and compaction degrees, but it is significantly enhanced when the H/D ratio is beyond 0.65. The compression curves of the remolded loess samples with different H/D ratios fit the Gompertz equation well, and the transition point of the compression curves becomes difficult to identify with the increase in the H/D ratio. The compression index (Cc) decreases exponentially as a function of the H/D ratio, while the compressive yield stress (Pc) increases linearly with the increase in the H/D ratio. SEM analysis showed that, with the increase in the H/D ratio, the compression deformation of the sample changed from the whole sample involved in compression to the part of the soil in contact with the pressure. The compression of loess is mainly dominated by the compression of macropores and mesopores. The development of the soil–ring wall friction on the compression deformation induced by the H/D ratio was analyzed based on the change in microstructure, and a three-stage development of the soil–ring wall friction was established. These results and analysis established the connection between the specification and the literature, providing strong support for the rationality of the confined compression test results.

Investigation on microstructure evolution of clayey soils: A review focusing on wetting/drying process

Variability in moisture content is a common condition in natural soils. It influences soil properties significantly. A comprehensive understanding of the evolution of soil microstructure in wetting/drying process is of great significance for interpretation of soil macro hydro-mechanical behavior. In this review paper, methods that are commonly used to study soil microstructure are summarized. Among them are scanning electron microscope (SEM), environmental SEM (ESEM), mercury intrusion porosimetry (MIP) and computed tomography (CT) technology. Moreover, progress in research on the soil microstructure evolution during drying, wetting and wetting/drying cycles is summarized based on reviews of a large body of research papers published in the past several decades. Soils compacted on the wet side of optimum water content generally have a matrix-type structure with a monomodal pore size distribution (PSD), whereas soils compacted on the dry side of optimum water content display an aggregate structure that exhibits bimodal PSD. During drying, decrease in soil volume is mainly caused by the shrinkage of inter-aggregate pores. During wetting, both the intra- and inter-aggregate pores increase gradually in number and sizes. Changes in the characteristics of the soil pore structure significantly depend on stress state as the soil is subjected to wetting. During wetting/drying cycles, soil structural change is not completely reversible, and the generated cumulative swelling/shrinkage deformation mainly derives from macro-pores. Furthermore, based on this analysis and identified research needs, some important areas of research focus are proposed for future work. These areas include innovative methods of sample preparation, new observation techniques, fast quantitative analysis of soil structure, integration of microstructural parameters into macro-mechanical models, and soil microstructure evolution characteristics under multi-field coupled conditions.

Microstructure of compacted low-plasticity soils: the initial fabric and its evolution on stress and suction paths

Soils used in earthworks undergo different hydro-mechanical paths due to the compaction and construction process, the change in climatic conditions or the groundwater level oscillations. Their hydromechanical behaviour is greatly affected by their initial microstructure set on compaction that evolves differently in compliance with the stress paths. The current study investigates the differences in the initial microstructure in a low-plasticity clayey silt compacted at the dry and wet of the optimum. Themicrostructure was characterized by mercury intrusion porosimetry. The definition of a microstructural void ratio (em) inside the soil aggregates and its ratio to the total void ratio (em /e) allowed plotting contours of equal em and em /e in the Proctor compaction plane for the as-compacted states. Additionally, the evolution of the initial microstructure along different stress and suction paths was evaluated. The microstructural voidratio reached after the hydro-mechanical paths did not reproduce the contours of the as-compacted states in the compaction plane. In fact, the microstructural void ratio inside saturated soil aggregates follows Terzaghi’s effective stress through a microstructural compressibility parameter, which provides a straightforward approach for predicting the evolution of the microstructure of compacted low-plasticity soils subjected to different stress-suction paths.

Study on Microstructure and Silting Characteristics of Soft Soil: A Case Study of Nansha District, Guangzhou

With the development of society and economy, land subsidence in soft soil areas has become a prominent geological disaster in our country’s coastal area. Land subsidence has the characteristics of irreversibility, wide influence range, long aging, etc., and these characteristics are closely related to the characteristics of soft soil, such as physical and mechanical parameters, microstructure characteristics, etc. The Nansha District of Guangzhou was taken as the study area in this paper. Through the collection of preliminary data, two-dimensional scanning electron microscopy, and three-dimensional Computed Tomography (CT), the micro-structure characteristics of soft soil are expounded and its plugging characteristics are further analyzed. The results show that the study area’s soft soil has a honeycomb, granular, and flocculent structure. From the perspective of pore scale distribution, the number of macropores and ultramicro pores is small, and the continuity is poor. In most small and medium pores, namely the aggregate and granular pores. The siltation of soft soil is closely related to the microstructure of soil and can be treated according to the effect of siltation characteristics on the consolidation settlement of soft soil. The research results of this paper can provide targeted prevention and control suggestions for the soft land subsidence disaster in the Nansha area, and also provide a reference for other similar research.

A microstructure-based procedure to simulate the effect of wetting-drying cycles on the soil water retention curve

Analysing unsaturated soil response often requires the soil-water-retention-curve (SWRC). The SWRC depends upon the soil microstructure, which evolves with hydromechanical loading such as in-situ exposure to wetting-drying cycles. If in-situ response is of interest and studied in the laboratory, it is essential specimens have a structure representative of in-situ conditions. Simulating wetting-drying cycles in the laboratory is possible albeit time-consuming and a faster alternative procedure would be preferred, which is the focus of this paper. Mixtures of two soils were prepared in the laboratory by either: exposure to three simulated wetting-drying cycles, or one of two compaction approaches. The microstructure and drying-path SWRC of the specimens prepared with each method were measured. Most of the compacted specimens achieved similar pore size distributions to the cycled samples though the outcomes in terms of achieving a target SWRC, which was the objective of the study, are mixed. The SWRCs of most compacted samples had similar gravimetric water contents yet significantly higher saturation degree at every suction measured. This is explained by the compacted samples containing less macro pores than cycled samples. The compaction procedure, designed to produce specimens having a SWRC similar to that of cycled materials, seems promising but needs modification.

Large cell triaxial tests of a partially saturated soil with vegetation

The use of vegetation roots as a nature-based solution against landslides and erosion requires the definition of sample preparation protocols and adoption of equipment that allows testing representative elementary volumes of the whole soil-root system. For this purpose, large cell triaxial compression tests were carried out on fallow and vegetated samples at different degrees of saturation. Samples were prepared by static compaction of a silty sand and seeded with Cynodon dactylon. The hydraulic state during plants growth was controlled and reproduced on bare soil samples. After isotropic compressions, the shearing phase was carried out at very low confining stresses (i.e., below 50 kPa). Tests were deemed to be comparable by assessing the normalised volume of roots with respect to soil, after shearing. For a given confining stress, soil samples with higher matric suction exhibited higher shear strength, furtherly increased by roots. The stress-strain behaviour observed in the vegetated soil systematically changed, when comparing tests at low and high matric suction values, due to the different mechanisms of vegetation reinforcement depending on the hydraulic state at the soil-root interface. The results were successfully interpreted within a failure criterion and skeleton stress framework for partially saturated soils, considering soil suction, degree of saturation, soil microstructure and the normalised volume of roots.

Pore scale investigation of unsaturated granular soil behaviour by means ofin situCT experiments

With continuing evolution of imaging techniques from medical applications and materials science, non-destructive imaging experiments have also become an important method to investigate soil specimens. Amongst other methods, computed tomography (CT) has developed to a tool to visualise and better understand the microstructure of different soils based on 3D image data. Furthermore, the acquisition of a temporal series of CT images allows to study processes in soils on the microscale, e. g., during mechanical loading. In order to study the hydraulic and mechanical behaviour of unsaturated granular soils, we combine different custom-built miniaturised experimental set-ups with geomechanics background with computed tomography, yielding so-calledin situCT experiments. By means of image reconstruction and further image analysis based on segmented CT images acquired during different hydraulic and mechanical experiments, we study the drainage and imbibition process as well as the shear process of unsaturated sand and glass bead specimens on the pore or grain scale. The analysis of data on the microscopic level, including the phase distribution, interfacial areas, contact lines as well as radii of curvature of capillary menisci, allows to obtain insights into the macroscopic water retention behaviour and shear behaviour of granular soils.

Developments in the seismic CPT and links to the Ménard Pressuremeter Test

The seismic cone penetration test (SCPT) was developed in the early 1980s and significant developments have been made in its use and application. This paper provides a review of these developments with particular emphasis on its application to identify soil microstructure. The Ménard pre-bored pressuremeter test (PMT) is one of the most popular in situ tests in France and links between SCPT results and the PMT are suggested.

Experimental study on solidified dredged sediment with MgO and industrial waste residue

To improve the engineering performance of dredged sediment, a solidification study using reactive MgO in combination with desulfurization gypsum (DG) and steel slag (SS) was carried out. The results indicated that the optimum contents of MgO, DG and SS in the solidified sediment were 7.62%, 11.31% and 5.80%, respectively. The encapsulation, cementation and filling of hydration products of MgO-DG-SS made the soil microstructure more stable, thus significantly improving the mechanical properties of sediment. Meanwhile, the MgO-DG-SS could effectively immobilize the heavy metals in sediment through chemical bonding and adsorption of hydration products.

Effects of Dry–Wet Cycling and Temperature on Shear Strength and Microscopic Parameters of Coal Measure Soil

Exposed coal measure soil (CMS) found in the mountains of Southern China is significantly affected by the seasonal climate, which makes this region prone to frequent shallow landslides. In this regard, very few studies have focused on the shear strength and microscopic characteristics of CMS subjected to dry–wet cycling and temperature. The aim of this study was to experimentally investigate the effects of dry–wet cycling and temperature on shear strength and microscopic parameters of CMS. We carried out an unconsolidated undrained triaxial test and scanning electron microscopy of CMS obtained from the K209 slope on the Chang-li highway. Our results indicated that the soil shear strength and microstructure parameters significantly decreased before three dry–wet cycles. Above 35 °C, the temperature affected mainly the mean fractal dimension. The soil cohesion was negatively correlated with the fractal dimension and positively correlated with the probability entropy. The surface-crack occurred once the stress value of high temperature was greater than 0.57 MPa. Strain-softening, swelling–shrinkage, low soil strength, and high soil temperature formed the main factors underlying rainfall-induced K209 shallow landslides.

Theoretical and experimental bases for the equivalent circuit model for interpretation of silty soil at different temperatures

The exploitation of underground space is accompanied by complex geotechnical problems. The development of electromagnetic exploration technology provides a new perspective for preventing and avoiding these problems. In this work, electrochemical impedance spectroscopy (EIS) was used to test the single-phase and mixed-phase medium. Based on the unsaturated soil theory and the dual-water conductivity theory, an equivalent circuit model to describe the electrochemical characteristics and microstructure of silty soil with temperature changes through comparative research. The results indicate that the resistance of near-water layer is not affected by temperature, the resistance of silty soil increases mainly results from the influence of the far-water layer until which increases significantly after freezing. The capacitance change of silty soil is mainly affected by the slowing down of the orientation movement of polar molecules in the far-water layer. Based on the fitting data, a mathematical model for calculating the unfrozen water content of frozen soil was proposed, which reasonably verified the relationship between the unfrozen water content and electrical resistance. By improving the testing conditions of electrochemical impedance spectroscopy, this method may provide new insights for future research of soil electromagnetic testing technology.

Novel Insights into the Influence of Soil Microstructure Characteristics on the Migration and Residue of Light Non-Aqueous Phase Liquid.

Understanding the influence of soil microstructure on light non-aqueous phase liquids (LNAPLs) behavior is critical for predicting the formation of residual LNAPLs under spill condition. However, the roles of soil particle and pore on LNAPLs migration and residue remains unclear. Here, the experiment simulated an LNAPLs (diesel) spill that was performed in fourteen types of soils, and the key factors affecting diesel behavior are revealed. There were significant differences between fourteen types of soils, with regard to the soil particle, soil pore, and diesel migration and residue. After 72 h of leakage, the migration distance of diesel ranged from 3.42 cm to 8.82 cm in the soils. Except for sandy soil, diesel was mainly distributed in the 0−3 cm soil layer, and the residual amounts were 7.85−26.66 g/kg. It was further confirmed from microstructure that the consistency of soil particle and volume of soil macropores (0.05−7.5 μm) are important for diesel residue in the 0−1 cm soil layer and migration distance. The large soil particles corresponding to 90% of volume fraction and volume of soil mesopores (<0.05 μm) are key factors affecting diesel residue in the 1−3 cm soil layer. The result helps to further comprehend the formation mechanism of residual LNAPLs in the soil.

Microstructure properties of soft soils under marine, interactive marine, and river-lake facies sedimentary environments

The marine environment has an important influence on the microscopic and macroscopic properties of soft soils, which are different from other types of soft soils. This paper discussed the microscopic properties of marine, interactive marine, and river-lake facies soft soils and their relationship to macroscopic parameters. Geotechnical tests and field emission scanning electron microscope (FESEM) tests for three types of soft soils were carried out to study the influences of sedimentary environments on microstructure and macroscopic properties; the microstructure parameters were obtained by image processing technology and fractal theory; quantitative relationships between microstructure parameters and the macroscopic physical parameters were analyzed by multiple regression analysis methods. The results show that three sedimentary environments lead to significant differences in the microstructure of soft soils, particularly in the marine environment, with the soil particles being mostly flake stacked under marine sedimentation, flocculated in interactive marine, and agglomerated in river-lake facies. The fractional dimensions of the pores, soil particles distribution and the pore shape are 1.54, 1.78 and 1.43, respectively, the largest for the interactive marine soft soils. The probability entropies of soil particles in marine, interactive marine, and river-lake are 0.43, 0.47, and 0.53, respectively, which get bigger in order. The highest pore complexity is found in the interactive marine soft soils. The relationship formula between macroscopic physical parameters and microstructure parameters was established. It reveals that the physical parameters are expressed by joint action of pore distribution fractional dimension, soil particles probability entropy, pore shape fractional dimension and soil particles shape fractional dimension.

Small strain stiffness, microstructure and other characteristics of an allophanic volcanic ash

Small strain mechanical properties are a prerequisite for seismic analysis and design. Corresponding studies on volcanic soils are by themselves limited while research on some volcanic ashes which intrinsically have high fines content is even rarer. Presence of clay minerals like allophane and imogolite confers uncharacteristic properties to the soils which somehow seems underappreciated. This is evidenced, for instance, by the near absence of literature related to the 2016 Kumamoto Earthquake explicitly mentioning the influence of these particular clay minerals. Against this background, this study aimed at partially filling the gap with respect to the small strain shear modulus, Gmax of an allophanic volcanic ash and underline the influence of allophane and imogolite on some of the soil properties. Remoulded black volcanic ash, kuroboku was sampled from two sites in the Kyushu area in the south of Japan. As part of the experimental work, a series of index properties, engineering properties, electron microscope (Scanning Electron Microscope (SEM) and Field Emission Scanning Electron Microscope (FE-SEM)) examinations and bender element (BE) tests was performed on these soils. The paper elaborates on the peculiar characteristics of the allophanic volcanic ash, including presence and effect of water of crystallization and influence of electrostatic bonding on grain size distribution and liquefaction resistance. Based on the SEM and FE-SEM images, the microstructure of the kuroboku soils was assessed in terms of its various components, the contact relation and the pore space. Observations from the microscopy analysis like presence of diatoms and intra-elemental pores were key to understanding the slight variation in the Gmax among the tested soils. An attempt was also made at situating the Gmax of the kuroboku soil vis à vis other types of soils. Empirical relationships for estimating the Gmax of kuroboku separately and with other volcanic soils were proposed. Findings from this research could rekindle an interest into further understanding the particular characteristics of allophanic materials as well as encourage more research on their dynamic properties, which is essential considering that these materials are mostly found in seismically active regions.

Effects of Cement Treatment on Mechanical Properties and Microstructure of a Granite Residual Soil

A proper treatment of granite residual soil (GRS) in geotechnical practices requires both macro and microscopic evaluations. In this study, uniaxial and oedometric compression tests were conducted to investigate the mechanical properties of the saturated untreated and cement-treated GRS. Meanwhile, XRD, SEM, and MIP tests were conducted to identify the presence and types of C–S–H and the changes in the pore structure after cement treatment. The effects of cement treatment on the uniaxial compressive strength, secant modulus, compressibility, and vertical yielding pressure were revealed and the mechanisms of the soil structure to be modified through cement treatment were clarified based on the test results. A threshold volumetric cement content of 2–3% was determined based on the mechanical properties and microstructural characteristics of the saturated cement-treated GRS. Cement contents below this threshold would produce inadequate cementation between the soil particles. In contrast, cement contents above this threshold are considered inefficient because the transformation of the soil structure from single-porosity to dual-porosity increases the total porosity and retards the strength and stiffness gains.