Changes In Soil Microstructure Research Articles

Analysis of liquid–vapor mixed migration mechanism in unsaturated soil based on the effect of temperature on soil microstructure

Moisture migration in unsaturated soils is a result of the interaction between temperature and soil microstructure. In order to reveal the mechanism of moisture increase of subgrade soils under diurnal cycle conditions, a series of macro and microscopic tests were carried out on the unsaturated silty clay and sand soil, including liquid–vapor mixed migration tests simulating a one-dimensional subgrade, environmental scanning electron microscope (ESEM), and matrix suction test. Then, the soil microstructure in microscopic images was investigated using the particle (pores) and cracks analysis system (PACS). Next, the relationship between the thermal effects of the soil–water characteristic curve (SWCC) and changes in soil microstructure was analyzed. Finally, the change mechanism of liquid–vapor mixed migration based on the change in soil microstructural under thermal effects was analyzed. The results showed under the diurnal cycle, both the silty clay and sand soil columns appeared in the phenomenon of a “diurnal cycle of water vapor migration”, which led to moisture accumulation at the top of the soil layer. In silty clay soil column, moisture was primarily driven by water vapor pressure and migrated upwards. Additionally, moisture redistribution led to changes in soil microstructure, which in turn influenced the process of moisture migration. The moisture content in the upper soil layer increased making both inter-aggregate and intra-aggregate pores decrease. The moisture content in the lower soil later decreased, leading to the water-holding capacity of the lower soil layer to increase. So, the moisture migration gradually decreased at night. In the sand soil column, moisture migration was mainly driven by gravity potential and migrated downwards. Moisture redistribution made inter-aggregate pore and matrix suction of the upper soil layer increase, leading to an increase in moisture migration at night.

Physicochemical effect on soil in sliding zone of reservoir landslides

The fate of reservoir landslides is mainly dictated by the mechanical behavior of soil in the sliding zone. This soil may be altered under changing environmental conditions of both physical and chemical nature. In this paper, we present a laboratory study on the changing soil properties under complex physicochemical interactions. The soil specimens are subjected to 1, 2, 3, 5, 7, and 10 wetting-drying cycles. The soil strength is determined by triaxial tests. These tests are performed with three types of pore water, namely, acidic groundwater (AGW), normal groundwater (NGW), and distilled water (DW). The deterioration of soil strength, the change in soil microstructure and mineral composition are observed and correlated. Interesting findings for reservoir landslides are drawn: acidic underground water considerably damages the soil microstructure and deteriorates the shear strength compared to normal underground water, while distilled water causes minimal disaggregation. A three-stage micromechanism associated with four physicochemical interactions is revealed, providing insights into microstructure damage and mineral transformation.

Laboratory and modelling investigation on chemically stabilized native expansive soil as a CNS material

Expansive soils challenge the stability of several civil engineering structures. Because of the presence of Montmorillonite clay mineral, they have the inherent property of swelling and shrinking upon wetting and drying. Researchers proposed several ground improvement and foundation techniques to enchase the properties of these soils. Cushion materials, particularly Cohesive Non– Swelling (CNS) cushions, attracted the attention of civil engineers due to their simplicity and long-term stability. To avoid the problem of the non-availability of required soil, a technique is presented in this study to convert local expansive soils into suitable CNS materials via chemical stabilisation. The study uses clayey sand from the Tirupati region of Andhra Pradesh. Admixtures such as fly ash and lime chemically modify native soils into CNS materials. The effectiveness of these prepared CNS materials in reducing expansive soil heave is investigated using model test tanks. SEM and XRD analyses are also used to determine changes in soil microstructure. According to the findings, 5% lime and 20% fly ash are optimal for preparing CNS material. The microstructure of the stabilised soil revealed the formation of cementation compounds, which improved the strength of the native soil stabilization using fly ash.

Changes in soil microstructure and physical characteristics of peat soils under pineapple plantation

In Malaysia, peat soils have been successfully planted with various crops such as pineapple, oil palm, and sago. However, the development of peat soils to plantations may raise problems to the environment and alter their physical and morphological characteristics. Hence, this study was conducted to differentiate peat soils microstructure between natural peat and pineapple cultivated peat. The soil samples were taken at 0-50 cm, 50-100 cm, and 100-150 cm depth, and the Von post scale method was conducted in the field. The samples were tested in the water content, organic content, and rubbed fiber content for physical analysis. The samples were then sent to the scanning electron microscope (SEM) and energy dispersive x-ray (EDX) analysis to determine peat soil microstructure and elemental analysis. The results showed that the pineapple cultivated peat has a higher humification degree than natural peat. The fast degradation process of cell walls also can be seen in SEM images in higher humification degrees. It is concluded that the disturbance of peatlands due to human activities may accelerate the decomposition process in peat soils.

Microstructure Evolution of Saturated Clay Under Cyclic Shearing

Abstract Changes in soil microstructure are important for understanding the macro-behavior of soil. Studies in the past have mainly focused on analysing variations under isotropic and one-dimensional consolidation. The present study aims to investigate the microstructure evolution of saturated clay under cyclic shearing. Cyclic triaxial tests are carried out under different magnitudes of effective confining pressure, cyclic deviator stress, and overconsolidation ratio (OCR). The soil microstructure is determined through the scanning electron microscope and mercury intrusion porosimetry tests. It is observed that there is a threshold cyclic deviator stress, above which the soil specimens show significant plastic strain and eventually fail. The threshold value is found to increase almost linearly with the increase of effective confining pressure and OCR. On the other hand, when the cyclic deviator stress is higher than the threshold value, some large pores appear even though the total pore volume is kept constant (undrained condition under cyclic loads). With the appearance of some larger pores, the soil skeleton becomes weaker and eventually fails.

Geotechnical properties of sandy clayey soil contaminated with lead and zinc

Previous studies have shown that pore fluid influences the geotechnical properties of soils. The aim of this paper is to investigate the effects of various concentrations of lead nitrate and zinc nitrate hexahydrate heavy metals in pore fluid on geotechnical properties of kaolinite–sand mixtures based on laboratory tests. Remolded samples of kaolinite–sand mixed with 5 and 25 cmol/kg-soil concentrations of $${\text{Pb}}\left( {{\text{NO}}_{3} } \right)_{2}$$ and $${\text{Zn}}\left( {{\text{NO}}_{3} } \right)_{2} \cdot 6{\text{H}}_{2} {\text{O}}$$ solutions were prepared. One-dimensional consolidation tests, direct shear tests, unconfined compression tests, consistency tests, sediment tests, adsorption tests, and X-ray diffraction tests were conducted on samples to investigate the changes in geotechnical properties of samples in the presence of heavy metals in pore fluid. The results indicate that the presence of Pb(II) and Zn(II) would decrease the initial void ratio and sedimentation of samples which is attributed to the noticeable changes in soil microstructure. With increasing Pb(II) concentration from 0 to 25 cmol/kg-soil, the void ratio of kaolinite containing 10, 25 and 40% sand decreased 18, 24 and 32% respectively. In addition, the reduction rates in similar samples containing Zn(II) were 14, 19 and 23%. The liquid limit of samples also decreased when the concentration of heavy metals increased to 25 cmol/kg-soil. The reduction in liquid limit was 21%, 15% and 14% for Pb(II) and 16, 12 and 7% in the presence of Z(II) for samples with 10, 25 and 40% of sand respectively. Significant decrease in the amount of cohesion also observed according to the test results. In the ratio of 40% sand the cohesion decreases 51% and 24% for Pb(II) and Zn(II) when the concentration of heavy metal increased from 0 to 25 cmol/kg-soil. The results of unconfined compression tests show that there is noticeable reduction up to 52% due to Pb(II) with the concentration of 25 cmol/kg-soil in the ration of 60% kaolinite and 40% sand.

Mechanism of Microstructural Variation Under Cyclic Shearing of Shantou Marine Clay: Experimental Investigation and Model Development

Soil microstructure has been extensively studied in recent years, because it can provide insights into macro behaviour. Previous studies in this area mainly focus on the change in soil microstructure during isotropic and one-dimensional consolidation. The principal objective of this study is to investigate the mechanism of microstructure evolution of soft marine clay under cyclic shearing. Cyclic triaxial tests were carried out at various confining pressures, cyclic deviatoric stresses and over-consolidation ratios (OCR). Soil microstructure was determined using the scanning electron microscope (SEM). Artificial neural networks (ANN) were used to develop a model for understanding the interaction effects of different parameters (number of cycles, cyclic deviatoric stress) on axial strain at different confining pressures (60, 80, 100 and 150 kPa). It was found that there is a threshold cyclic deviatoric stress, above which soil specimen shows significant strain and eventually fails. The threshold value is found to increase almost linearly with an increase in confining pressure and also, increases with increasing OCR. On the other hand, the change of soil microstructure during the subsequent cyclic shearing depends on the magnitude of cyclic deviatoric stress. When the cyclic deviatoric stress is below a threshold value, the soil microstructure maintains almost unchanged. When the cyclic deviatoric stress exceeds the threshold value, some large pores appear even though the total pore volume is kept constant (undrained condition under cyclic loads). With an increase in pore non-uniformity, soil skeleton becomes weaker and eventually collapses.

Experimental Evidences of the Strengthening of Dredged Sediments by Electroosmotic Consolidation

Nowadays dredged materials are increasingly seen as a resource, and strategies and methodologies for their beneficial reuse are being developed throughout the world. When dealing with fine grained sediments, electro-osmotic treatment is very promising to this aim, being able to speed up dewatering and to remove contaminants. The paper presents the first results of an experimental activity started to get an insight on the ability of electric treatment to strengthen fine grained dredged soils. A special oedometer was adopted to this aim, capable of applying different combinations of mechanical and electric loads to the soil, and large enough to allow the final retrieval of specimens to be tested in a triaxial cell. Both the oedometric and the triaxial test results obtained on a soft dredged material at a very high water content indicate that the application of a low voltage produces a negligible volumetric reduction but a complete change in the mechanical behaviour of the treated soil. Upon electro-osmotic consolidation, the soil—still having a very high void ratio—behaves as being over-consolidated, with a brittle behaviour in the triaxial tests and a large increase of the yielding strength in the oedometric tests. It is argued that such a sharp change in the macro-mechanical response of the treated soil is not related to the amount of water removed but to the change in soil microstructure caused by the removal of part of the electrically bonded water during the electroosmotic process.

Restoration of former agricultural fields on acid sandy soils: Conversion to heathland, rangeland or forest?

Three restoration strategies on agricultural fields with acid sandy soil were evaluated after 18 and 25 years: conversion to heathland, rangeland and forest. Changes in soil microstructure, chemical characteristics, availability of N and P, and vegetation composition were analyzed in agricultural soils, three undisturbed reference sites and five types of restored former agricultural fields.Agricultural soils were characterized by organic slurry without much soil life. Soil nutrients were especially high for P, mostly in mineral form, and P-desorption rates were high. Partial and complete topsoil removal, aiming at heathland restoration, led to (much) lower soil organic matter and nutrients, but not to recovery of soil life, nor to P-limited soils. Heather was accompanied by many grassland species, even with complete topsoil removal. Conversion to rangeland did not decrease nutrient stocks, but led to improved soil life, although different from reference grasslands due to the higher pH. P-availability remained high, but net N-mineralization and plant N-content were clearly lower after 25 than after 18 years. Plant diversity was relatively high, and cover of eutrophic grasses decreased to 8–39% in intermediate and productive rangelands. Nutrient-poor species remained absent, but the slightly higher pH improved conditions for many grassland herbs. Afforestation did also not lead to nutrient-poor conditions, but soil life clearly increased and nutrients were used for rapid tree growth. Undergrowth species however remained eutrophic.It was impossible to retrieve the P-limited reference ecosystems within 25 years of restoration, not even with complete topsoil removal. Differences in plant diversity between expensive topsoil removal and much cheaper conversion to rangeland were also relatively small. For restoration on a landscape scale, it may thus be better to focus on conversion to semi-natural grasslands and afforestation. The half-open, nutrient-, mineral-, and species-rich landscape offers opportunities for large grazers, but also for many insects and birds.

Microstructural study of deformation zones during cone penetration in silt at variable penetration rates

During conventional cone penetration testing in silt, the soil will normally be partially drained. If the penetration rate varies, time for drainage is altered and therefore the measured cone resistance and pore pressure will change. This paper studies the change in soil microstructure around the probe during cone penetration carried out at different penetration rates to investigate the failure mechanism and the processes controlling drainage in silt. Backscattered electron images of polished thin sections prepared from frozen samples at the end of penetration were used. Making use of advanced image-processing techniques, the statistical distribution of particle orientations and the local porosity were investigated for zones around the cone tip and shaft. The spatial distribution of the measured microscale parameters in the region near the probe indicates that the soil deformation during a piezometric cone penetration test (CPTU) in silt leads to the formation of both contractive and dilative zones. The macro response of the material, presented by the pore pressure and cone penetration resistance measured during the test, results from the competition between these zones during penetration, which is shown to be dependent on the penetration rate.

Changes in Soil Micro-Structure for Natural Soft Clay under Accelerated Creep Condition

The accelerated rheological properties of natural soft clay are the external power and predisposing factors of geological and geotechnical disasters. To determine the changes in soil micro-structure for natural soft clay under accelerated creep condition, a detailed study has been carried out using consolidated undrained tri-axial tests under static and cyclic loads, as well as experimental data from mercury intrusion porosimetry and a scanning electron microscope. In the tri-axial tests, creep of the soft clay would be accelerated under dynamic loads, the sample would be destroyed faster, and the strain-rate would increase clearly along with the increase of the vibration frequency. Based on a comparison of micro-structure behaviors between the intact sample and the test soil samples under static and dynamic loads in the creep condition, an interpretation of the changes in soil micro-structure for natural soft clay under an accelerated creep condition was proposed. The research indicated that the large pores of the soil sample gradually changed into small pores, the large particles continuously changed into tiny particles, and the pore distribution gradually became uniform under dynamic loads. It was found that the fractal dimension of the pores decreased with the increase of the vibration frequency. The test results not only showed that the accelerated creep characteristics depended on the vibration frequencies of the applied dynamic loads, but the accelerated creep was actually an unceasingly self-adjusting and reengineering process of the internal structure in soil under dynamic loads. In addition, these results revealed that there was a close relationship between the vibration frequencies and the micro-structure parameters.

Stress-Dependent Soil-Water Characteristic Curves of Lime-Treated Expansive Clay

AbstractProper understanding of the field performance of lime-treated expansive soils requires the evaluation of their unsaturated properties under stress conditions similar to that in the field. This paper studied the influence of net vertical stress on the soil-water characteristic curves (SWCC) of lime-treated expansive clay. The first phase of this study investigated the influence of lime content and curing period under low net vertical stress (7 kPa) using axis translation and filter paper techniques with simultaneous measurements of sample volume change. The second phase evaluated the SWCCs of untreated and lime-treated clays under vertical stresses of 100 and 600 kPa. Different representations of SWCCs were considered in the interpretation of test results. Test results indicated that lime content had a significant effect on the shape and air entry value of SWCCs because of changes in soil microstructure, whereas curing period had a far less significant effect. Furthermore, the application of net ve...

MODIFICATION OF KAOLIN MINERALOGY AND MORPHOLOGY BY HEAT TREATMENT AND POSSIBILITY OF USE IN GEOTECHNICAL ENGINEERING

This paper presents an experimental work on the effect of heat treatment on the mineralogy and morphology of kaolin. Kaolin, which is moderately expansive, was preheated at temperatures ranging from 200°C to 800°C by increment of 200°C and cooled at room temperature before it was mixed with peat. Peat represents a compressible soil and has low bearing value. The main aim of the work is to study the effect of heat treatment on shear strength parameters of peat-kaolin mixture. The mixtures are prepared at different proportions, and at each proportion triaxial samples were prepared and tested in the undrained condition to study the shear strength parameters. Untreated kaolin-soil mix was also prepared to provide a comparison between the treated and untreated soil mix in terms of the shear strength behaviour. XRD and FESEM were also carried out to investigate the change in soil micro-structure due to heat treatment. The tests showed that there was a slight improvement in the strength of peat-kaolin mixture when kaolin was preheated to a temperature of up to 400°C. Finally results were discussed and conclusions were made for the preheated kaolin-peat mixture.

Sequestration of chlordecone in the porous structure of an andosol and effects of added organic matter: an alternative to decontamination

The context of this study is the pollution of soils and water by a persistent organo‐chlorinated insecticide, chlordecone, in a tropical environment. The application of chlordecone to control the banana black weevil has led to continuing diffuse pollution of soils, and to its being a source of contamination for cultivated plants, as well as for terrestrial and marine ecosystems. Chlordecone is toxic and stable and is considered to be a persistent organic pesticide. Consequently, the amounts of chlordecone that could migrate through the environment and contaminate agricultural products need to be controlled. We measured the impact of two composts (5% weight) on chlordecone sequestration in andosols. To this end, we first characterized the transfer of chlordecone from soil to water, and then its transfer from soil to plants. After 3 months of maturation, soil‐water and soil‐plant transfers were reduced by a factor of from 3 to 10. We also showed that adding compost to contaminated soils increases chlordecone sequestration because it leads to changes in soil microstructure in the form of pore collapse and closure of the fractal structure of the allophane content.

Role of Iron Oxides in Changes in Soil Microstructure with Drying and Reducing Treatments

Role of Iron Oxides in Changes in Soil Microstructure with Drying and Reducing Treatments

Change in soil microstructure caused by shear movements : Klukanova, A; Modlitba, I Proc 6th International Congress International Association of Engineering Geology, Amsterdam, 6–10 August 1990V3, P2211–2215. Publ Rotterdam: A A Balkema, 1990

Change in soil microstructure caused by shear movements : Klukanova, A; Modlitba, I Proc 6th International Congress International Association of Engineering Geology, Amsterdam, 6–10 August 1990V3, P2211–2215. Publ Rotterdam: A A Balkema, 1990