Unsaturated Samples Research Articles

Carbon dioxide emissions at local scale linked to soil heterotrophic activity from an experimentally simulated drained peatland in Western Patagonia (Tierra del Fuego, Chile)

AbstractGreenhouse gas emissions from managed peatlands have not been extensively studied in Western Patagonia. The objective of this study was to assess the annual CO2 emission from microbial carbon (C) mineralization in a peatland site under not saturated conditions at Tierra del Fuego. The annual CO2 emissions were measured from unsaturated soil samples (n = 41) under soil incubation at seasonal local temperatures to simulate CO2 emissions for a year, using a non‐dispersive infrared gas analyser. Spatial models for total soil C and CO2 were calculated using discrete and continuous variables. The annual mean of measured cumulative CO2 was 1,358 µg CO2 g soil−1, lower than Northern peatlands, and 82% of the C mineralization occurred in the warmer season. The modelled CO2 in the warmer season showed levels of CO2 as high as 4 mg CO2 g soil−1, but 66% of the area showed between 600–2000 µg CO2 g soil−1, which is 28%–92% and 9%–32% of CO2 values reported for crop rotations. Consequently, after a potential habilitation of the study area for agricultural use, the soil CO2 emissions from heterotrophic activity would become a C source to the global CO2 emissions. This ecosystem is highly exposed to the effects of the land‐use change, and global temperature increase.

A finite element method based on modified Alonso unsaturated soil constitutive model

Based on the revised ALONSO model, combined with the ABAQUS software platform, this paper compiled the USDFLD subroutine and obtained the stress-strain curve and the axial strain-volume deformation by simulating the triaxial shear test of the unsaturated soil to control the suction and confining pressure. The curve verifies the accuracy and reliability of the algorithm. The research results show that: (1) When the suction is the same, the higher the confining pressure is, the higher the yield point of the soil sample is. According to the comparison between experiment and simulation, the modified Alonso model can reflect the nonlinearity of the stress-strain relationship of the reshaped unsaturated soil sample and its plastic flow characteristics; (2) Compared with saturated soil, unsaturated soil has a higher yield point as a result of the suction. And the larger the suction is, the higher the yield point is, which proves that the model can simulate the influence of unsaturated soil liquid relative to the solid phase.

Test Apparatus for Rapid Determination of Soil-Water Retention Curve under Isotropic Loading Condition

Abstract In the present study, a custom-designed test apparatus comprising a plexiglass cell chamber with a base pedestal to accommodate a cylindrical soil sample with the water potential and volumetric water content sensors was developed to measure the soil-water retention curve (SWRC) of unsaturated soils under isotropic loading condition. The developed test apparatus is capable of rapidly measuring isotropic stress-dependent SWRC utilizing preequilibrated unsaturated soil samples. The methodologies for obtaining normalized volumetric and gravimetric stress-dependent SWRC using continuous and spot measurements were discussed in detail. In addition, the need for the soil-specific sensor calibration was illustrated, and an alternate method of developing calibration curves for sensors was discussed. To understand the efficacy of the developed apparatus, stress-dependent SWRC for two different statically compacted low-plasticity clays at isotropic confining pressures of 10, 100, and 200 kPa were evaluated. The stress-dependent SWRC obtained from the developed test apparatus is in good agreement with the results obtained from the suction-controlled triaxial test apparatus, which proves the ability of the developed apparatus to replicate stress-dependent SWRC without involving the axis translation technique. In addition, the apparatus can also be used to obtain the void ratio of unsaturated soil under various isotropic confinements at constant water content. Hence, the developed test apparatus has the potential to simulate both water content and void ratio surfaces with respect to suction under rapid isotropic loading condition, which can be used for constitutive modeling in unsaturated soils.

Determining the effect of dissolved CO2 on solution phase Li+ diffusion in common Li-ion battery electrolytes

Electrolyte decomposition in Li-ion batteries (LIB) and its entailing gas evolution significantly impacts cell performance. CO2 is one of the most abundantly evolved gases from common LIB electrolytes and could potentially affect Li+ solution transport during LIB operation. To this end, a comparative analysis of the Li+ diffusion coefficient (DLi+) which governs mass transport is required. Herein a methodology is established to saturate common battery electrolytes with a soluble gas without introducing other contaminants, so as to determine DLi+ in saturated and unsaturated samples. As a proof of concept, the values of DLi+ are determined in 1 M LiPF6 and 1 M LiClO4 in ethylene carbonate (EC):dimethyl carbonate (DMC) with and without dissolved CO2, confirmed by gas chromatography–mass spectrometry (GC–MS). The results obtained by pulse field gradient nuclear magnetic resonance (PFG-NMR) diffusion measurements agree for both electrolytes; saturating the electrolyte with CO2 has no measurable effect on DLi+ and therefore does not hinder Li+ mass transport.

Cyclic shear property and seismic runout analysis for pumice fall deposit

This paper presents the physical properties of pumice fall deposit sampled at Takanodai, Minami-Aso village, Kumamoto Prefecture in JAPAN, where long runout landslide took place due to the main shock from the 2016 Kumamoto earthquake. A series of direct box shear tests under various conditions such as constant-stress, constant-volume and cyclic-shear loading conditions was conducted for undisturbed and unsaturated samples of pumice fall deposit to evaluate shear strength properties and seismic behaviors. Based on the slope stability analysis followed by the soil runout analysis at Takanodai, the pumice fall deposit layer shows a large horizontal displacement. While pumice fall deposit layer has enough stability under a normal condition, the strength reduction induced by the soil particle crushing is considered to be one of the factors caused a long runout landslide at the above site.

The influence of soft clay saturation characteristics on unconfined compressive strength in Guangdong-Hong Kong-Macao Greater Bay Area

In order to study the influence of saturated characteristics of soft clay on unconfined compressive strength, the soft clay of Guangdong-Hong Kong-Macao Bay Area is taken as the research object, comparing and contrasting on unconfined compression test of saturated clay and unsaturated clay in laboratory, studying the variation law of unconfined compressive strength and sensitivity of unsaturated and saturated soft clay. The test results show that: 1. During the failure of unsaturated soft clay samples, oblique fractures appear, showing brittle shear failure, while the saturated clay samples appear constant bulging in the middle, and finally a “cross” is broken in the central bulging part, showing plastic shear failure.2. The unconfined compressive strength of unsaturated clay is about 10kPa higher than that of saturated soft clay, and its corresponding sensitivity is also about 0.4 higher. It can be shown that the soil saturation has a certain influence on the soil strength. The difference between saturated soil and unsaturated soil is the existence of gas phase. To be more precise, the existence of gas phase in unsaturated soil, i.e. the existence of suction, makes the soil stronger and presents the brittle shear failure form, while the saturated soil basically presents the plastic shear failure form.

Creep behaviour monitoring of short-term duration for fiber-glass reinforced composite cross-arms with unsaturated polyester resin samples using conventional analysis

The leading objective of this experimental study is to perform a Short-term Creep Testing on samples (coupon test) obtained from a private company in order to learn the durability of the Unsaturated Polyester Resin (UPR) material with and without Calcium Carbonate (CaCO3) on the applied cross-arm application according to ASTM E139. Through the method of flexural test jig (three point bending test) along with the usage of the furnace chamber HK160, samples then examined with initial temperature of 30°C until it breaks down. Result has been evaluated using conventional method for predicting the life-long purpose of the samples for future reference and analysis. The configurations or the patterns of the failure through (conventional) method recorded for short term test inside HK160 Chamber furnace fails at temperature of 120°C for samples without CaCO3 the samples fails and crack. Therefore, the samples with due to the material of UPE with FRP in a bar shape are said to have Ultimate Temperature for Failure of 120°C. Further details are crucial for advance analysis in the future research purposes.

Analysis of unsaturated shear strength and slope stability considering soil desalinization in a reclamation area in China

The unsaturated soil shear strength is a crucial parameter for the evaluation of erosion and slope stability analysis. In this study, we collected unsaturated and undisturbed soil samples in Aug 2018 and investigated the relationship between the unsaturated soil shear strength, soil salt content (SSC) and particle content in coastal reclamation areas. A combination method of the direct shear test and soil water characteristic curve (SWCC) was used on undisturbed soil to determine the unsaturated soil shear strength in 79 soil sampling sites. The shear strengths of four gradient salt content soils with different matric suctions were selected to perform a slope stability analysis using the strength reduction method (SRM) for an agricultural engineering slope model considering soil desalinization. The shear strength of unsaturated soils increased with increasing matric suction, especially when the soil salt content was high. The soil effective cohesion was found to be significantly positively correlated with the SSC and clay content, while the soil effective internal friction angle was significantly negatively correlated with the SSC and clay content. The internal friction angle related to matric suction was independent of the soil properties. The location of the critical failure surface was deeper, and the factor of safety (FS) was higher on a slope with a high SSC at all slope gradients than that with a low SCC. The FS decreased with increasing saturation and decreasing SSC. This implies that the agricultural engineering slope, especially for drainage ditch slopes rammed with high salt content soils, was at the stable stage after its establishment but had higher potential to fail during desalinization and saturation. This study aims to evaluate slope stability using unsaturated soil shear strength and increase the current understanding of the soil desalinization of saline-sodic soils and their efficacy in soil shear strength in coastal reclamation areas.

Damage effects on coal mechanical properties and micro-scale structures during liquid CO2-ECBM process

The utilization of liquid CO2 for enhanced coalbed methane recovery has great environmental and commercial potential to sink greenhouse gas emissions and enchance CH4 extraction. A comprehensive understanding of mechanical properties and micro-scale changes to coal saturated with liquid CO2 would fill the knowledge gap regarding CO2 utilization and be greatly beneficial to industry. The present study employed extensive experimentation using uniaxial compression test, acoustic emission (AE) monitoring, Brazilian splitting test, scanning electron microscopy, and energy dispersive X-ray diffraction spectrometry to determine the damage features of coal microstructure, mechanical characteristics, failure modes, and minerals variation between original and liquid CO2-saturated coal samples. The pre-existing cracks were enlarged and more cracks were generated in the saturated coal samples than in the raw samples. The liquid CO2-saturated coal samples exhibited larger plastic deformation and lower compressive strength in unstable crack propagation stage and more AE counts in rupture stage than those of unsaturated samples. The average compressive strength and tensile strength of the saturated samples were 21% and 27% lower than that of the raw samples. More ragged failure cracks were formed in the liquid CO2-saturated coal specimens than in the unsaturated coal samples. The failure modes of the saturated coal samples were inclined to ductile fracture as opposed to brittle fracture in the unsaturated coal samples. The width and number of micro-cracks on the saturated samples were both increased, and the amount of partial minerals that filled in the micro-cracks reduced after liquid CO2 saturation, which indicated an interaction between minerals in coal and liquid CO2 during the saturation process.

An Electrical Resistivity Method of Characterizing Hydromechanical and Structural Properties of Compacted Loess During Constant Rate of Strain Compression.

Hydromechanical and structural properties of compacted loess have a significant impact on the stability and reliability of subbase and subgrade, which needs to be quickly determined in the field and laboratory. Hence, an electrical resistivity method was used to characterize the hydromechanical and structural properties of compacted loess during constant rate of strain compression. In the present work, compacted loess samples with a dry density of 1.7 g/cm3, a diameter of 64 mm, a height of 10 mm and different water content ranging from 5–25% were prepared. The constant rate of strain (CRS) tests were conducted by a developed oedometer cell equipped with a pair of horizontal circular electrodes (diameter of 20 mm) and vertical rectangular electrodes (width of 3.5 mm) to determine the electrical resistivity of compacted loess. The results showed that as average water content increases, plastic compression indices increase from 0.220 to 0.350 and the elastic compression indices increase from 0.0152 to 0.030, but they decrease to 0.167 and 0.010 and yield stress decreases from 381.28 kPa to 72.35 kPa. Moreover, as vertical strain increases, the variation trend of average formation factor and average shape factor for the lower water content decreases but increases for the maximum water content, and the anisotropy index first decrease and then tend to increase slightly, which indicates that the structural properties of unsaturated and saturated samples during compression exhibits different trend and the anisotropy of samples tend to be stable as vertical strain increases. As the water content increases, the average formation factor and average shape factor decrease, but the anisotropy index first decreases then increases, suggesting that water content has a significant impact on these electrical indices. More important, The coefficients of average formation factor decrease from 33.830 to −1.698 and the coefficients of average shape factor decrease from 8.339 to −0.398 as water content increases, whereas there is less variation for the coefficient of anisotropic index with a value of 2.190. An equation correlating average formation factor and water content and vertical strain is regressed to characterize the hydromechanical properties of compacted loess by measuring its impedance, which can be used to evaluate the stability of compacted loessic ground and subgrade.

Effective Stress and Compressibility of Unsaturated Clayey Soil under Drying and Wetting Cycles

Naturally, soil moisture reduces during dry seasons when the soil is in drying state; while it increases during wet seasons when the soil is in wetting state. Previous studies have shown that for an unsaturated soil sample, soil-water characteristic curves (SWCCs) do not match in wetting and drying paths. The differences between wetting and drying paths are called the hydraulic hysteresis. The hydraulic hysteresis plays an important role in mechanical properties of soil such as shear strength, volume change, and settlement. The objective of this research is to study the effects of drying and wetting on the effective stress and compressibility of unsaturated clayey fine-grained soils. To this end, saturated and unsaturated triaxial tests were performed on the soil samples under various normal mean stresses, and matrix suctions in drying and wetting paths. It was found that soil samples bear higher levels of effective stress in the drying path than the wetting path under a same level of suction. The attained values of effective stress parameter (χ) showed that the Bishop’s effective stress parameter (χ = Sr) is not properly applicable for the clayey soil. Moreover, the resulting loading-collapse curve (LC) revealed that the effective pre-consolidation pressure in the drying and wetting stages changed even with same degrees of matrix suction.

Influence of different strain rates on hydro-mechanical behaviour of reconstituted unsaturated soil

The hydro-mechanical behaviour of a reconstituted unsaturated soil under different suctions and strain rates was studied through various rate-controlled unsaturated/undrained triaxial tests. The fully saturated reconstituted specimens were desaturated to four different initial suctions (s0 = 0, 100 kPa, 200 kPa and 300 kPa) and then triaxially sheared (conventional triaxial compression) at three different strain rates in undrained conditions ( $$\dot{\varepsilon }_{1} = 0.001$$ h−1, 0.01 h−1, and 0.1 h−1). The observed hydro-mechanical behaviour during shearing including the volumetric strain, deviatoric stress, degree of saturation and suction is presented and discussed in this paper. The results indicate that when the strain rate rises at the given initial suctions (or pore water pressures), the maximum deviatoric stress (qmax), critical net stress ratio (M) and critical state suction (sc) increase but the degree of saturation (Src) and volumetric strain at the critical state (evc ) reduce. The critical effective stress ratio (M′) is not dependent on the strain rate for saturated and unsaturated samples. The critical state lines for unsaturated soils with the constant strain rates are parallel with each other in the e − lnp′ space.

Titanium isotopic fractionation during magmatic differentiation

To better investigate the behavior of titanium (Ti) isotopes during magmatic processes, we report high-precision Ti isotope compositions for 60 terrestrial igneous rocks from different geological settings worldwide. Based on their major element compositions and petrographic descriptions, these samples can be subdivided into two groups: Fe-Ti oxide unsaturated and Fe-Ti oxide saturated. The Fe-Ti oxide unsaturated group samples show a narrow δ49/47Ti (δ49/47Ti = [(49Ti/47Ti)sample/(49Ti/47Ti)OL-Ti] × 1000) range (− 0.036 ± 0.043‰ to 0.082 ± 0.021 ‰), and no correlation between δ49/47Ti and the degree of differentiation is observed. By contrast, Fe-Ti oxide saturated group samples show a remarkable δ49/47Ti variation, ranging from 0.005 ± 0.018‰ to 1.914 ± 0.006 ‰, which are positively correlated with SiO2 contents, and negatively correlated with MgO contents. In particular, multiple SiO2 vs. δ49/47Ti trends are observed in Fe-Ti oxide saturated group, which are controlled by crystal fractionation degrees, magma SiO2 compositions, and Fe-Ti oxide compositions during magma differentiation.

Applications of Electrical Resistivity Surveys in Solving Selected Geotechnical and Environmental Problems

Standard test methods may not be suitable or sufficient for determining the geotechnical conditions of a structure’s subsoil and the effects of the designed structures on the environment. Geophysical test methods, validated with other methods, may prove useful. In recent years they have found many new applications in engineering practice, both geotechnical and environmental. The advantages of geophysical methods include the non-destructive and non-invasive nature of the tests, their low costs and quick results, as well as compatibility with different materials, including soils, solid rocks, wastes and anthropogenic formations. The paper presents the analysis of laboratory and field investigations including research in a modified oedometer, resistivity chamber, electrical resistivity tomography (ERT) and resistivity cone penetration test (RCPT). Laboratory tests allowed for the assessment of the degree of saturation and porosity of sandy and clayey soils. The tests were carried out on saturated and unsaturated soil samples and allowed for the determination of some relationships between electrical conductivity and porosity. The proposed equations were used to assess parameters in in situ studies using RCPT tests and showed good agreement with reference values based on undisturbed soil samples. ERT tests confirmed the usefulness of electrical measurements in the quality assurance of subsoil and hydrotechnical structures. The tests showed weakening zones in the levee body, discontinuity of the vertical sealing system on the modernized section of the embankment, and location of the top of clay deposits.

A Suction-Controlled Ring Device to Measure the Coefficient of Lateral Soil Pressure in Unsaturated Soils

A suction-controlled ring device has been developed to continuously measure the coefficient of lateral soil pressure in deformable unsaturated soil samples from the at-rest to the active condition under application of increasing vertical pressure and controlled matric suction. The device incorporates a thin aluminum specimen ring equipped with horizontal strain gages for recording the lateral soil strains. In addition, a sensor recording water volume changes is utilized to continuously monitor the degree of saturation of the soil sample during tests. The matric suction within the soil texture is controlled using the axis translation technique. In order to verify the performance of the ring device, twenty suction-controlled tests under application of five different constant matric suctions have been performed on two different unsaturated soil types, namely Firouzkouh Clay and Sand-Kaolin mixture, each with two different initial densities. During tests, the lateral soil pressure and corresponding lateral strains have been continuously recorded from the at-rest to the active state of the soil. Accordingly, the coefficients of the lateral soil pressure for the at-rest and the active states of the soil are calculated based on the experimental data and by considering the effective stress concept in unsaturated soils. Finally, the observed responses have been analyzed and the effects of initial density and matric suction on the ratio of lateral to vertical soil pressure have been discussed.

Consolidation and creep phenomena in a sand-bentonite mixture under controlled suctions

ABSTRACT Investigating the creep of expansive unsaturated soils is necessary to simulate the long-term deformations that can cause structural failures. Hence, this paper presents a laboratory investigation of the consolidation and creep of an artificially prepared sand-bentonite mixture consisted of 40% commercial bentonite at controlled suction levels of zero, 100 kPa, 200 kPa, 400 kPa, and 800 kPa. A modified osmotic oedometer was utilised to apply vertical net stress and the desired matric suctions. After conducting the consolidation tests on samples, the last loading step was kept for extra 72 h to assess the creep behaviour of expansive unsaturated samples, and the creep coefficients (Cα) at different suctions were calculated. The results showed that the swelling index (Cs) gradually decreased with an increase in the amount of suction and a negligible value of the parameter is observed at higher levels of suction. Also, the creep coefficient decreased with an increase in the amount of suction value.

Investigating effects of temperature and confining pressure on dynamic elastic properties and permeability—An experimental study

Unconventional reservoirs refer to hydrocarbon reservoirs that require special stimulation treatments, such as hydraulic fracturing, to economically produce hydrocarbon. These reservoirs are tight with very low permeability, less than 0.1 milli Darcy (0.1 mD). Shale oil and shale gas reservoirs are prime examples of unconventional reservoirs. Effects of temperature on unconventional reservoirs’ dynamic elastic properties and permeability are usually overlooked during laboratory measurements and reservoir simulations leading to erroneous results and unsuccessful stimulation operations.To investigate effects of temperature and confining pressure on dynamic elastic properties and permeability of unconventional core samples, two series of experiment were conducted on outcrop core samples from three prominent unconventional basins (Barnett, Wolfcamp, and Eagle Ford) in the United States. In the first set of experiments, eleven unsaturated outcrop core samples (four Barnett, four Eagle Ford, and three Wolfcamp) were used to assess effects of temperature and confining pressure on their dynamic elastic properties. In the second set of the experiments, seven core samples (two Barnett, two Eagle Ford, and three Wolfcamp) were used to study effect of temperature on their permeability at a constant confining pressure. Also, X-ray diffraction (XRD) analysis was conducted on core samples to determine the mineral compositions them. The results were then used to study effect of mineralogy on the rock samples’ dynamic elastic properties and permeability.The results of this experimental study show that increase in temperature makes the core samples more ductile and less permeable. The results also demonstrate that the degree of the effect of the temperature on the dynamic elastic properties of the samples is directly related to the presence and volume of the clay minerals.

Effect of soil structure disturbance on the shear strength of black volcanic ash soil

The effect of soil disturbance on the shear strength of black volcanic ash soil was investigated using a constant volume direct shear apparatus. Disturbance of soil structure was considered as the pore size distribution which obtained from the soil-water characteristic curve (SWCC). The disturbed sample was used as a representation of soil structure disturbance due to earthquake shakes. A series of cyclic tests were conducted under unsaturated and saturated samples. It was found that the undisturbed samples exhibit a unimodal pore structure, and the disturbed samples indicate to a bimodal pore structure. Since the pore structure of the disturbed sample is unstable, the degradation index value is higher than that of the undisturbed sample and increases with the increasing number of cycles. In other words, the cyclic normalized vertical stress of disturbed samples degrades faster. Furthermore, the degradation index value in the normallyconsolidated samples was found to be larger than the overconsolidated. It might be attributed to increasing of the pore water pressure during shearing. Where in the over-consolidated samples is lower than normally-consolidated. On the other hand, the normalized shear stress of unsaturated samples, it is slightly larger due to the suction forces in the total strength of soils.

Very-small to large strain dynamic behaviour of unsaturated sand in a wide range of suction

Shear modulus (Gmax at very small strain and G at large strain) and constraint modulus at very small strain (M) are important soil parameters for static and dynamic analysis in geotechnical applications. However, these dynamic properties of unsaturated soil are rarely reported. In this study, a cyclic simple shear apparatus was newly-modified for allowing both the shear and constrained moduli at both very small and large strains to be measured. Benders or ultrasonic sensors were embedded in an unsaturated soil sample for transmitting/receiving shear- and pressure-wave, respectively. Two very-small-strain tests were conducted to determine the Gmax, M and soil damping ratio of a sand for a wide range of suction covering from the boundary-effect, transition and residual zone of the water retention curve of the sand. In addition, six large-strain cyclic simple shear tests were carried out to investigate G. The test results showed that Gmax and M were approximately constant before reaching the air-entry value, but there was a significant increase in Gmax as the sand dried further. Yet, M dropped within the transition zone, and interestingly when the suction was beyond the residual value, M increased. M along the wetting path was higher than that along the drying path. The damping ratio, on the other hand, first reduced before reaching the air-entry value, but it increased at the transition zone and then decreased within the residual zone. At large strain, G/Gmax also increased as suction increased until reaching the residual zone, beyond which the normalised value show substantial decreased.

Field and laboratory investigation of rainfall-triggered slope failure in unsaturated loess soils, New Zealand

Shallow rainfall triggered slope failures occur frequently in loess and loess-derived deposits across the South Island, New Zealand. These failures, which occur in both natural slopes and engineered cuttings, impact road infrastructure, residential housing and rural land use. When dry, the loess can form near vertical cuttings. However, with increase in moisture content loess slopes become susceptible to shallow slope failures. To date, the influence of negative pore-water pressure (suction) on the stability of loess slopes in New Zealand has not been well understood. In this paper, data from long term in situ field monitoring of rainfall, suction and volumetric water content from a loess slope in Banks Peninsula, Canterbury are presented with laboratory triaxial test results undertaken on undisturbed unsaturated loess samples. Field and laboratory soil responses to wetting and drying are compared, and the characteristics of rainfall events which reduce suction in situ and therefore slope stability are discussed.