Soil-water Characteristic Curve Tests Research Articles

Study on environmental characteristics of sustainable biomimetic soil preparation and its application in gangue reclamation

To address the key challenges in achieving large-scale ecological reclamation of coal gangue in situ, this study utilized loess from a mining area in Northwest China to prepare a biomimetic soil. A new inorganic curing agent (ISA) was developed using coal-based solid waste and cement. The study investigates the influence of different ISA dosages and their interaction with Polyacrylamide (PAM) on the environmental characteristics of biomimetic soil, including geotechnical, ecological, and heavy metal stabilization properties. Results indicate that the addition of ISA significantly enhanced the geotechnical performance of the in-situ soil, with unconfined compressive strength exceeding 350 kPa when ISA content was above 6 %. Soil evaporation experiments and soil water characteristic curve tests demonstrated that a 6 % ISA dosage combined with 0.05 ‰ PAM could maintain soil's good water retention capability while preserving a bimodal pore structure. The biomimetic soil exhibited effective stabilization of heavy metals, including Pb, Zn, Cu, and As. The particles of PAM and the hydration products of ISA not only filled soil pores and increased soil strength but also adsorbed, encapsulated, and immobilized heavy metals. “Biomimetic soil-gangue” leaching test results confirmed that the gangue samples treated with biomimetic soil met the standards for Class III underground water in China regarding heavy metal leaching values. This study provides an effective solution for the large-scale in-situ disposal and resource utilization of gangue in Northwestern mining areas of China.

Expansive soil improvement using industrial bagasse and low-alkali ecological cement

Cement is the primary material used for mitigating expansive soil hazards due to its easy availability and relatively low cost. However, the most commonly used ordinary Portland cement renders the soil strongly alkaline after treatment, and the development of cracks in the soil is not effectively controlled under the influence of wet-dry cycles following cement stabilization. These two drawbacks significantly impact both the ecological environment and the strength of the cement-stabilized expansive soil. This study innovatively proposes an improvement to the method of cement stabilization of expansive soil by incorporating low-alkali ecological cement and industrial sugarcane bagasse (a byproduct of sugar production). After assessing the basic physical properties of the expansive soil, it was improved by mixing with industrial bagasse and low-alkali ecological cement. Through experiments on dry-wet cycles, soil-water characteristic curve tests, direct shear tests, and electron microscopy scans, we explored the enhancement of the expansive soil. Our focus was on the development of cracks, soil moisture migration patterns, changes in shear strength, and microstructural alterations. The following conclusions were drawn from the experiments: i. Compared to the method of ordinary Portland cement stabilization, the pH of expansive soil improved by low-alkali ecological cement combined with industrial sugarcane bagasse is more suitable for plant growth. ii. The unique tubular structure of industrial sugarcane bagasse provides elastic space for soil deformation, inhibiting the development of soil cracks. iii. The water retention capacity of expansive soil significantly increases after improvement with low-alkali ecological cement and industrial sugarcane bagasse. Within the range of matrix suction for plant growth, soil moisture changes more slowly, providing a more stable water supply for plants. iv. The combination of 1.2% industrial sugarcane bagasse and 7% low-alkali ecological cement exhibits the highest shear resistance, maintaining strength exceeding that of untreated expansive soil after five wet-dry cycles. v. SEM analysis confirms the effectiveness of industrial sugarcane bagasse and low-alkali ecological cement in improving expansive soil. This study improved the widely used cement stabilization method for expansive soil in practical engineering applications. Additionally, it introduced an additional utilization pathway for industrial bagasse residue waste. This research provides a new direction and reference for the combination of waste utilization and natural disaster management in the treatment of expansive soil hazards.

Response of soil–water characteristics to pore structure of granite residual soils

Granite residual soil is a special regional soil with special mineral composition and pore structure characteristics, which is easy to induce serious geological disasters or engineering problems, so it is particularly important to study its mechanical properties of unsaturated soil and its control mechanism. However, the effects of dry density and initial water content on soil–water characteristic curve (SWCC) and their mechanisms are still unclear. Therefore, samples with different dry densities (1.30 g/cm3, 1.50 g/cm3, 1.70 g/cm3) and initial water content (14 %, 20 %, 22 %) were set up in this paper. SWCC test was conducted on the two groups of samples under the humidification path and dehumidification path using filter paper method. Combined with scanning electron microscopy (SEM) test and pore-size distribution (PSD) test, the influence mechanism of different micro-pore structure on SWCC and hysteresis characteristics of granite residual soil was analyzed qualitatively and quantitatively. The results show that the samples with different dry densities basically coincide with each other in the high suction segment. The larger the dry density is, the smaller the range of transition zone in the low suction segment is. As the initial water content of the sample increases from 14 % to 22 %, SWCC changes from a single increase curve to a double increase curve, and the corresponding pore-size distribution curve (PSDC) changes from a trimodal curve to a bimodal curve. The SWCC of granite residual soil has obvious hysteresis effect, and the hysteresis area becomes smaller with the increase of dry density. The inflection point exists in the hysteresis area of SWCCS with higher initial water content. The distribution range of macropore is determined by dry density, and the distribution range of small pore is determined by initial water content. The transformation of SWCC from a single increase curve to a double increase curve is mainly controlled by the distribution range of small pores. The bottleneck effect in the migration process of pore water in the soil and the pore redistribution during water intake and water loss are the main reason for the hysteresis of SWCC. The results of this work provide some guidance for the study of unsaturated soil mechanical properties of granite residual soils.

Modified Equation of Shear Strength with Respect to Saturation

Unsaturated soil shear strength is a significant topic in geotechnical engineering. The measurement of unsaturated soil properties such as shear strength and matric suction could be costly, hard, time-consuming, and often impractical to obtain. The purpose of this research is to propose a new shear strength model for unsaturated soil and to predict the shear strength by using the degree of saturation directly because the water saturation is easy to obtain. More specifically, this study focused on Bishop’s shear strength theory and van Genuchten’s soil–water characteristic curve to describe the effect of saturation on shear strength. The new shear strength model was expressed as a function of saturation degree, residual degree of saturation, effective shear strength indices, net normal stress, and five parameters. The performance of the presented model in this paper was verified by fitting it to data obtained by laboratory tests on silty sand including the triaxial shear test and soil–water characteristic curve test. From these two laboratory tests, we obtained the variation in the matric suction and shear strength with the degree of saturation under various dry densities. This proposed model was also validated against the shear strength results of the clayed-silty sand and Ankara clay recorded in the literature. Comparing the result of the shear strength under the variation of one parameter, this model was much more sensitive regarding parameter m, which was related to the material characteristic. The comparison between the predicted values and experimental points was particularly fine and showed the ability of this model to be applied to a wide range of soils.

Investigation on hydraulic-mechanical properties in bentonite-sand mixture with consideration of salinity water

Bentonite with fine components much such as extremely expansive soil has been commonly required as buffer/backfill materials in order to prevent seepage for construction of further deep geologicalrepository for disposal of high-level radioactive waste (i.e. HLW). The buffer received significant hydromechanical efforts from host rock at underground environment. While the construction/maintenance with long-time operation of a repository, it was predicted that high compacted bentonite will be progressively hydration due to pore water flow or infiltrated from surrounding host rock. The purpose of this study is to establish the connection among the basic mechanical properties, soil retention characters and chemical couple properties under unsaturated condition and saturated conditions for a bentonite-sand mixture. This conducted out some testing programs, which are soil-water characteristic curve test, unconfined compression test, creep test and triaxial compression test for unsaturated sodium bentonite sand mixture and saturated bentonite sand mixture. In addition, the influence of salinity water on mechanical properties is considered with comparison to saturated bentonite sand mixture subjected to swelled due to distilled water.

Investigation of soil-water characteristic curves for compacted bentonite considering dry density

Deep geological repositories are often considered for the disposal of high-level radioactive wastes (HLW). HLW should be disposed of safely and permanently using two barrier systems: engineered barrier systems (EBS) and natural barrier systems (NBS). An EBS is an artificial concept comprising various components. A buffer is one of the most important components because it plays a crucial role in the safety of the entire disposal system, and compacted bentonite is considered a highly adequate buffer candidate material. The compacted bentonite is located between a canister with HLW packages and rock-mass. In compacted bentonite, the dry density varies in actual repository environments, and it may be necessary to consider this when evaluating the disposal system safety. As groundwater flows into the rock mass and buffer, the degree of saturation of compacted bentonite increases, with the saturation behavior represented by a soil-water characteristic curve (SWCC). In this study, SWCC tests of Korean compacted bentonites were conducted based on the chilled-mirror dew point temperature condensation (CMDPTC) method, incorporating the dry density variation. The van Genuchten fitting parameters (α, n) were obtained; α decreased exponentially, and n increased linearly according to the dry density. Furthermore, using the obtained SWCC datasets, a simple empirical model was proposed to estimate the van Genuchten fitting parameters.

Experimental Study on Permeability Anisotropy of Unsaturated Compacted Loess

This study investigates the permeability characteristics of unsaturated compacted loess by focusing on the anisotropy parallel and perpendicular to the compaction. Three tests are conducted on compacted loess: permeability test, soil-water characteristic curve test, and scanning electron microscope (SEM) test. Samples are maintained and tested at different dry densities under optimal moisture conditions. Test results showed that the horizontal saturated permeability coefficient of compacted loess was larger than in the vertical direction, showing obvious anisotropy. Based on the saturated permeability coefficient, the permeability coefficient of unsaturated compacted loess can be predicted according to the soil-water characteristic curve fitted using the van Genuchten model combined with the Childs and Collis-Geroge model. The relational formula was established between the unsaturated permeability anisotropy ratio and the matrix suction through introducing a pore connectivity-tortuosity parameter, which represents the relationship between unsaturated permeability anisotropy ratio and matrix suction. The difference of pore characteristics between horizontal and vertical directions of compacted loess is the main reason for the permeability anisotropy.

Soil Water Characteristic Curve Test of Waste under Different Compaction Densities and Analysis of the Multi-Factors that Affect the Instability of Landfill Slopes

Soil Water Characteristic Curve Test of Waste under Different Compaction Densities and Analysis of the Multi-Factors that Affect the Instability of Landfill Slopes

Nutrients adsorption characteristics and water retention capacity of polyurethane–biochar crosslinked material modified filler soil in stormwater treatment

Bioretention systems are frequently used practice in stormwater treatment to remove pollutants from runoff and reduce waterlogging. The additive materials of filler soil in bioretention facilities play a key role in function realization, while the traditional additives have some problems of unstable removal efficiency on nutrients and decreasing hydrologic performance. Polyurethane–biochar crosslinked material (PCB) is a new polyurethane composite material, having low nutrients leaching quantities and strong adsorption capacity. To evaluate the feasibility of PCB as an additive material of filler soil, physicochemical properties tests, adsorption experiments, column experiments and soil water characteristic curve tests were conducted. With the addition of PCB, the particle grading and physicochemical properties of filler soil were optimized, increasing the specific surface area and cation exchange capacity. Through electrostatic attraction and chemisorption, PCB improved the adsorption quantities and further prolonged the lifespan of the filler soil owing to high cation exchange capacity and hydrophilicity of the PCB. Due to its porosity, the matric suction of the PCB modified filler soil increased significantly, as well as the permeability coefficient and saturated water content. The application of PCB expanded the contact area and time between stormwater and filler soil, and increased the infiltration channels, providing better conditions for adsorption reaction. The results suggest the addition of PCB optimized adsorption and hydrologic performance of the filler soil in bioretention facilities, showing promising potential for stormwater treatment.

Study on Soil Water and Suction Stress Characteristics for Unsaturated Clay Soil of Airport Engineering Based on Laboratory Tests

For the unsaturated soil in Feidong China, this study examined the suction stress characteristics based on the soil-water characteristic curve (SWCC), which was different from traditional research ideas. At the same time, the unsaturated consolidation device was adopted for SWCC tests, with consideration of the influence of yielding stress of soil, which was different from the traditional test approach of the soil-water characteristic curve. The results were estimated using the van Genuchten model, which was revealed that this is well-fit for the studied unsaturated soil, and the triaxial shear-strength tests were conducted with suction control. Then, the suction stress characteristic curve (SSCC) was analyzed, and SWCC-predicted data were compared with triaxial test-derived suction stress data. For the studied unsaturated soil, the deviatoric stress increased with the net inner stress p − u a at the same matric suction. At the same net inner pressure, the deviatoric stress increased with the matric suction, which verified the hardening activity of matric suction on the tested unsaturated soil strength. Besides, triaxial test-derived suction stress data greatly conformed to SWCC data-derived SSCC that was determined using identical parameters used in the SWCC model.

Tensile strength of unsaturated coarse and fine-grained soils

Soils at the ground surface experience multiple cycles of drying and wetting. On drying, the soils experience shrinkage and cracks may appear. The development of cracks depends on the tensile strength of the soil. Such cracks increase the permeability of the soil and can cause slopes and earth structures to fail due to rainfall. Several tensile strength models have been proposed for unsaturated soils considering the effect of matric suction. However, the tensile strength models proposed are for either cohesionless (coarse-grained) or clayey (fine-grained) soils. The tensile strength models were shown to be different in their definition of suction stress and the presence or absence of a cohesion term. As tensile strength data of fine-grained soils with the same soil structure and soil–water characteristic curve (SWCC) data are lacking in the literature, Brazilian tensile tests and SWCC tests were conducted on compacted fine-grained soils from two residual soil formations. The test data highlighted the problem in the friction angle used in existing tensile strength models. Using a general form of the suction stress and the extended Mohr–Coulomb criterion with the Brazilian test Mohr circle, a new tensile strength model applicable to both coarse-grained and fine-grained soils was proposed. The proposed model was shown to perform better than existing models using independent data.

Effect of water content on near-pile silt deformation during pile driving using PIV technology

Piles are widely used in structural foundations of engineering projects. However, the deformation of the soil around the pile caused by driving process has an adverse effect on adjacent existing underground buildings. Many previous studies have addressed related problems in sand and saturated clay. Nevertheless, the failure mechanism of pile driving in unsaturated soil remains scarcely reported, and this issue needs to be studied. In this study, a modeling test system based on particle image velocimetry (PIV) was developed for studying deformation characteristics of pile driving in unsaturated silt with different water contents. Meanwhile, a series of direct shear tests and soil-water characteristic curve (SWCC) tests also were conducted. The test results show that the displacement field shows an apparent squeezing effect under the pile end. The installation pressure and displacement field characteristics are sensitive to the water content. The installation pressure is the largest and the total displacement field is the smallest, for specimens compacted at water content of 11.5%. These observations can be reasonably interpreted according to the relevant unsaturated silt theory derived from SWCC tests and direct shear tests. The variation characteristics of the soil displacement field reflect the macroscopic mechanical properties of the soil around the pile.

Elastoplastic Behavior of Compacted Kaolin under Consolidated Drained and Shearing Infiltration Conditions

Abstract This study presents a comprehensive laboratory study to characterize the elastoplastic behavior of compacted kaolin under consolidated drained (CD) and shearing-infiltration (SI) triaxial conditions. The laboratory tests include a soil-water characteristic curve (SWCC), isotropic consolidation (IC), CD, and SI tests under different net confining stresses to consider in situ stress state and matric suctions that describe the volume change characteristics of unsaturated soil with respect to two stress state variables (i.e., net normal stress and matric suction). Consistently prepared specimens of statically compacted kaolin were used in this study. The results of the SWCC tests demonstrated that the air-entry value and yield suction (so) increased with an increase in net confining stress. The IC tests indicated the strong influence of matric suction on compressibility and stiffness of the compacted kaolin. The results of SI tests indicated that water infiltration reduced the matric suction of the soil and was accompanied by a degradation in deviator stress. It was also found from the CD and SI tests that the failure envelope of compacted kaolin was unique. For practical purposes of transient analyses, therefore, the CD and SI tests, as well as the SWCC and IC tests, might be performed to obtain more rigorously elastoplastic behavior of unsaturated soil under CD and SI conditions.

Effect of initial gravimetric water content and cyclic wetting-drying on soil-water characteristic curves of disintegrated carbonaceous mudstone

Abstract The soil-water characteristic curve (SWCC) is often used to estimate unsaturated soil properties (e.g. strength, permeability, volume change, solute and thermal diffusivity). The SWCC of soil samples is significantly affected by cyclic wetting-drying. To examine how water content and cyclic wetting-drying affect the SWCC of disintegrated carbonaceous mudstone (DCM), SWCC tests were implemented using a pressure-plate apparatus. In addition, SWCC models for DCM considering the initial gravimetric water content and cyclic wetting-drying were developed. The test results showed that the volumetric water content (θ) of the DCM first decreased rapidly and then became stable as matric suction (s) increased. The initial water content affected the SWCC by altering the pore structure of the DCM. For a given number of wetting-drying cycles, the higher the initial water content, the higher the stabilized θ. At a given s value, θ decreased as the number of wetting-drying cycles increased, which suggests that cyclic wetting-drying reduces the water-holding capacity of DCM. The Gardner model for DCM was constructed considering initial water content and cyclic wetting-drying, and was effective at describing and predicting the SWCC model for DCM.

Prediction of soil-water characteristic curves using basic soil properties

A model for predicting the soil–water characteristic curve (SWCC) using easy-to-measure basic soil properties has been proposed. The model uses the widely used SWCC function by Fredlund and Xing. This function has three parameters: a, n, and m. a is related to the air-entry value (ψaev) of a soil, n mainly influences the desaturation rate in the transition zone of a SWCC, and m controls the residual moisture content. By analyzing the results of a series of laboratory SWCC tests as well as the results in the literature, explicit empirical equations were established for evaluating the values of a, n, and m using the saturated permeability (ks), grain size distribution (GSD), and plasticity index (PI). The basic soil properties were selected based on the conceptual relationship between each SWCC function’s parameters and those basic soil properties. The model has been verified by applying it to nine SWCCs in the literature, which were not included in developing the model. The proposed model showed better predictions than the existing prediction models. It is suggested that the model can be used to estimate the preliminary drying SWCC of soils.

Soil water characteristic curve test and saturated-unsaturated seepage analysis in Jiangcungou municipal solid waste landfill, China

In this study, an analysis of municipal solid waste (MSW) is undertaken at the Jiangcungou landfill in Xi’an, China. Using waste specimens gathered from the landfill, the soil water characteristic curve (SWCC) is determined at the laboratory, and the effects of landfill age and dry density on the SWCC parameters are analyzed. Leachate level trends during the landfill lifecycle are examined by performing saturated-unsaturated seepage analysis with the consideration of precipitation, as well as the inner source of water. These results are then summarized into three primary points. First, the shallow waste in the Jiangcungou landfill lost its water relatively quickly at small negative pressure heads, while the deep waste lost its water much more gradually. In addition, the deeper waste exhibited a lower volumetric water content when the matrix suction was less than 1.4kPa, but the result was opposite when the matrix suction was more than 2.0kPa. Second, the saturated volumetric water content and steepness of the SWCC decreased as the dry density increased, and the residual volumetric water contents increased and the inverse of the air-entry pressure decreased with increasing landfill age. Third, layers three, five and six demonstrated the highest leachate levels in the eighth landfill stage especially layer five, and these merit the greatest attention in monitoring and construction. These findings may provide a basis for the management of the Jiangcungou landfill.

Study on the soil water characteristic curve and its fitting model of Ili loess with high level of soluble salts

Soil water characteristic curve (SWCC) is a critical parameter for unsaturated soil. However, few study can be found in literature for SWCC of loess with high soluble salt content, in particular SWCC fitting model. This paper targets on Ili loess in Xinjiang in China, which has a high level of soluble salts and is a typical loess in central Asia. Centrifuge method and filter paper method are used to conduct SWCC tests of Ili loess under different salt conditions. The total and matric suction of Ili loess under different soluble salt contents are obtained. The results of two methods are compared. The corresponding influence of soluble salt on SWCC is studied. At the same time, based on the Gardner model, a new SWCC fitting model of total and matric suction for Ili loess is proposed, which is able to consider the influences of soluble salts. The test results show that the compression during centrifuge method contributes certain increase of matric suction, especially at high volumetric water content conditions. Soluble salts have large effect on total and osmotic suction of Ili loess, however limited effect on matric suction. The alteration of total and osmotic suction is caused by the change of solution concentration in the sample. Total suction is mainly affected by osmotic suction. The alteration of matric suction is caused by the precipitation of the salts. It is proved that the proposed SWCC model is capable of characterizing the total and matric suction by simultaneously considering volumetric water content and soluble salt content.

Reliability analysis of unsaturated slope stability considering SWCC model selection and parameter uncertainties

Soil-water characteristic curve (SWCC) is usually estimated by fitting a prescribed parametric model to a limited number of test data. Uncertainties in estimated SWCC are unavoidable due to a lack of test data, including uncertainty arising from selecting a parametric model and uncertainty in fitting parameters. This paper develops a reliability analysis approach for unsaturated slope under the Bayesian model averaging (BMA) framework. The proposed approach accounts, simultaneously, for SWCC parameter uncertainty and model selection uncertainty, and allows shedding light on their effects on unsaturated slope reliability analysis. The proposed approach is illustrated through an unsaturated cut slope example. Results show that, with a limited number of SWCC test data, explicit quantification of SWCC parameter uncertainty provides more insights into the variability of SWCC in comparison with only considering the best-fit of test data. More importantly, the model selection uncertainty might be considerably large when only a limited number of test data is available. In such a case, betting on a single model might result in an improper estimate of unsaturated slope failure probability. On the contrary, the proposed approach provides a model-independent estimate of unsaturated slope failure probability that is less sensitive to the selection of SWCC model among the pool of candidates.

A New Soil-Water Characteristic Curve Model for Unsaturated Loess Based on Wetting-Induced Pore Deformation

The soil-water characteristic curve (SWCC) is the basis for describing seepage, strength, and constitutive model of unsaturated soil. The existing SWCC models do not work accurately for evaluating loess, because they do not consider the pore deformation that is induced by wetting. The present study develops a new SWCC model for unsaturated loess. The model considers the effect of wetting-induced pore deformation (WIPD) on the SWCC. The new model includes 6 parameters, which could be confirmed by laboratory tests. The pore volume function (PVF) was described by the WIPD. The shift factor ξ1i and the compression factor ξ2i were introduced into the model. The relationship between the void ratio e and ξ1i and ξ2i was established using the average pore radius. The new SWCC model for saturated loess was improved based on the classical van Genuchten (V-G) model. If the WIPD had not been considered, the new model would regress into the classical V-G model. SWCC tests of unsaturated loess with different void ratios were carried out to verify the new model. The model parameters were calibrated in the original state, and the SWCCs of different void ratios were predicted by the new model and found to be in good agreement with the test results.

Structural properties of unsaturated compacted loess for various sample moisture contents

This paper investigates the structural properties of compacted loess by focusing on the influence of structural changes incurred by varying sample moisture content on matric suction and stress-strain characteristics of unsaturated compacted loess. Two tests are conducted on unsaturated compacted loess with different structural properties: soil-water characteristic curve test and triaxial shear test. Samples are maintained and tested at the same moisture content and dry density through humidification or dehumidification. Test results show that structural changes incurred by sample moisture content markedly influence the soil-water characteristic curve of unsaturated compacted loess. Below the plastic limit, matric suction increases with increasing sample moisture content. At the optimal sample moisture content, unsaturated compacted loess exhibits a homogeneous micro-pore aggregation structure and thus a relatively high suction. In the optimal structural state, the stress-strain curve was located at the top of the stress-strain graph. For moisture contents below the optimal sample moisture content, the degree of structural weakening is relatively low, and the stress-strain curve gradually moves down the stress-strain graph. For moisture content above the optimal sample moisture content, the degree of structural weakening of the soil sample is relatively high, and the stress-strain curve moves to the bottom of the stress-strain graph. A structural parameter mɛr is defined that could reasonably reflect the structural properties of compacted loess and is of great significance for evaluating the engineering quality of compacted loess.