Unsaturated Soil Properties Research Articles

Enhancing the Evaporation Method for Accurately Determining Unsaturated Soil Properties in the Wet Range

Enhancing the Evaporation Method for Accurately Determining Unsaturated Soil Properties in the Wet Range

Modified Complex Electrical Resistivity Technique: Applications to Saturated and Unsaturated Soils

Abstract Direct current (DC) electrical resistivity is a common laboratory soil characterization method to support geotechnical infrastructure design and to supplement site investigations. The complex electrical resistivity method has the potential to provide additional electrical soil properties to enhance electrical soil characterization. Both methods are conventionally performed under fully saturated soil conditions; however, many environments exist where soil is not fully saturated, such as ballast structure supporting railways. In this study, a new experimental setup featuring a current enhancing agent (agar) for complex electrical resistivity testing is evaluated by testing five different soil specimens reconstituted at saturated and unsaturated conditions. Results showed that the new experimental setup is valid and can be used to obtain repeat measurements, particularly for specimens reconstituted in the unsaturated conditions where the traditional DC electrical resistivity setup yields results that are unreliable. This is one of the very few studies where tolerances for triplicate specimens are reported to establish differences in measurements from sample preparation versus discernable variability between different geomaterials. Additionally, all results are supported by a Cole-Cole model. The results show that the additional data collected in a complex electrical resistivity test can be used to differentiate different soil types that are ambiguous with the DC electrical resistivity method. The additional data have the potential to more fully characterize the electrical properties of saturated and unsaturated soils, as well as to help distinguish unique geophysical signatures of various geomaterials to enhance a geophysical site investigation for identifying soil variability in the subsurface.

Measuring shrinkage of expansive soils using a novel automated high‐frequency setup

AbstractSoil shrinkage characteristic curves are used to describe the shrinkage behavior and hydraulic properties of unsaturated soils. To construct soil shrinkage characteristic curves, a high‐data‐density measurement method is needed that relates water content to soil volume changes. We present a fully automated soil shrinkage measurement setup, based on the simplified evaporation method, to characterize the shrinkage behavior of undisturbed natural expansive clay soils. The high data density creates the opportunity to produce soil shrinkage characteristic curves without the need for a mathematical model. The technique allows for resaturation of the samples after drying, enabling differentiation between reversible and irreversible shrinkage. The setup consists of the commercialized HYPROP2 apparatus combined with optical distance sensors to measure the horizontal and vertical dimensions of the samples, yielding data on the sample volume, weight, and soil water suction. The measurement frequency is once per 10 min, and the measurement period is up to 4 weeks, providing a detailed time series of the drying and shrinkage characteristics. The setup can capture the different shrinkage phases and offers the opportunity to relate the soil shrinkage characteristic curve to soil water suction. The measurement data acquisition rate and accuracy enable detailed interpretation of soil water retention curves for nonrigid soils and are shown to be essential for understanding and quantifying the shrinkage potential of several types of deposits.

Preliminary Experiences in Determining the Soil–Water Characteristic Curve of a Sandy Soil Using Physical Slope Modeling

Relating soil moisture content to soil suction, the soil–water characteristic curve (SWCC) represents an essential feature in unsaturated soil mechanics that enables estimation of different unsaturated soil property functions and modeling of the macro-scale soil behavior. However, depending on the soil and processes under consideration, proper hydraulic characterization of a soil through direct laboratory measurements can be difficult, time-consuming, and involve many uncertainties. In the case of uniformly graded sands, there is a highly nonlinear and steep shape of the SWCC, with only a few kPa of soil suction separating saturated and residual soil moisture conditions, which makes measurements for determinations of SWCC especially challenging. This study encompasses an investigation of the sandy type of soil’s behavior and presents some preliminary results and experiences on the determination of SWCC through the use of physical slope model tests. The 30 cm deep slope, inclined at 35 degrees and instrumented with soil moisture and pore water pressure sensors, was exposed to series of rainfall intensities, ranging from 37 up to 300 mm/h. The results indicated that the data on hydraulic response in monitored points are not only useful for the determination of SWCC, but that the approach is useful for investigation of hydraulic hysteresis phenomena, as well as its effects on soil moisture and pore water pressure conditions, which also affects the stability conditions of a slope. In particular, the best-fit parameters of the van Genuchten model suggested air entry values of 1.6 and 1.1 kPa for the drying and the wetting curves of the SWCC, respectively, with the two branches shifted by about 1 kPa of soil suction.

A novel framework for deriving water retention behavior of multimodal unsaturated soils based on pore size distribution data

The soil water retention curve (SWRC) strongly influences the hydro-mechanical properties of unsaturated soils. It plays a decisive role in geotechnical and geo-environmental applications in the vadose zone. This paper advances a novel framework to derive the water retention behavior of multimodal deformable soils based on the pore size distribution (PSD) measurements. The multiple effects of suction on the soil pore structure and total volume during SWRC tests are considered. The complete picture of soil microstructure is quantitatively described by the void ratio (for the pore volume) and a newly defined microstructural state parameter (for pore size distribution) from a probabilistic multimodal PSD model. Assuming a reversible microstructure evolution, a unique PSD surface for wetting and drying links the SWRC and PSD curves in the pore radius-suction-probability space. A closed-form water retention expression is obtained, facilitating the model's implementation in particle applications. The model is validated using the water retention data of four different soil types, showing a strong consistency between the measurement and the reproduced curve. The proposed method provides new insights into the pore structure evolution, the water retention behavior and the relationship between them for multimodal deformable soils.

A Novel Numerical Model Considering Unsaturated Soil Properties and Computational Study on Heat and Moisture Transfer Characteristics of Helix Ground Heat Exchanger

Abstract A novel three-dimensional numerical simulation model for the helix ground heat exchanger was proposed in this paper, which takes into account unsaturated soil properties. This model is more suitable for real working conditions. To validate its accuracy, a miniature model heat transfer experimental platform was constructed. Additionally, the study conducted simulation research using three types of soil with significantly different thermal and moisture characteristics. Moreover, the comprehensive thermal conductivity and water diffusion coefficient of these three soil types were determined by relevant literature and experimental tests. The aim was to comprehensively explore the impact of different soil types on the heat and mass transfer of the helix ground heat exchanger. The results indicate that the numerical model developed in this paper accurately captures the heat and mass transfer characteristics of the helix ground heat exchanger to a certain extent. Increasing the comprehensive thermal conductivity and water diffusion coefficient of the soil can significantly enhance the heat exchange capacity of the exchanger. For instance, under sandy loam conditions, the heat exchange capacity is approximately 20.73% higher compared to clay loam conditions. The study also identifies two distinct areas around the helix ground heat exchanger: the severe change region and the soft change region. In the severe change region, there is a notable decrease in soil water content near the exchanger, which inevitably weakens the thermal conductivity of the soil. It is advised to minimize this effect through measures like active water spraying.

Effect of volumetric deformation on saturation of unsaturated soils under constant suction

The soil–water retention curve (SWRC) represents a fundamental relationship in the study of unsaturated soils, where volumetric deformation plays a crucial role in shaping the water retention characteristics by impacting the pore size distribution. This study conducted a thermodynamic analysis to derive a relationship between the effective stress parameter and the water retention properties of unsaturated soils. Consequently, a streamlined formula was introduced to forecast the impact of volumetric deformation on saturation at constant suction. This formula, incorporating a singular parameter from the effective stress parameters equations, can be seamlessly integrated with any prevailing SWRC model to characterize the influence of volume deformation on the SWRC. The effectiveness of the proposed model was corroborated through seven sets of experimental data spanning various soils, affirming its capability to precisely describe the water retention behavior of deformable soils under deformation.

Hydrothermal Interaction Properties of Unsaturated Frozen Soil Studied Using a Modified Soil Freezing Characteristic Curve

Hydrothermal Interaction Properties of Unsaturated Frozen Soil Studied Using a Modified Soil Freezing Characteristic Curve

A modified model predicting the hydraulic properties of variably saturated soil accounting for film and capillary flow

Accurate knowledge of hydraulic properties of unsaturated soil is vital to modelling water and solute transport process and addressing soil salinisation problems. Due to the defect that the van Genuchten (VG) model cannot represent soil water characteristics (SWC) during the drying stage, we have introduced a modified model, which combines capillary flow with film flow models to describe the unsaturated hydraulic conductivity. The capillary SWC is obtained by assuming the specific water capacity function with lognormal distribution. The film SWC is modified using the logarithmic nonlinear expression by introducing a modification coefficient into the Campbell equation, and the film relative hydraulic conductivity (RHC) is obtained based on the logarithmic nonlinear relationship. The calculated modification coefficient values range from 0.99 to 2.49. Finally, the proposed model’s rationality and applicability are validated through root mean square error (RMSE) analysis with measured values and comparison with the VG model. While the calculation RMSEs of SWC in the two models are basically consistent, the RMSEs of RHC in the proposed model are around 1–10 times smaller than those of the VG model, indicating its superiority in calculating the hydraulic conductivities over the entire matric head.

Microstructural investigation of the unsaturated hydraulic properties of hydrochar-amended soils

Hydrochar is an urban soil amender that supports plant growth on slopes. It is a biomass-derived carbon-rich material produced by the hydrothermal carbonation process; thus, it has a different pore structure from biochar produced by pyrolysis. The effects of hydrochar on the hydraulic properties of unsaturated compacted soils and the underlying pore-level soil-hydrochar interaction mechanisms are unclear. In this study, silty-clay sand was amended by grass-derived hydrochars produced at two hydrothermal carbonisation temperatures (180 and 240 °C, denoted as H-180 and H-240, respectively) with different mass proportions (fH\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${f}_{{\ ext{H}}}$$\\end{document}). The pore and throat size distributions, water retention curves (WRCs) and hydraulic conductivity functions (HCFs) of the soils with and without amendment were measured. The results showed that hydrochar improved the soil water retention capability as the smaller hydrochar particles filled the soil pores with diameters larger than 290 μm. Compared with the increase in air-entry value made by H-180, the one made by H-240 was more substantial as this type of hydrochar altered the soil pores to be smaller in size. Hydrochar also increased the hydraulic conductivity of the soil by half to one order of magnitude due to the increase in the throat frequency and the presence of hydrochar intra-pores. One exception was the amendment by H-180 at fH\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${f}_{{\ ext{H}}}$$\\end{document} of 2.5%, wherein the HCF was reduced due to pore clogging at throat diameters less than 90 μm and more than 250 μm. Finally, amending the soil with either H-180 or H-240 at fH\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${f}_{{\ ext{H}}}$$\\end{document} of 5% always improved the WRC and HCF, thereby benefiting plant water uptake.

Bioelectricity generation in plant microbial fuel cells: Influence of vegetation density and unsaturated soil properties

Plant microbial fuel cells (PMFCs) have promising potential in various geoenvironmental engineering applications such as green roofs, biosensors, water purification and soil remediation. However, the vulnerability of PMFCs to drought stress and the unknown interplay between bioelectricity, planting density, and unsaturated soil properties pose significant challenges for their applications. The research objective is to investigate the effects of planting density on bioelectricity generation under drought conditions. The research was conducted using vegetation Hydrocotyle vulgaris at four different planting densities in silty soil. Bioelectricity and unsaturated soil properties, including water content and suction, were monitored. The results indicated that moderate increase in planting density could enhance the power density of PMFCs up to 4.5 times. This is because of the increased availability of root exudates for microbial activity with increasing root biomass and decreased internal resistance by extending the soil pore space. It was also found that plant roots could affect bioelectricity generation by altering unsaturated soil properties, such as the air-entry value of soils and soil pore structure. However, high planting densities reduced bioelectricity due to increased internal resistance from roots occupancy and evapotranspiration-induced crack initiation. This research provides valuable insight into optimizing PMFC performance and sustainability.

Study on the Effect of Soil Type and Salinity on Water–Soil Characteristic Curves of Saline Soils Based on the Vapor Balance Technique

The soil–water characteristic curve (SWCC) describes the relationship between water content and soil suction, which is an important tool for determining the engineering properties of unsaturated soil. By measuring SWCC through vapor equilibrium technique, which could obtain soil suction up to 309 MPa, this paper explored the influences of the soil type and salt content on SWCC of saline soil. The experimental results illustrate that fine‐grained saline soil had a good water‐holding capacity, which had a higher air entry value and slower dehydrate rate. The study also brings out the observation that salt had negligible influence on soil suction and parameters effecting SWCC in low suction range; however, this effects was slight until it disappeared with the suction increasing.

High-suction polymer sensor for measurement of soil suction under freezing and thawing conditions

Soil-water characteristic curves (SWCC) are one of the most essential elements of unsaturated soil properties, yet the instruments employed for acquiring them have distinct limitations, for example, the tests are time-consuming, tedious and costly. High-suction polymer sensor (HSPS) with modified polyacrylamide (PAM) polymer is an alternative of measuring soil suction that relies on osmosis pressure rather than capillary pressure alone. The sample examined originated in Astana and was defined as clayey sand (SC). The ambient temperature of the Astana soil (sample 1) was used to assess soil suction using HSPS, which was verified by typical unsaturated equipment such as the Tempe Cell and dew-point potentiometer (WP4C). Once confirmed and SWCC was formed, the HSPS was utilized to detect soil suction during freeze-thaw cycles to simulate the climate in Astana. Based on the durability test, with the anticipated outcome lasting up to a year, PAM can resist the pressure for >200 days. Additionally, the modified PAM generated a stable response compared to the conventional PAM. The results from laboratory testing indicated that sample 2 has a greater average suction during freezing, while sample 3 has a higher average suction during thawing. In general, freezing cycles should produce more suction than thawing; yet, in sample 3, the water was still in supercooling stage, resulting in freezing having a lower suction average than thawing.

Determination of soil water retention curves from thermal conductivity curves, texture, bulk density, and field capacity

The soil water retention curve (SWRC) is frequently expressed using the van Genuchten (VG) model, which has four parameters: saturated water content (θs), residual water content (θr), α, and m (1–1/n). Soil thermal conductivity (λ), which is linked to the hydraulic properties of unsaturated soil, has been a proxy variable used to estimate SWRC. In this study, we present a new approach to estimate the VG model parameters. Parameters θs, α and m are calculated from the information of soil texture, bulk density (ρb), and a measured water content at field capacity (θfc, at −33 kPa or −10 kPa), and θr is estimated from the thermal conductivity versus water content curve, λ(θ), based on similarities between SWRCs and λ(θ) curves. The new approach was evaluated with laboratory and field measurements on 23 soils of various textures, ρb values, and θ values. Results showed that for repacked core samples, intact core samples, and in situ field soils, the new approach estimated SWRCs with average root mean square errors (RMSEs) of 0.042, 0.030, and 0.049 m3 m‐3, respectively. The new approach offers a quick and effective way to estimate SWRCs accurately with measured λ(θ) curves, texture, bulk density, and θ at field capacity.

Fractal thermal conductivity of unsaturated soils considering pore heterogeneity

Accurately predicting the thermal conductivity of unsaturated soils is crucial for assessing and simulating the long-term environmental impact of subterranean engineering structures. This study presents a fractal thermal conductivity model for unsaturated soils that considers the influence of heterogeneous pore topology, water content and volumetric deformation. Based on the results from the scanning electron microscopy (SEM) and mercury intrusion-extrusion cyclic porosimetry (MIECP) tests on Shanghai clay and Kaolin, we introduce the pore-throat structure to describe the soil heterogeneity and deal with the coupling of solid-pore thermal properties through the mixing law at the pore scale. Together with the self-similar fractal law, the thermal conductivity model is upscaled from the pore scale to the representative element volume (REV) scale. During the process, heterogeneity factors are introduced to describe differences between local and bulk properties, such as porosity and saturation. Thermal property tests of Shanghai clay and Kaolin with varying porosities are then conducted to validate the proposed thermal conductivity model. Additionally, nineteen published datasets on thermal conductivity are also utilized to confirm the adaptability and robustness of the model. The results show that the proposed model predicts well the thermal conductivity of unsaturated soils. Moreover, when compared to existing models, the proposed model offers superior performance in describing the thermal properties of unsaturated soils.

Holistic multiphysics simulation of climatic responses of cold region pavements

In cold regions, the environment dynamics lead to variations of soil temperature, water content, and deformation, which are characterized by highly coupled physical interplay. The hydraulic and thermal properties of unsaturated soils are highly nonlinear, which is further complicated when subjected to freezing. This paper presents a comprehensive multiphysics coupling model to evaluate these complex processes. The model considers the behaviors of unsaturated frozen soils. It accounts for the influences of meteorological, geothermal, and hydrological factors. The model is validated through two pavement case studies using Long-Term Pavement Performance (LTPP) road section data. The first case analysis is performed for a pavement section in Vermont, and the simulation lasted for 30 days during a non-freezing season on an hourly basis. The results validated the performance of the model considering unsaturated soil behaviors. The second case study is based on a daily analysis of a pavement section in South Dakota over a freezing–thawing cycle over 194 days. The results validated the model in considering the frozen unsaturated soil behaviors. Both case studies demonstrate the performance of this comprehensive model in quantifying the spatial and temporal variations of soil temperature and water content in response to environmental stressors. The capability of the model in accurately predicting the responses of pavement to the meteorological factors unleashes the potential of this model to assess the effects of climate and climate change on cold region pavement, as well as other types of geo-structures.

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.

Feasibility of vibration mitigation in unsaturated soil by periodic pile barriers

Periodic wave barriers have been widely used to mitigate ambient vibration. However, almost all the studies on the performance of periodic wave barriers are conducted under the assumption that soil is a single-phase or saturated medium. In fact, soil often consists of three phases: solid, liquid and gas, and it is more reasonable to be considered as an unsaturated soil. Existing research demonstrates that the dynamic properties of unsaturated soil have been significantly changed by gas phase, or the dynamic performance of unsaturated soil is completely different from that of single-phase soil and saturated soil. Especially, the dynamic performances of unsaturated soil cannot be approximated by the two extremes (saturated soil and single-phase soil). In this paper, the feasibility of vibration mitigation in unsaturated soil by periodic pile barriers is numerically validated. Firstly, based on the periodic theory, the complex band structures of P-SV waves in the periodic pile-unsaturated soil system are calculated. The influence of the saturation on the complex band structures is also investigated. Secondly, a numerical model for analyzing ambient vibration isolation is established. The responses of the numerical model are carried out in both frequency and time domains. The results indicate that periodic wave barriers in unsaturated soil can be designed with a wide attenuation zone covering the dominant frequencies of P1 waves, which is just a dream in saturated soil. Furthermore, S waves can also be greatly shielded in the target band. The present study removes the obstacles for the vibration mitigation in unsaturated soil by periodic pile barriers.

Sustainable Lignin to Enhance Engineering Properties of Unsaturated Expansive Subgrade Soils

New ways to apply sustainable materials, such as biomass components, are essential for reducing dependence on fossil fuels. This work investigated the engineering properties of unsaturated expansive subgrade soils stabilized by bio-based energy coproducts containing lignin. Lignin is a waste by-product of the paper and pulp industry that is frequently burned. Highway subgrade could consume lignin as an environmentally benign, low-cost, and energy-efficient chemical substance for soil stabilization. Swell and shrink behavior of expansive subgrade soils complicates highway construction and causes damage to existing highways. However, research on the hydromechanical properties and volume change behavior of lignin-stabilized expansive soil is limited, and better insight is required into its unsaturated behavior for safe and economical pavement design practices. In this research, a series of geotechnical laboratory tests were conducted to characterize expansive subgrade soils treated with lignin by determining the Atterberg limits, compaction and consolidation behaviors, swelling characteristics, and water retention properties. The mechanisms influencing the changes in engineering properties of lignin-treated expansive soils were further investigated using soil pH, scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy analysis. The study shows that the optimal lignin content contributed to an acceptable degree of soil stabilization. The lignin-based cementing material effectively bonds soil particles together and fills pores, thereby preventing water infiltration into the soil and reducing the swell–shrink potential of stabilized soils.

A Trans-Scale Study on the Influence of Water Content and Particle Size on Matric Suction

Exploring the water retention properties of unsaturated soil from the perspective of a liquid bridge has been a popular issue in recent years. This study first measures the soil–water characteristic curves (SWCCs) of granular specimens to determine the influence of particle size on matric suction from a macroscopic perspective. Then, the internal mechanism of the influence of particle size and volumetric water content on matric suction is analyzed from the mesoscopic perspective by using the Young–Laplace (Y–L) equation to calculate matric suction between two equal spheres. The macroscopic and mesoscopic experiments both show that matric suction decreases with an increase in particle radius. Moreover, identifying the internal mechanism of SWCC from the liquid bridge perspective is only applicable when the influence of gravity can be disregarded or is in the transitional stage. The influence of volumetric water content and sphere radius on matric suction is mostly caused by the variation in the outer radius of the liquid bridge ( r 1 ) and the neck radius of the liquid bridge ( r 2 ). With an increase in volumetric water content and sphere diameter, the increasing rate of r 1 is much higher than r 2 , and the macroscopical matric suction gradually decreases.