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• The vertical kinematic response of a pipe pile in unsaturated soil under P-waves is studied theoretically. • Introducing the fractional standard linear solid (FSLS) model describes the flow-independent viscosity of soil skeleton. • The effects of soil parameters on the seismic response of pipe pile are evaluated with parametric analysis. • The effects of slenderness and radius ratios for pipe pile on its seismic response are investigated in detail. • The physical mechanism behind the theoretical results is stated and discussed. This work investigates the kinematic response of a pipe pile embedded in a fractional-order viscoelastic unsaturated soil stratum over a rigid base subjected to vertically-propagating seismic P-waves based on a continuum model. The fractional standard line solid model is introduced to the governing equation of unsaturated soil to refine the frequency-dependent viscoelastic behavior of the soil skeleton. The Rayleigh-Love rod theory is utilized to characterize the vertical motion of the pipe pile. A rigorous series form solution for the vertical kinematic response of the end-bearing pipe pile under vertically incident P-waves is yielded theoretically with boundary conditions of pile-soil system. The proposed solution is then verified by comparing with the numerical result of the reported solution. Based on the given rigorous analyzed solution, the effects of physical parameters in pile-soil system on the vertical kinematic response of the end-bearing pipe pile under harmonic P-waves are studied in both the frequency and time domains through calculation example and parametric analysis. Finally, relevant meaningful conclusions are included and indicate that soil parameters and pipe pile dimensions have significant effect on the vertical kinematic response of the pipe pile. [ABSTRACT FROM AUTHOR]

For production of clean drinking water, the processes governing bacterial remobilization in the unsaturated zone at transient water flow are critical. Although managed aquifer recharge is an effective way to dispose of pathogens, there are concerns about recontamination after heavy precipitation. To better understand how bacteria that were initially retained in porous media can be released to groundwater due to transient water content, transport experiments and modeling for Escherichia coli and Enterococcus moraviensis were conducted at the soil column scale. After inoculating dune sand columns with a bacteria suspension for 4 h, three rainfall events were performed at 24-h intervals. The effluent from sand columns was collected to analyze bacteria breakthrough curves (BTCs). After the rainfall experiments, the bacteria distribution in the sand column was determined. The collected BTCs and profile retentions were modeled with HYDRUS-1D, using different model concepts, including one-site kinetic attachment/detachment (M1), Langmuirian (M2), Langmuirian and blocking (M3), and two-site attachment/detachment (M4). After inoculation, almost 99% of the bacteria remained in the soil. The M1 and M2 bacteria models had a high agreement between observed and modeled concentrations, and attachment and detachment were two significant mechanisms for regulating bacteria movement in a porous medium with fluctuations in water flow. At the end of the experiment, the majority of bacteria were still found within the depth range of 5 cm to 15 cm. Our experiments show that E. coli is more mobile in sandy soils than E. moraviensis. The results of this study also suggest that the unsaturated zone is an important barrier between microbial contamination at the soil surface and groundwater. Follow-up studies are needed to completely understand the variables that regulate bacteria remobilization in the unsaturated zone of dune sands. IMPORTANCE At managed artificial recharge sites in the Netherlands, recontamination of infiltrated water with fecal indicator bacteria has been observed. The results of this study suggest that the unsaturated zone is an important barrier between microbial contamination at the soil surface and groundwater. Bacteria that accumulate in the unsaturated zone, on the other hand, can multiply to such an extent that they can be released into the saturated zone when saturation increases due to major rain events or a rise in groundwater level. [ABSTRACT FROM AUTHOR]

Traditional soil mechanics idealizes geomaterials as being in a dry or fully water-saturated state. However, there is a zone between the ground surface and the groundwater table, where the soil is unsaturated, which means the pore space is filled with air and water. This zone is affected by environmental changes during wetting and drying seasons through precipitation, evaporation, or evapotranspiration. The influence of water content, for instance, the degree of saturation, on the characteristics of soil under dynamic loading i.e. at small to intermediate strains, for example, small-strain shear modulus (Gmax), modulus degradation [G(γ)], and damping ratio [D(γ)] remain of interest in the geotechnical community due to the practical relevance of the problem. This study attempted to cover the existing gaps in the dynamic characteristics of soil with different degrees of saturation. Therefore, Hostun sand (HS) was mixed with 30% Amberger kaolin (AK) and compacted to Proctor density at the optimum water content. They were saturated in a special saturation cell applying back pressure. Then, the saturated samples were dried to different degrees of saturation. Resonant column (RC) tests were carried out on the prepared samples to determine the effect of the degree of saturation on Gmax, G(γ), and D(γ). Of note, in this series of tests, suction was not strictly controlled; however, the necessary volume change corrections were performed via various noncontact transducers. To describe the experimental observations and to evaluate the impact of the degree of saturation on Gmax, G(γ), and D(γ), the soil water characteristic curve (SWCC) of the adopted mixture was determined. The results showed a significant increase of Gmax with the degree of saturation following an S-phaped curve. Based on the new data, previous models to predict Gmax were inspected under a significantly wider range of the degrees of saturation and suction. In addition, the effect of the degree of saturation on the intermediate strain characteristics of the adopted mixture [G(γ)/Gmax] and D(γ) was evaluated, where modulus degradation was found almost unaffected by Sr while a moderate increase of damping ratio with Sr was observed. [ABSTRACT FROM AUTHOR]

In this paper, a parametric study is done with various removal and replacement materials to study the effectiveness of the removal and replacement method on the wetting depth in the expansive soil and the amount of differential heave caused by climate conditions and common irrigation scenarios for the southern region of Syria. Soil suction changes and associated soil deformations are analyzed using finite element codes, VADOSE/W and SIGMA/W. The paper concludes that the optimum thickness for replacement with high permeability soil should be at least 1 m. In addition, it concludes that replacing soil with a permeability coefficient lower than the permeability coefficient of the site soil contributes to a 56% and 79% reduction in total and differential heave, respectively. [ABSTRACT FROM AUTHOR]

The classical shotcrete design approaches often ignore the effect of suction, which enables higher strength and, accordingly, a cost-effective temporary support. This paper addresses a numerical approach to design an economic tunnel temporary support considering suction effect on the mechanical characteristics of a simulated soil and shotcrete. Various suction profiles at: fully saturated, high degrees of saturation, transition zone and residual saturated zone, at various shotcrete thicknesses (t), strength (cc) and cohesion values were analysed. Nonlinear stability–suction relationships were obtained. The role of suction on (t) for the simulated soil was more significant in the transition zone and then in the zone of high degrees of saturation. Reduction in (t) increased with suction increase until beyond the residual saturation. An increase of suction from 58.86 to 78.48 kPa (in the transition zone) at cc = 400 kPa was sufficient to cause a reduction of 61.4% in (t). [ABSTRACT FROM AUTHOR]

The tensile strength of unsaturated soils is a fundamental property in various geotechnical designs. Reliable estimation of the tensile strength of unsaturated soils, in particular fine-grained soils, is required in both theoretical research and engineering practice. Although several tensile strength models have been proposed in the literature, an overestimation may occur over a wide suction range, when applying them in the modelling of tensile strength of fine-grained soils. In this paper, the tensile strength of an unsaturated lean clay has been measured over a wide range of void ratio and water content by employing the Brazilian tensile strength test. A critical degree of saturation can be observed for specimens with different void ratios, at which the soil tensile strength reaches the peak. In addition, a predictive tensile strength model considering the effect of initial void ratio has been subsequently proposed for both coarse-grained and fine-grained unsaturated soils based on the interaction mechanisms between the adsorptive and capillary soil water. Finally, the proposed model has been demonstrated to be capable of modelling the tensile strength characteristic curve of various soil types ranging from clean sands to silty and clayey soils. [ABSTRACT FROM AUTHOR]

Plant microbial fuel cell (PMFC) newly emerges as a sustainable technology in green infrastructures such as green roofs, reactive barriers, and environmental biosensors. PMFCs generate bioelectricity driven by microbial redox reactions in the rhizosphere. However, the interactions between the bioelectricity and soil–water characteristics remain unknown, and PMFCs are vulnerable to drought stress. Therefore, this study aims to reveal the coupled bio-hydrological behaviors of PMFCs and propose biochar amendment on PMFCs for improving bioelectricity generation under drought conditions. Biochar-aided PMFCs were prepared by growing green-roof vegetation, Hydrocotyle vulgaris, on sandy lean clay with biochar at a mass ratio of 0, 5, and 10%. Bioelectricity (i.e., electrical potential and current) and unsaturated soil properties (i.e., suction and water content) were monitored simultaneously. Results showed that biochar increased the electrical power output of PMFCs by up to 30 times in unsaturated conditions. This may be because biochar-enhanced catalytic activities in redox reactions decreased soil electrical resistance and improved water retention. The bioelectricity was highly correlated with the suction, as the bioelectrical potential and current decreased by 90% with increasing suction. It was also found that the bioelectrical potential dropped rapidly by 67.6–87.2% when suctions were larger than the air entry values. This might be because air invasion led to the discontinuity of the liquid phase, hindering the ion transport of microbial redox reactions. The study implies that bioelectricity could be applied for field monitoring of soil–water characteristics, and biochar addition would enhance the drought resistance of PMFCs. [ABSTRACT FROM AUTHOR]

A new method and instrument were developed for in situ measurement of the gas diffusion coefficient (Dg) and gas coefficient of permeability (kg) of unsaturated soil. Air containing tracer gas of volumetric concentration of 5%–10% was continuously pumped into a point source at a constant flux rate (q0) until approaching steady state. Then, the pseudo-steady-state tracer gas concentration (Cr0) and gas gauge pressure (ΔPr0) were recorded at a distance (r0) away from the point source. The Dg and kg were determined through the Cr0−q0 and ΔPr0−q0 relationships, respectively. Soil column tests (silty sand) and field tests (sandy lean clay) were conducted to validate the new method. The in situ measured Dg and kg were comparable with those of undisturbed specimens measured by a two-chamber apparatus. The in situ measurement of Dg and kg ranged from 2×10−8 m2 s−1 to 4×10−6 m2 s−1 and 1.9×10−8 ms−1 to 3.9×10−7 ms−1 , respectively. The measurement accuracy increased at deeper depth, due to reduced boundary effect at soil surface. The measurement of in situ Dg was more susceptible to the boundary effects than that of kg at shallower depth (i.e., 20 cm deep) in general. The field test results showed that the ratio of in situ Dg (or kg) to that measured by the element tests was generally between 1.2 and 3.3 at or below depth of 0.30 m, due to the in situ heterogeneity of soil. [ABSTRACT FROM AUTHOR]

Most soils in the natural state are unsaturated. At present, the theoretical formulas for the shear strength of unsaturated soils are difficult to apply in engineering practice due to the difficulty in obtaining the physical parameters. While one of the factors determining the fundamental physical indices of soils is moisture content, is convenient to be obtained. Based on the indoor consolidation undrained shear test, analysis was done on how changing moisture contents and confining pressure affected the strength and deformation traits of remolded silty clay. Results show that the shear strength is affected by the combined effect of moisture content and confining pressure, the increase of confining pressure can significantly improve the shear strength of soil. Under the condition that the moisture content is maintained constant, the perimeter pressure has a positive relationship with the percentage increase in shear strength of unsaturated chalky clay. So, the formula for the total stress strength of unsaturated soil with the moisture content as the independent variable is established by curve fitting. The findings of this study can serve as a valuable guide for estimating the shear strength produced by the local change in groundwater level in silty clay layers. [ABSTRACT FROM AUTHOR]

Assessing the effect of rainfall on the short-term and long-term stability of earthen structures warrants studying the earth pressures of unsaturated soils under transient flow. This paper presents a variational approach applied to the limit equilibrium method for calculating the resultant force of active and passive earth pressures in unsaturated soils subject to transient flow conditions. The method is developed using the unified effective stress representation and utilizes a log-spiral slip surface. The latter eliminates the need to make any static assumptions in the derivation and leads to a more comprehensive failure mechanism than the planar failure mechanism used in most alternative methods. The proposed analytical solution accounts for the infiltration rate, batter angle, backslope inclination, and surcharge. For verification purposes, the results from the proposed formulations are compared against those from several existing analytical solutions. A set of parametric studies is performed to examine the effects of rainfall on active and passive earth pressures for different soil types. The results show that the soil type, mechanical parameters, and shape of the failure surface will significantly affect the earth pressure coefficient. The proposed formulations provide a valuable tool for evaluating the effect of rainfall on the behavior of earthen structures and forensic studies of failed earthen structures. [ABSTRACT FROM AUTHOR]

The tensile strength of unsaturated soils is essential to assess the safety and reliability of earthen structures over a prolonged period. This specific property of unsaturated soils plays a significant role in desiccation cracking and tensile failure of earth structures. The tensile strength of unsaturated soils can be measured through various direct and indirect testing methods in the laboratory and through different models available in the literature. These models can rapidly predict the tensile strength of unsaturated soils over a wide suction range. However, a practical framework to capture the tensile strength behavior with variations in soil type, tensile strength testing methodologies, and sample preparation methodologies has not been probed. In the present study, a novel tensile strength framework for both coarse- and fine-grained soils is proposed to determine the tensile strength characteristic curve (TSCC) over the entire saturation range from the respective soil water retention curves. A critical assessment of the variation of the parameters for the TSCC model for various types of soils, tensile strength testing methods, and sample preparation methodologies was also performed in this study. Further, the proposed framework was validated with several experimental data sets available in the literature for both coarse- and fine-grained soils. A good agreement was found between the experimental data and predicted results in most cases, with variation in soil type, sample preparation, and testing method. Therefore, the proposed model can be effectively used to determine the tensile strength of unsaturated coarse- and fine-grained soils over the entire saturation range and can be widely utilized for various engineering assessments in real-field situations. [ABSTRACT FROM AUTHOR]

Currently, climate change has several negative impacts on the worldwide environment. One of them is land sliding due to rainfall precipitation. Commonly, mountainous areas have a high possibility of having such consequences. Unsaturated soil characterization is presented in this paper to demonstrate that the shifting of ground occurs at the surfaces of any slopes. The paper presents the simulation analysis on clayey soil slopes under extreme rainfall in Kazakhstan. The different slope heights and angles are selected to perform seepage and stability analysis in GeoStudio software. The results show that the slope is prone to failure due to the lower safety factor. It means the driving force is increasing during rainfall due to the increase in pore water pressures. As a result, the shear strength of the soil decrease. According to the results of the seepage analysis, the pore-water pressure has increased to almost 80 kPa at the mid-slope for each simulation. It leads to a decrease in suction and shear strength. The change of a factor of safety for the gentlest slope with 27 degrees angle is the lowest for 10 m slope height and the highest for 30 m slope height, whereas the change of safety factor for a slope with 45 degrees angle (9%) is almost the same for all slope heights. In other words, the factor of safety tends to reduce in all simulation cases, which confirms the loss of strength of soil due to a large amount of infiltration into the ground. [ABSTRACT FROM AUTHOR]

Stratigraphic uncertainty is widely present in nature, but it has not been well considered in the stability analysis of unsaturated soil slopes in the past. In this study, the stability of the unsaturated soil slope is evaluated based on borehole data considering stratigraphic uncertainty. Firstly, an enhanced coupled Markov chain model is used to simulate stratigraphic uncertainty. Then, a finite element algorithm for automatically calculating the safety factor (FS) and the average groundwater table (AGT) of the unsaturated soil slope is developed. At last, a hypothetical slope located in the stratum from Perth, West Australia is analyzed using the proposed algorithm under different borehole schemes. The results show that with the increase in the borehole number, the statistics of FS and AGT will not monotonically increase or decrease. But the trend is that the mean values of FS and AGT gradually approach and eventually converge to the real values, and the standard deviations of FS and AGT decrease. There is a linear relationship between the standard deviation of FS (or AGT) and the average information entropy. The FS and AGT are negatively correlated considering stratigraphic uncertainty. [ABSTRACT FROM AUTHOR]

Abstract Numerous important earthen heritage sites in northwestern China have survived until today because of the arid and semi-arid climate. However, most of them suffer from various types and degrees of deterioration, with many questions not answered. Scaling off is one of the main types of deterioration widely observed at the Site of Yar City. To clarify the underlying mechanisms of cracked surface crust formation, previous experimental achievements are reviewed in the framework of unsaturated soil mechanics in this paper. A series of laboratory tests on the properties of local soil during desiccation, including water retention, volumetric shrinkage, cracking and tensile strength, are presented and analyzed, with a particular emphasis on salinity effect. Results show that suction is the key factor controlling the soil behavior. With the decrease in water content, all suction components increased gradually. The presence of NaCl led to a large increase in total suction, but a negligible change in matric suction. The soil shrinkage characteristic curve, which can be divided into three zones, was not affected by NaCl. The suction-loading curve indicates that the change in void ratio was governed by matric suction. In addition, most of the drying shrinkage of soil took place in saturated and near-saturated conditions. According to the quantitative analysis of final crack patterns, the highly fragmented surface morphology tended to be induced in thinner, less salinized soil specimens. The soil tensile strength also increased during drying, which was further enhanced by the cementation of salt crystals between particles. These preliminary results can provide some insights in understanding the on-site deterioration of earthen heritage sites.

Soil-water characteristic curve (SWCC) is an essential parameter in unsaturated soil mechanics, and it plays a significant role in geotechnical engineering to enhance theoretical analysis and numerical calculations. This study investigated the effects of key factors, such as the percentage of sand, initial degree of saturation, and initial dry unit weight, on the SWCC of expansive soil by measuring the matric suction using a pressure apparatus method. The empirical equation of SWCC was obtained using the Van Genuchten and Fredlung Xing models, and the processing of experimental data checks the fitting of the two empirical models. The findings revealed that the Fredlung Xing model fit the relationship between matric suction and volumetric water content of expansive soil better than the Van Genuchten model, indicating that the pressure apparatus approach’s experimental data are correct and acceptable. The study also found that the matric suction increased with decreasing percentage of added sand at the same volumetric moisture content, and the increase in initial dry unit weight increased the matric suction, with the water retention capacity decreasing significantly after adding 20% sand. Moreover, as the initial degree of saturation increased, the volumetric water content decreased, and the characteristic curves became identical when the initial saturation degree reached 90%. Finally, to minimize the water retention capacity of expansive soils, the study recommended adding a percentage of sand not less than 30% to the expansive clay sample.

An elastoplastic analysis scheme for the cylindrical cavity expansion in offshore islands unsaturated soils considering anisotropy is established. The hydraulic properties and anisotropy caused by stress of unsaturated soils are coupled in an elastoplastic constitutive matrix for unsaturated soil to obtain the governing equations for the cylindrical cavity expansion problem, with an analytical solution that utilizes the original hydro-mechanical state of the soil as the initial conditions. Through a comparative analysis with other analytical solutions, the effectiveness of the new solution is verified. Moreover, the swelling response of the cylindrical cavity expansion in unsaturated soils is examined by systematically analyzing different parameters of the surrounding soil. The findings reveal that the development and rate of anisotropy in normal consolidated soil and over-consolidated soil exert a significant impact on the soil’s mechanical characteristics. Nevertheless, the alteration in the model constant h has little effect on the soil’s mechanical characteristics. The analytical solution introduces anisotropy and broadens the expansion theory of unsaturated soils to yield a more comprehensive theoretical framework for the comprehensive analysis of offshore islands’ unsaturated soils.

This work investigates the rainfall infiltration process within homogeneous and stratified colluvial deposits caused by short (1–3 h) and intense (40–90 mm/h) rainfall, using both analytical and numerical infiltration modelling. The findings of the investigation demonstrate that the classic Green–Ampt model can be employed effectively to study homogeneous colluvial covers with permeability equal to or lower than kw = 10−5 m/s and that are subject to a 1 h rainfall with intensity I ≥ 45–50 mm/h. In these circumstances, a top-down saturation front forms within the colluvial deposit, leading to the saturation of a 70–100 cm-thick layer. This critical condition occurs every 5–10 years in the mountain area of the Friuli Venezia Giulia Region (NE Italy), which corresponds to a lower return period of critical hydrologic events when compared with other mountain basins in the Alps due to the higher initial degree of saturation characterising colluvial covers in this area (70–95%). When analysing stratified colluvial covers, the Dagan–Bresler approximate model, as well as the numerical modelling, emphasised the strong influence that abrupt variations in the permeability of the various soil layers have on the infiltration process at depth. In particular, the presence of a top organic soil horizon that is rich in macro-pores and is characterised by a higher permeability (k = 10−4 m/s) actually reduces the possibility of surficial ponding, which is the basic condition of the “piston” models. The highly permeable top soil allows for a rapid downward infiltration up to contact with the underlying colluvial material, which is less permeable (k = 10−5 m/s). Therefore, a perched water table forms starting from the organic soil–colluvium interface, originating pore–water overpressures within the colluvial deposit, with maximum values in the order of 5–10 kPa.

Describing the hydraulic conductivity of unsaturated soil is very important in predicting water transport. Most current models have complex forms and generally need to be calibrated by the measured unsaturated hydraulic conductivity curve. A simple model, by which it is possible to conveniently predict the unsaturated hydraulic conductivity, is proposed in this study. The soil–water characteristic curve and hydraulic conductivity curve are separated into three parts. The soil–water characteristic curve is represented by Fredlund and Xing’s equation. A simple model composed of three lines is proposed for estimating the hydraulic conductivity of unsaturated soil. The model parameters can be conveniently calibrated from the measured soil–water characteristic curve and saturated hydraulic conductivity. Finally, the proposed model is validated by the experimental data from different soils. The proposed model provides a simple approach to estimating the hydraulic conductivity of unsaturated soil, which is more convenient for practical application.

Unsaturated soil covers a significant part of the world, and studying the behavior of deep foundations in this medium is an important step in increasing accuracy and economic efficiency in geotechnical studies. This paper presents an analytical solution to investigate the load-carrying characteristics of single piles embedded in unsaturated soils, accounting for the effect of groundwater level on the pile’s response. For this purpose, relationships for shear modulus and Poisson’s ratio for unsaturated soils were collected from the literature to consider their effects as key parameters on pile performance. A parametric study was conducted to evaluate the effect of soil moisture content on the behavior of the pile-soil system for different soil types, and the effect of pile slenderness on its load-settlement behavior was studied for varying soil moisture contents. The results indicate that the pile stiffness increases as the soil suction increases while below a critical slenderness value, hence increasing the pile load capacity. However, this improvement occurs within a limited range of soil suction that is narrower for coarse-grained soils. The pile settlement corresponding to soil failure was also evaluated by modifying the existing solutions for unsaturated soils. The developed solutions were verified against the predictions of published solutions as well as the results of finite element analysis and pile load tests. It was found that the system stiffness decreases by 50% when the water table rises from the pile toe level to the ground surface in the studied soil.

The study of the rainfall infiltration mechanism of unsaturated soil has always been a hot issue in the field of geotechnical engineering. It is worth studying which parameters should be introduced to characterize the infiltration characteristics of unsaturated soil in the calculation and analysis of rainfall infiltration. In this paper, the Fredlund–Xing model was quoted in the SEEP/W module of the Geostudio software, and the transient numerical calculation of rainfall infiltration under the same rainfall duration T and different rainfall intensity I was carried out for a soil column. Three infiltration characteristic parameters were introduced: rainfall infiltration front depth WF, suction reduction depth MRn, and section infiltration rate IR. The variation of these three parameters and rainfall intensity I during rainfall were sorted out and analyzed; it is indicated that WF increases with the extension of rainfall duration. MRn decreases with the increase of suction reduction rate n%, and when the rainfall duration is 24 h, the maximum depth of the soil column affected by rainfall is approximately 35% of the total depth. IR is mainly affected by the rainfall intensity I and the saturation permeability coefficient ks. There is a limit value for the influence of I on WF, MRn, and IR, and the limit rainfall intensity under the calculation conditions in this paper is I = 2.5ks.

Because of factors such as plant root action, soil capillary action, and evapotranspiration, the soil near the surface of a slope tends to be drier and in an unsaturated state. However, when rainfall infiltrates the soil, the soil near the surface can become saturated, creating a wetting band that may trigger shallow landslides on the slope. This study performed transient finite element seepage and slope stability analyses on unsaturated soil slopes during rainfall. The analysis considered a representative rainfall pattern observed in Taiwan from 2010 to 2022, accounting for potential impacts of climate change. The objective of this study was to explore the impact of wetting band development on the stability of unsaturated soil slopes under rainfall conditions. Additionally, the investigation aimed to analyze the variations in pore water pressure and volumetric water content throughout the rainfall process. These analyses were conducted to determine the slope safety factor and investigate the relationship between the wetting band and the potential sliding surface. The results showed that the soil pore water pressure and water content near the surface of the slope were highly influenced by rainfall. The wetting band gradually extended downward from the surface with increasing rainfall duration, resulting in an increased wetting band depth (WBD), while the potential sliding surface approached the slope surface. When the safety factor was at its lowest, the potential sliding surface intersected the edge of the wetting band and was included within the range of the wetting band. This observation indicates a significant relationship between the potential sliding surface and the evolution of the wetting band during slope failure at the moment of minimum safety factor. The study also observed that water infiltration near the toe of the slope was more rapid, leading to a deeper wetting band at this particular location.

Soil improvement is often done through mixing the granular soils with fine-grained materials to obtain densified soil matrix and eventually increase the bearing capacity of the soil. In this paper, sand was mixed with three different quantities of bentonite. Then, the hydromechanical behaviour of sand-bentonite mixtures was evaluated at different matric suctions. Suction-controlled triaxial test and the pressure plate extractor were used to obtain the experimental data needed to derive the stress-strain curves and the Soil Water Characteristics Curve (SWCC), respectively. The results showed that the apparent cohesion that developed due to the existence of matric suction was increasing with the bentonite content increase. Residual suction value was increasing as the bentonite content increasing. It was concluded that the increase of water retention capacity has made the sand-bentonite samples responds very significantly with changing of matric suction. Therefore, addition of fine materials to granular soils would make the soil prone to wetting collapse upon reaching the saturation condition during the rainy seasons.

The frame beam anchor plate (FBAP) is a new supporting structure to reinforce backfill. The current studies of slope stability concentrate on homogeneous isotropic dry or saturated slopes according to two-dimensional assumptions. In engineering practice, however, the soils are heterogeneous and anisotropic and exhibit evident unsaturated characteristics, and the failure surface shows obvious three-dimensional (3D) characteristics universally. A new method for evaluating the 3D stability of unsaturated soil slopes reinforced with FBAPs is established in this paper. The upper-bound analytical solution of slope safety factor (Fs) is derived based on the energy balance equation incorporated with the 3D spiral failure mechanism and the gravity increase method (GIM). Comparisons verified the effectiveness of the methodology and the optimization program. The influence of unsaturated characteristics, support structure design parameters, and seismic force on the 3D slope stability is evaluated through parameter analysis. The results show that the shear strength prediction model is crucial in slope stability assessments. The stability of slopes can be significantly improved by about 29%–52% by using FBAPs. Seismic action has a significant negative impact on the slope stability. When kh increases from 0 to 0.3, Fs generally decreases by about 40%–55%. [ABSTRACT FROM AUTHOR]

Soil suction plays an important role in governing the stability of slopes. Environmental sustainability could be jeopardized by hazards, such as slope failures (forest destruction, landscape alteration, etc.). However, the quantification of the suction effect on slope stability is a challenging task as the soil suction is usually affected by the precipitation and evapotranspiration. Numerical simulation plays an important role in the estimation of contour in soil suction due to rainfall and evapotranspiration as long-term and widespread monitoring is rarely conducted. The result of numerical simulation is highly dependent on the accuracy of the input parameters. Hence, suction monitoring plays an important role in verifying the result of numerical simulation. However, as a conventional tensiometer is limited to 100 kPa soil suction, it is hard to verify the performance of numerical simulation where suction is higher than 100 kPa. The osmotic tensiometer developed by Nanyang Technological University (NTU) can overcome this problem. It is now possible to monitor changes in soil suction higher than 100 kPa (up to 2500 kPa) for an extended period in the field. In this study, a procedure was proposed to estimate suction changes in residual soil based on rainfall and evapotranspiration data. Numerical simulation was carried out based on the soil properties and geometry of a residual soil slope from Jurong Formation Singapore. Changes in soil suction due to rainfall and evaporation were simulated and compared with the readings from the NTU osmotic tensiometers installed at 0.15 m and 0.50 m from the slope surface in the field. It was observed that numerical simulation was able to capture the variations of suctions accurately at greater depths. However, at shallow depths, erratic suction changes due to difficulties in capturing transpiration. [ABSTRACT FROM AUTHOR]

Compacted soils used as formation layers of railways and roads continuously undergo water content and suction changes due to seasonal variations. Such variations, together with the impact of cyclic traffic-induced loads, can alter the hydro-mechanical behaviour of the soil, which in turn affects the performance of the superstructure. This study investigates the impact of hydraulic cycles on the coupled water retention and cyclic response of a compacted soil. Suction-monitored cyclic triaxial tests were performed on a compacted clayey sand. The cyclic response of the soil obtained after applying drying and wetting paths was different to that obtained immediately after compaction. The results showed that both suction and degree of saturation are required to interpret the cyclic behaviour. A new approach was developed using (a) a hysteretic water retention model to predict suction variations during cyclic loading and (b) Bishop's stress together with a bonding parameter to predict accumulated permanent strain and resilient modulus. The proposed formulations were able to predict the water retention behaviour, accumulated permanent strains and resilient modulus well, indicating the potential capability of using the fundamentals of unsaturated soils for predicting the effects of drying and wetting cycles on the coupled soil water retention and cyclic response. [ABSTRACT FROM AUTHOR]

Slope disasters often include soil erosion and shallow landslides. These types of slope failure can occur in unsaturated soil. In this study, artificial rainfall is applied to a compacted soil slope in a laboratory to investigate the effects of rainfall intensity, initial moisture content and relative compaction on infiltration and failure behavior of an unsaturated soil slope subjected to rainfall infiltration. Soil moisture probes installed in the soil slope were used to monitor soil moisture content during the experiment. Test results show that the soil saturation at the toe of the slope was observed to be higher than other areas of the slope following the onset of rainfall. Gradually, the saturation advanced towards the crest, resulting in the formation of a wetting band along the slope surface. With increasing rainfall duration, the wetting band progressed further downwards along the slope. The failure of the slope began at the crest and junction of the slope, and the primary cause was soil erosion resulting from high-intensity rainfall. The most significant variable affecting infiltration behavior is relative compaction, followed by the initial moisture content of the compacted slope. During the experiment, the low hydraulic conductivity of the saturated soil caused most of the rainfall on the slope to become surface runoff, which minimized the impact of rainfall intensity on infiltration behavior. Specifically, the hydraulic conductivity of soil compacted at optimal moisture content and 90 R.C. is only 7.041 × 10-5 cm/sec or 2.53 mm/h, which is much lower than rainfall intensities of 80 mm/h and 160 mm/h. In addition, soil saturation contours facilitated visualization and quantification of the infiltration behavior of slopes, enabling a more detailed analysis of experimental results. These results help understand the behavior of unsaturated soil slopes under artificial rainfall conditions and aid in designing effective slope stabilization measures to prevent slope failure and minimize the risk of landslides. [ABSTRACT FROM AUTHOR]

The water retention properties of soils present in formation layers of roads and railways continuously vary due to repeated traffic loads and periodic rainfall events. This is important because the accumulated deformation and resilient modulus of such soils under cyclic loading are profoundly affected by water retention properties. This paper discusses the cyclic and water retention response of a clayey sand subjected to repetitive cyclic loading and wetting stages. The results show that the accumulated permanent strains and resilient modulus of the tested soil are dependent on the suction level while the main wetting water retention curve of the soil dictates the variation of the suction measured during cyclic loading and wetting. This bounding effect of the water retention curve is found to be dependent on the void ratio where the suction can even increase due to the accumulation of strains under cyclic loading while the degree of saturation increases. This contradicts the suction reduction typically observed with an increase in the degree of saturation. A void ratio dependent water retention model is developed accounting for the observed bounding effect and employed to predict the measured suction during repetitive cyclic loading and wetting. The suction values predicted by the void ratio dependent water retention model are in good agreement with the experimental data. The predicted suctions are then used in semi-empirical formulations to obtain the accumulated permanent strains and resilient modulus. A better correlation between model predictions and experimental data is achieved where the suction values predicted by the void ratio dependent water retention model are used. The results imply that predictive frameworks proposed for the cyclic behaviour of road and railway formation layers require water retention counterparts that incorporate the bounding effect of void ratio on soil water retention curves. [ABSTRACT FROM AUTHOR]

The lateral response of piles is governed by the soil–pile interaction in the shallower layer of surrounding soil, which may be above the water level and thus is unsaturated. There is no comprehensive analytical model for the soil–pile interaction under lateral loads considering both linear and nonlinear behaviors in unsaturated soils. In this study, an analytical solution is proposed to simulate the linear and nonlinear load–displacement variations for laterally loaded piles supported by unsaturated soils. Analytical formulations are developed to determine the stiffness per unit pile length considering the influence of soil suction on the soil modulus of elasticity. Since the current p–y curves available in the literature are unable to adequately address the unsaturated soil reaction to piles, analytical p–y relations for three types of soil are developed regarding the soil suction parameter. In order to expand the scope of application of presented method, the developed solution is extended to step-tapered piles and to piles subjected to simultaneous horizontal and vertical loads. The developed solutions are validated with the results obtained from laterally loaded pile tests reported in the literature. Parametric studies evaluated the effects of pile length and cross-section configuration and applied vertical load on the pile lateral response under various soil unsaturation conditions. The results revealed that the stiffness of the pile–soil system and its load-deformation behavior are improved significantly as the soil suction pressure increases. Furthermore, the lateral pile displacement in unsaturated soil decreases as the pile length and diameter of the upper pile portion increase, and by considering the applied vertical load. Considering the positive effect of soil suction on reducing the lateral response of piles in unsaturated soil (in arid areas) in design may help shorten the pile length or reduce its diameter while achieving a desired force–deflection demand. [ABSTRACT FROM AUTHOR]

For unsaturated soils, Terzaghi's effective stress σ = σ tot - u has been generalized by Bishop to the form σ B = σ tot - χ u . Factor χ , for example χ (S) ≈ S , takes into account the degree of saturation S. However, σ B is unable to characterize strength and deformation of soil unless applied with suction. A new stress σ E = σ tot - χ E u with χ E (S , n) is proposed. It is likewise insufficient if applied without suction, but it has (at least) a clear physical meaning: σ E represents contact forces between grains in the static equilibrium in the same manner as the effective stress does in saturated soils. All grains must be entirely surrounded by free or capillary water. [ABSTRACT FROM AUTHOR]

This paper attempts to investigate the bearing capacity of a strip footing that rested on unsaturated homogenous sands and used upper bound finite-element limit analysis in association with linear optimization. The analysis is based on suction stress-based (σs) effective stress (σ′) analysis. Unlike the saturated soil, the size of the yield surface varies with the vertical position, and therefore, proper modifications are carried out in the upper bound formulations to incorporate the unsaturated soil property functions. A detailed investigation is carried out to estimate the load-carrying capacity of a strip footing that vary the mechanical strength, unsaturated soil properties, water table depth (hw), and climatic conditions. The numerical solutions are presented in terms of bearing capacity factors [BCF (Nγs, Nqs)] for the unit weight (γw) and surcharge pressure (qs). The nodal velocity patterns are drawn for several cases, and the presented solutions are compared with the available results. The design charts provided in this paper might be useful to design engineers. [ABSTRACT FROM AUTHOR]

An analytical study of the problem of water infiltration into a homogeneous unsaturated soil showed that, in the case of periodical water recharge through the soil surface, the character of the flow will tend to uniform with the depth. The stabilization of the flow was found to be due to the effect of two factors: capillary dissipation and nonlinearity. The role of each factor was studied by constructing appropriate exact solutions. An estimate was proposed for the depth at which flow variations become insignificant. This estimate takes into account the joint effect of both these factors. An explicit expression for it contains the hydraulic characteristics of the soil and the main characteristics of the surface recharge regime. The pumping effect was also studied and it was showed that, at some assumptions regarding the soil hydraulic characteristics, it manifests itself in that the time-averaged water content in the top soil layers is less than that at greater depth. [ABSTRACT FROM AUTHOR]

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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. [ABSTRACT FROM AUTHOR]

Shallow landslides in Souk Ahras region, NE Algeria, occur usually after prolonged rainfall events. Three localities which are Zaarouria, Hammam Tassa, and Mechroha were chosen to study these landslides and the conditions of their occurrences. They have been observed to affect mainly agricultural lands, main supplies, and the road network. This paper aims to find the relationship between the rainfall, the soil engineering properties, the geology, and the geomorphology. A number of field trips were organized to the study areas where landslide masses were described and mapped as well as some field measurement such as natural moisture content and sampling. In the laboratory, we establish gain size distribution, Atterberg limits, density, and effective cohesion and internal friction. Then we established the unsaturated soil property functions such as the soil water characteristic curve (SWCC) and the hydraulic function. The stability computation was undertaken using, GeoStudio software 2020, a coupled analysis where the Seep/W simulates the seepage and the Slope/W for the safety factor. The results show that the sector study containing: silty sand (SM), well graded sand with silt and gravel (SW-SM), and clayey sand with Gravel (SC) while hydraulic conductivity varies between 4.5. 10−6 for 4.10−5 and 10−3 m/s. The factor of safety varies between 0.8, 1.16, and 0.8. In conclusion, our study confirms the field observations as shallow landslides have been numerous in mars 2019 when rainfall reaches 110 mm. Moreover, high permeability soils, in Mechrouha, show a more dramatic decrease in the factor of safety as the 100 = -mm rainfall amount is reached while it decreases gradually for the other two less permeable soils of Zaarouria and Hammam Tassa. [ABSTRACT FROM AUTHOR]

Understanding colloid transport in porous media under transient-flow conditions is crucial in understanding contaminant transport in soil or the vadose zone where flow conditions vary constantly. In this article, we provide a review of experimental studies, numerical approaches, and new technologies available to determine the transport of colloids in transient flow. Experiments indicate that soil structure and preferential flow are primary factors. In undisturbed soils with preferential flow pathways, macropores serve as main conduits for colloid transport. In homogeneously packed soil, the soil matrix often serves as filter. At the macroscale, transient flow facilitates colloid transport by frequently disturbing the force balance that retains colloids in the soil as indicated by the offset between colloid breakthrough peaks and discharge peaks. At the pore-scale and under saturated condition, straining, and attachment at solid–water interfaces are the main mechanisms for colloid retention. Variably saturated conditions add more complexity, such as immobile water zones, film straining, attachment to air–water interfaces, and air–water–solid contact lines. Filter ripening, size exclusion, ionic strength, and hydrophobicity are identified as the most influential factors. Our review indicates that microscale and continuum-scale models for colloid transport under transient-flow conditions are rare, compared to the numerous steady-state models. The few transient flow models that do exist are highly parameterized and suffer from a lack of a priori information of required pore-scale parameters. However, new techniques are becoming available to measure colloid transport in real-time and in a nondestructive way that might help to better understand transient flow colloid transport. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality.

This study investigates the thermal soil-geosynthetic interaction mechanisms of reinforcing geotextiles confined in compacted silt that may be encountered when using mechanically-stabilized earth (MSE) walls as geothermal heat sinks. A thermo-mechanical geosynthetic pullout device was used that incorporates standard components for geosynthetic pullout or creep testing but also heating elements at the top and bottom of the soil box to apply boundary temperatures and dielectric sensors embedded in the soil layer to monitor distributions in temperature and volumetric water content. Two test series were performed: the first involves monotonic pullout of woven polypropylene geotextiles after reaching steady-state conditions under different boundary temperatures without a seating load, and the second involves monotonic pullout of woven polyethylene-terephthalate geotextiles after reaching steady-state conditions under different boundary temperatures with a seating pullout load. The results indicate that the pullout resistance of both geotextiles decreased with increasing temperature. Although heating led to drying of the unsaturated silt layers as expected, measurements from the second test series indicate accumulation of water at the silt-geotextile interface. An effective stress analysis considering thermal softening of soils indicates that the increase in effective saturation at the silt-geotextile interface was the cause of the decrease in pullout resistance with heating.

In this work, a series of unconfined compression tests at different water contents were performed to investigate the mechanical behaviour of clay–sand mixtures compacted in standard Proctor conditions. For studying the effect of water content and suction on unconfined compressive strength (UCS) and on strain secant modulus (E50 modulus) of these mixtures, drying–wetting paths were defined by measuring the soil–water characteristic curves (SWCCs) using osmotic and salt solution techniques and filter paper method. The results highlighted that an increase in sand content of the mixture leads to an increase in the maximum dry densities and a decrease in the optimum water content of the materials. However, at the given state, when clay is mixed with 25% of sand, the UCS and E50 modulus increase to 37% and 70%, respectively, compared to those of clayey samples. But when clay is mixed with 50% of sand, the UCS and E50 modulus decrease to 38% and 46%, respectively, compared to those of clayey samples. The results also indicate that the UCS and E50 increase with a decrease in the water content and an increase in suction, irrespective of the sand content.

In order to reflect the creep characteristics of unsaturated silty clay, a triaxial compression consolidation drainage creep test was conducted under the condition of controlling the matric suction. According to the results of the creep test, combined with the empirical models, Mesri model and Log-modified model, the relationship between the initial tangent modulus and the matric suction was established, and two empirical models of unsaturated soil creep considering the effect of matric suction were constructed. The study confirmed the stress–strain through the ε/D–ε relationship curve, and determined the parameters F and n through power function. The methods for determining the strain–time relationship parameters of the two improved models are different. The improved Mesri model was obtained by fitting the ln ε–ln t relationship, while the improved Log-modified model was solved by the BFGS algorithm and the general global optimization method. By comparing the two improved models of unsaturated soil creep tests, it was found that the improved Mesri model can more accurately describe the creep characteristics of unsaturated soils, which confirms the rationality and feasibility of this model and method.

In arid and semi-arid regions, such as in northwestern China, unsaturated soils at high suction or over a wide suction range are widespread in various geotechnical designs. Studies of the soil water retention behavior are fundamental to a comprehensive understanding of the water flow, deformation processes, and shears strength for unsaturated soils. To investigate the soil water retention behavior of a silt over a high suction range, the suction is imposed on silt specimens using the vapor equilibrium technique, and the volume of irregular soil sample is measured using the Archimedean method. The soil water characteristic curves (SWCCs) with different initial void ratios at high suctions were simulated by the modified Van Genuchten’s model. The results show that (1) SWCCs in terms of water content versus suction relation with different initial void ratios are independent of the initial void ratio at high suctions. That is, soil water retention at high suctions is mainly related to adsorbed water content. (2) It was found that the fitting results of the existing equation based on Van Genuchten’s model can not accurately simulate the soil water retention behavior over a high suction range. A modified soil water characteristic equation is proposed by introducing a correcting function into the equation. (3) A modified relative permeability coefficient equation is proposed and it can be applied to predict the relative permeability coefficient with different initial void ratios. The predicted results are in good agreement with the measured values.