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Using numerical method to investigate the evolution of micro-structure in unsaturated soil is helpful to explain their physical mechanical. However, the evolution of micro-structure in unsaturated soil is not clear. For this reason, the capillary method is proposed to simulate compression test of unsaturated soil for analyzing soil-water interaction and evolution of pore spaces. In this research, an ideal micro-structure soil model of 540 μm×400 μm is established according to the shape and size of skeleton particles of loess. The capillary method is used to calculate the capillary water distribution and capillary force under different water content(5%, 10%, 18%), and the force then acts on the wetting surface among particles. The loading is applied to the soil sample. The compression test of unsaturated soil is simulated in this research. The simulation results show that the characteristics of compression curve are in good agreement with the experiment data, indicating that the numerical test can reflect the unsaturated soil deformation behavior under the condition of compress. Moreover, the saturation increases and matric suction decreases with the compression of the soil; the saturation increases most at high water content; the matric suction decreases least at middle water content; the deformation pattern is related with the moisture, namely, the pores are mainly shrunk at low water content while mainly disintegrated at high water content. The simulation results are a very useful supplement to laboratory test to explore the deformation mechanism of unsaturated soil.

The traditional SWCC test method is time-consuming, and it is of great practical significance to develop a method that can quickly determine the SWCC of unsaturated soil. In order to predict the SWCC of compacted loess rapidly, the water potential and moisture content of compacted loess with different dry densities were tested, and the pore size distribution curve was measured using nuclear magnetic resonance (NMR) technology. Based on the test results, a rapid prediction method of the soil-water characteristic curve of Yan’an compacted loess was established based on the void ratio, and its accuracy was verified by the measured data. The results show that the fractal dimension D in the prediction model can be determined by the cumulative pore volume of two points (peak point and half-width point) on the pore size distribution curve and the slope of the pore size in a double logarithmic coordinate; and it can be expressed by the void ratio based on the linear relationship between void ratio and dominant pore diameter in logarithmic coordinates. The inlet value of SWCC is controlled by the diameter of macropore; the slope of the transition section is controlled by the volume of mesopore. There is a critical pore size in compacted loess, the residual water content is mainly controlled by the pore volume whose pore size is smaller than the critical pore size, and an empirical method is proposed to calculate the residual volume water content. Compared with traditional methods, the proposed method can save a lot of time in determining SWCC.

In the common practice of geotechnical design, the state of soil is considered fully saturated, while recent examples show the importance of the consideration of unsaturated conditions as well due to the location of the groundwater table at deeper levels. However, it is problematic to design a foundation and calculate bearing capacity as the internal stress in unsaturated conditions needs to be quantified. The mechanics of unsaturated soils are critical in geotechnical engineering, especially when the case of the foundation design is required. This study aims to investigate the influence of suction on the shaft capacity of pile foundations based on the data obtained from in situ and laboratory testing. The three modified methods (i.e., modified α, β, and λ) were implemented in this study in order to calculate the shaft capacity of the pile foundation incorporating unsaturated soil mechanics principles. In addition, the effect of varying matric suction on shaft capacity was discussed. The results of obtained changes show the importance of rain infiltration in the design of piles in unsaturated conditions. It can be concluded that the most conservative method is found to be the modified β method in designing pile foundations incorporating the suction changes due to rainwater infiltration. However, cost optimization shall be considered while choosing the method for the design. As for the increase in GWT, the modified λ method depicted the most conventional behavior. The possible rain infiltration decreases matric suction, mostly in the upper layers. [ABSTRACT FROM AUTHOR]

Capillary rise tests were conducted on soil columns containing of three layers of sandy soils with coarser over finer over coarser sandy soil to investigate the effect of the relatively finer soil interlayer. The capillary rise height, rate, and water distribution were observed in laboratory tests of four layered soil columns, with two homogeneous (without the interlayer) soil columns serving as the controls. The final maximum height of the capillary rise in the soil column with the interlayer was larger than that of the column without the interlayer when the interlayer was laid around the water entry value of the underlying soil. The water content was not continuous in the entire soil profile with the interlayer, and a small matric suction gap was observed in the relatively fine soil between the soil column with and without the interlayer. [ABSTRACT FROM AUTHOR]

The soil moisture content at shearing and other factors, including dry density and grain size, influence its shear strength. This study investigated the effect of moisture content at compaction and grain size distribution on the unsaturated soil shear strength. Triaxial compression tests were performed in the laboratory using the modified triaxial apparatus on silica sands No. 3 and 6 without fines and with 20% fines to explore the unsaturated soil shear strength characteristics. Test samples were compacted and sheared at various combinations of the soil's optimum and residual moisture content. The analysis of the triaxial compression test results shows that moisture content at compaction and the grain size distribution influence the unsaturated soil shear strength. The test samples compacted at optimum moisture content showed higher peak shear strength when sheared at residual moisture content. Further, test results show that the test samples of soil without fines, when compacted at residual moisture content, show higher peak shear strength at optimum moisture content. The finding of this study endorses considering the moisture content at compaction for the geotechnical design of structures while predicting the soil shear strength. [ABSTRACT FROM AUTHOR]

The coexistence of air and water as a mixture in void spaces in unsaturated soils, makes its mechanical behaviour relatively complex and difficult to be addressed by classical saturated soil mechanics principles. In addition, many engineering problems encountered in practical situations had some part to deal with unsaturated soils and therefore the clear understanding of various behavioural patterns of unsaturated soils is crucial. To date, a large number of attempts are reported in evaluating the mechanical behaviour of unsaturated soils in different stress spaces. The present study evaluates the deviatoric behaviour of compacted unsaturated soils in deviator stress–specific water volume–mean net stress ( q , v w , p ) space. The effect of moisture content, confinement and the stress history on the behaviour of compacted unsaturated kaolin subjected to triaxial compression was assessed and the results showed that there exists a critical state surface for compacted unsaturated soils in q - v w - p space. [ABSTRACT FROM AUTHOR]

Unsaturated permeabilities of six different kaolin-sand mixtures named as 100K0S, 90K10S, 80K20S, 50K50S, 20K80S, and 10K90S were directly measured using a modified flexible wall triaxial permeameter system. 90K10S means that the soil mixture contains 90% kaolin and 10% sand by dry mass. Soil–water characteristic curves (SWCC) of the kaolin-sand soil mixtures were also measured for a wide range of suctions using a Tempe pressure cell, 5-bar pressure plate, 15-bar pressure plate, and saturated salt solutions. The unsaturated permeabilities of the soil mixtures were also estimated using ten different theoretical models using (1) the measured SWCC data over a complete suction range and (2) the measured SWCC data up to 100 kPa suction in combination with the estimated SWCC data at higher suction values. The directly measured unsaturated permeability of the soil mixtures was used to validate the estimated values of unsaturated permeability. The results showed that the unsaturated permeability estimation models could reasonably fit the directly measured unsaturated permeability data when both the data from the complete measurement of SWCC and the measured SWCC data up to 100 kPa suction in combination with the estimated SWCC data at higher suction values were used. [ABSTRACT FROM AUTHOR]

The rhizosphere is the region of soil directly influenced by plant roots. The microbial community in the rhizosphere includes fungi, protists, and bacteria: all play significant roles in plant health. The beneficial bacterium Sinorhizobium meliloti infects growing root hairs on nitrogen-starved leguminous plants. Infection leads to the formation of a root nodule, where S. meliloti converts atmospheric nitrogen to ammonia, a bioavailable form. In soil, S. meliloti is often found in biofilms and travels slowly along the roots, leaving developing root hairs at the growing root tips uninfected. Soil protists are an important component of the rhizosphere system, able to travel quickly along roots and water films, who prey on soil bacteria and have been known to egest undigested phagosomes. We show that a soil protist, Colpoda sp., can transport S. meliloti down Medicago truncatula roots. Using model soil microcosms, we directly observed fluorescently labeled S. meliloti along M. truncatula roots and tracked the displacement of the fluorescence signal over time. Two weeks after co-inoculation, this signal extended 52 mm farther down plant roots when Colpoda sp. was also present versus treatments that contained bacteria but not protists. Direct counts also showed protists are required for viable bacteria to reach the deeper sections of our microcosms. Facilitating bacterial transport may be an important mechanism whereby soil protists promote plant health. IMPORTANCE Soil protists are an important part of the microbial community in the rhizosphere. Plants grown with protists fare better than plants grown without protists. Mechanisms through which protists support plant health include nutrient cycling, alteration of the bacterial community through selective feeding, and consumption of plant pathogens. Here, we provide data in support of an additional mechanism: protists act as transport vehicles for bacteria in soil. We show that protist-facilitated transport can deliver plant-beneficial bacteria to the growing tips of roots that may otherwise be sparsely inhabited with bacteria originating from a seed-associated inoculum. By co-inoculating Medicago truncatula roots with both S. meliloti, a nitrogen-fixing legume symbiont, and Colpoda sp., a ciliated protist, we show substantial and statistically significant transport with depth and breadth of bacteria-associated fluorescence as well as transport of viable bacteria. Co-inoculation with shelf-stable encysted soil protists may be employed as a sustainable agriculture biotechnology to better distribute beneficial bacteria and enhance the performance of inoculants. [ABSTRACT FROM AUTHOR]

Composite foundation is a widely used foundation treatment technology in geotechnical engineering. This study are derived governing equations to axisymmetric consolidation of unsaturated composite foundation (USCF) incorporating permeable columns and impermeable columns under instantaneous loading, and obtained the final solutions of excess pore pressures (EPPs) by using mathematical methods such as the variable separation, decoupling, finite Hankel transform and its inverse transform. The analytical solution obtained under free strain conditions is more general and representative, as it can be readily degraded to solutions for a variety of consolidation models and the EPPs can be conveniently calculated at any point in the calculation cell. Through degrading proposed solutions to the special case to verify their correctness, the comparative results were found well‐matched to the existing literature solutions. The effect of various parameters on the consolidation performance of USCF was analyzed, where the studied parameters include the compression modulus of columns, radius ratio for permeable columns, and the number of impermeable columns. The results show that increasing the compression modulus and area replacement rates of columns can accelerate consolidation and reduce settlement, and the latter is more effective. Compared to reinforced composite foundations with only a single form of the permeable or impermeable column, the combination of permeable and impermeable columns can significantly shorten the consolidation time and enhance the bearing capacity of the unsaturated ground. [ABSTRACT FROM AUTHOR]

Biochar, a type of soil amendment, is widely used in heavy metal–polluted soil. It is also effective in enhancing plant performance. However, how biochar affects the soil heavy metal toxicity considering the plant–soil interactions is not well summarized. Therefore, this paper aims to review and summarize the effects of biochar on heavy metal availability in vegetated unsaturated soil. First, this review summarizes the effects of biochar on key parameters of unsaturated soil, including pore size distribution, water permeability, water retention curve, nutrient cycling and soil biota. Then, the mechanism of biochar for managing heavy metals in soil with vegetation is discussed, with a focus on both physical and biochemical aspects. Additionally, the impact of biochar-induced plant growth on the availability of heavy metals in the plant–soil system is explored. Finally, suggestions for future research and applications of biochar are provided. This paper supplies novel outlooks for further study and practice. [ABSTRACT FROM AUTHOR]

This study proposes a new soil-water characteristic model to predict the variation of volumetric water content with the matric suction pressure for unsaturated soil conditions. The proposed model has been built utilizing the nondestructive electrical properties of the unsaturated soil. In addition, nine previous models have been collected and used in parallel with the new model to predict the soil-water characteristic relationships for four sets of different experimental data. The study has clearly revealed that the new proposed model agreed very well with the experimental data with R2 values of 0.90, 0.93, 0.95, and 0.92 for the four studied cases respectively. However, none of the collected nine models had the power of the new proposed model in terms of predicting the experimental data for all the studied cases. Furthermore, it was concluded that the new model parameters have been correlated strongly with the final volumetric water content. [ABSTRACT FROM AUTHOR]

This research presents a modified numerical simulation approach for predicting approximate values of unsaturated soil parameters with low water content. In order to understand how unsaturated soils behave and to create the soil–water characteristic curve, the suggested model attempts to predict the following parameters. First, the relationship between the water content and the matric suction in terms of particle separation distance. Secondly, the relationship between the effective stress parameter χ and the filling angle θ when the separation distance between soil particles is taken into account. Thirdly, the relationship between suction stress and filling angle θ , in terms of particle separation distance. Fourthly, the effect of particle separation distance on the curve of tensile strength in terms of the degree of saturation. The updated model assumes that all the particles are equally dispersed spheres of the same size. In this study, the updated model is exclusively applied to simple cubic packing order with zero contact angle. The model may also be used to predict matric suction, effective stress parameter, suction stress, and tensile strength while combining experimentally determined soil properties. This removes the requirement for sophisticated measuring devices or specific test procedures to illustrate how the separation distance affects the behavior of unsaturated soils. Compared to the experimental data, our model has shown positive results. In this paper, all curves have been programmed using MATLAB. [ABSTRACT FROM AUTHOR]

Extreme hydroclimatic events such as heavy rainfall and drought have occurred frequently in recent years, and their impacts on geotechnical systems must be understood for developing climate-adaptive design procedures. A fully coupled flow-deformation finite element method is best suited for accurately predicting the behavior of geotechnical structures under extreme hydroclimatic events. PLAXIS, a finite element software widely used by practicing engineers and researchers, models water flow accurately, but the deformation model is not fully coupled with the flow model because the constitutive models do not update the stiffness and failure criterion during water flow. Therefore, to improve the predictive capability, first, the elastic modulus and yield criterion of the Mohr-Coulomb (MC) model was modified, validated, and implemented as a user-defined constitutive model in PLAXIS. Then, the Modified Mohr-Coulomb (MMC) model was used to understand the hydromechanical behavior of foundations and earth slopes under site-specific extreme hydroclimatic events in a coupled manner. The temporal variation of the (a) safety factor of earth slopes, (b) skin and tip resistances and settlement of deep foundations, and (c) settlement of shallow foundations were estimated for the site-specific extreme hydroclimatic events. The results show that the traditional design limits are altered by the hydroclimatic events, and therefore the existing design methods must be updated. [ABSTRACT FROM AUTHOR]

Wildfire records demonstrate worsening patterns coupled with the spread to higher altitudes in several regions, raising the risk of post‐wildfire ground failures. This study investigates the post‐wildfire stability of unsaturated hillslopes against rainfall‐triggered shallow landslides. We developed a new physics‐based analytical framework incorporating wildfire‐induced changes in soil properties and near‐surface processes affecting the hillslope stability. A coupled hydromechanical infiltration model is integrated into an infinite slope stability analysis to simulate temporal changes in the depth profiles of soil water content, pressure head, and the resulting factor of safety (F.S.) of a vegetated slope. We consider the antecedent conditions of soil and vegetation cover, including the recovery phase after the fire, wildfire‐induced alterations in transpiration, and time‐varying infiltration rates. The model is verified against numerical simulations and employed in parametric studies evaluating the effects of wildfire severity and rainfall intensity‐duration. For the cases examined, it was shown that wildfire could reduce the F.S. of slopes by 25%. As a case study, the model successfully captured shallow rainfall‐triggered landslides that occurred in the Las Lomas watershed in California, USA, in 2019, 3 years after the Fish Fire burned the area. The proposed model uses measurable hillslope and wildfire characteristics and can be employed to evaluate the risk of shallow landslides in wildfire‐prone areas. Plain Language Summary: Several regions (e.g., Western U.S., Australia) globally experience higher wildfire activities and increased wildfire elevations. The worsening patterns of wildfires elevate the risk of post‐wildfire ground instabilities, which can threaten the integrity of communities and infrastructures in the affected areas. However, the literature is still scarce on a physics‐based yet practical slope stability framework capable of capturing wildfire‐induced alterations in soil and vegetation properties. Here we present a new physics‐based framework to evaluate the post‐wildfire stability of unsaturated hillslopes against rainfall‐triggered shallow landslides. The framework accounts for antecedent conditions of soil and vegetation cover, wildfire‐induced alterations in transpiration and root reinforcement, and time‐varying infiltration rates. For the cases examined, it was shown that wildfire could reduce the factor of safety by 25%. The efficacy of the proposed framework is illustrated through modeling a case study in the Las Lomas watershed in California, USA, which experienced several landslides during a heavy rainstorm in January 2019, 3 years after the Fish Fire burned the area. The proposed model uses measurable hillslope and wildfire characteristics and can be employed to evaluate the risk of shallow landslides in wildfire‐prone areas. Key Points: Increased activity and altitudes of wildfires have elevated the likelihood of post‐wildfire ground failuresA new physics‐based analytical framework is formulated to model the temporal evolution of hillslope stability following wildfiresWildfires can significantly lower the hillslope stability to rainfall‐triggered shallow landslides [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering Geology / Gongcheng Dizhi Xuebao is the property of Journal of Engineering Geology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Featured Application: Thermo-hydraulic modelling of engineering geomaterials consistently with vaporisation. The aim of this study is to assess the consistency in the modelling of thermo-hydraulic problems in clay-based engineered barriers. This study focuses on two aspects: the modelling of vapour pressure as a function of temperature, and the specific heat capacities of liquid water and water vapour in relation to the enthalpy of vaporisation and the internal energy of liquid water and water vapour. Regarding the first aspect, several formulations of the saturated vapour pressure have been inspected, evaluating their accuracy and information provided in the temperature range from 0 to 150 °C. Regarding the second aspect, the enthalpy of vaporisation and the internal energy of water were used to assess the consistency of pairs of specific heat capacity values in the same temperature range. Values from the literature were also inspected. An accurate and simple enough expression for the saturated water vapour pressure with temperature has been identified as the optimal option for modelling. Recommendations on specific heat capacity constant values for liquid water and vapour are suggested to maximise consistency in the studied temperature range. However, the loss of accuracy in the enthalpy or internal energy of vaporisation associated with the inspected specific heat capacity pairs is limited. [ABSTRACT FROM AUTHOR]

Most earth structures, such as embankments, roadways, foundations, slopes, and retaining walls, are commonly built above the groundwater table whenever possible, implying that the soil used in the construction of such structures is widely unsaturated in nature. The behavior of unsaturated soil is less predictable as compared to saturated soil. Usually, triaxial tests are carried out on unsaturated soils at various net axial stress and matric suction under constant confining pressure. There is limited research regarding the constant volume tests on unsaturated soil. In this study, a series of triaxial compression tests were performed under constant water content conditions using a double-cell triaxial test apparatus. The samples were prepared with an 80% degree of compaction and an optimum water content of 20%. All the soil specimens were consolidated isotropically under three different confining pressures of 100kPa, 300kPa, and 500kPa before being sheared with constant volume. The soil specimen's volume was controlled by manually altering the cell pressure during the shearing phase (depending on the volume change measurement). A pre-shear infiltration test was also carried out to understand the influence of matric suction in unsaturated soil when sheared with constant volume. The results show that the normally consolidated specimen exhibits anomalous behavior of sudden temporary drop in axial stress within the axial strain range of 0-2%, which can be attributed to the collapse of soil particles held together by suction. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Considering the flow‐independent viscosity of soil skeleton and the lateral inertia effect of pipe pile, the vertical dynamic behavior of a pipe pile embedded in unsaturated soil is investigated in this work when the pipe pile is subjected to the vertical dynamic loading. In order to model this problem, the fractional‐order standard linear solid (FSLS) model is introduced to characterize the flow‐independent viscosity of the soil skeleton, the Rayleigh‐Love rod model is employed to specify the lateral inertia effect of pipe pile, while both vertical and radial soil displacement are entirely considered. The analytical solution of the pipe pile complex impedance is obtained through the rigorous theoretical derivation. Numerical results with known analytical solution are finally presented to discuss the influence of the model parameters for the FSLS model, lateral inertia effect and geometrical characteristic for the pipe pile, and saturation degree and intrinsic permeability for the unsaturated soil on the vertical complex impedance of the pipe pile. The main results indicate that enlarging the fractional‐order index, raising the strain relaxation time, or shortening the stress relaxation time will reduce the oscillation amplitude and natural frequency for the dynamic stiffness and damping of the pipe pile but will improve their mean value. The lateral inertia effect of pipe pile has effect on its complex impedance only in the high‐frequency region. The outer and inner radii of pipe pile, pile length, soil saturation degree, and soil intrinsic permeability are highly correlated with the pipe pile complex impedance. [ABSTRACT FROM AUTHOR]

The soil-water characteristic curve (SWCC) is an essential tool to determine hydraulic and mechanical properties of unsaturated soils. As an inherent influencing factor, soil texture controls the characteristics of SWCCs. Fractal theory can quantitatively describe the physical characteristics of soil. This study used particle size distribution data and water content data contained in the UNSODA2.0 database to explore the fractal characteristics of 12 soil types with different textures under different matrix suctions. The SWCC fractal model was adopted to characterize the hydraulic properties of soil with various soil textures. The findings revealed that the mass fractal dimensions of particles from these 12 different soil types significantly differed and were closely related to the clay content. Fractal dimension increased with increasing clay content. The fractal dimension established a good relationship between soil structure and hydraulic properties. Fractal analysis can be used to determine the connection between physical properties and soil hydraulic parameters. The estimated results of the SWCC fractal model indicated that it had a good performance regarding the description of SWCCs for the 12 soil textures. The soil structure could be described through fractal dimensions, which can effectively indicate soil hydraulic characteristics. The estimated fractal dimension of this model could be obtained by particle size distribution. Furthermore, using the SWCC fractal model, we found that the SWCC of coarse textured soil changed sharply in the low suction stage and its residual water content was small, and the SWCC of fine textured soil changed gently with a large residual water content. The water retention capacity followed the order clay > silty clay > sandy clay > clay loam > silty clay loam > sandy clay loam > loam > silt loam > sandy loam > silt > loamy sand > sand. [ABSTRACT FROM AUTHOR]

The plate load test (PLT) is the most reliable in situ testing for studying the load–settlement behaviour of footings on unsaturated collapsible soils. In these soils, the suction profile is not constant along the depth, and the scale effect between the prototype and footing leads to different suction averages and, consequently, different data. One method to eliminate the effect of soil suction on the test data is to fully saturate the soil prior to the test, which is also recommended at the design process for footing on collapsible soils. However, the inundation process on PLTs is expensive and time-consuming, which makes this procedure difficult to incorporate into engineering practice. This study presents a device that can be attached to flat dilatometer (DMT) to allow local inundation of the soil as part of the in situ test campaign and obtain the DMT-constrained modulus (MDMT) for both natural and inundated conditions. The MDMT presented an average reduction of 56% from natural to inundated condition. This parameter can be used in a model to predict load–settlement curves by DMT data considering the suction influence on this behaviour. The curves obtained from the prediction model were compared to curves determined by PLT conducted under the same in situ conditions. Good agreement was found between the curves predicted by DMT and those measured by PLT for both conditions. The proposed procedure, which uses a device attached to the DMT blade, provides an investigation method to obtain the load–settlement curve under different suction conditions, which can help in the selection and performance prediction of shallow foundations, taking into account suction and collapse phenomenon-related problems. [ABSTRACT FROM AUTHOR]

Based on the theory of single-phase elastic medium and unsaturated porous medium, the vibration isolation effect of composite multilayer wave impeding block (WIB) in the unsaturated ground under an underground dynamic load is investigated. The results show that the best vibration isolation effect can be obtained by designing the wave impedance ratio between the composite multilayer WIB and unsaturated ground. The composite multilayer WIB improves the vibration-damping bandwidth compared with the homogeneous WIB. The vibration isolation effect is better the closer the embedded depth of the composite multilayer WIB is to the vibration source, and its vibration isolation effect increases significantly with the increase of thickness, but when the thickness of the WIB exceeds a certain critical thickness, its vibration isolation effect decreases with the increase of thickness. Soil saturation has a significant effect on the vibration isolation effect of composite multilayer WIB in the unsaturated ground, and the composite multilayer WIB can achieve a better vibration isolation effect at low saturation. [ABSTRACT FROM AUTHOR]

This study investigates the bearing capacity of strip footings resting on unsaturated soils subjected to combined loading using the lower-bound limit analysis coupled with the finite element discretization approach. In this regard, second-order cone programming (SOCP) is exploited to simulate the nonlinear form of the universal Mohr-Coulomb yield criterion during the process of stress field optimization. The significant influence of matric suction induced below the surface footing was accounted for by adopting the suction stress concept under the no-flow and steady-state infiltration/evaporation flow conditions. The eccentricity and inclination of the foundation loading are introduced into the equilibrium equations along the strip footing so as to render various combinations of moment (M), vertical (V) and shear (H) forces. The results stemming from the lower-bound finite element limit analysis are compared with several previous studies throughout the literature for verification of the model. The substantial contribution of suction stress to the evolution of failure loci and the distribution of subsurface stresses for the shallow foundation subjected to inclined and eccentric loadings is thoroughly discussed. A general three-dimensional failure envelope is presented for shallow foundations resting on partially saturated soils under combined vertical, horizontal and moment loadings. [ABSTRACT FROM AUTHOR]

This study presents a series of triaxial tests to highlight the effect of saturation degree ranging from 79.8% to 100% on the behavior of sand subjected to cyclic loading, the development of pore water pressure increment, the volumetric strain at liquefaction state, and the behavior of sand subjected to monotonic loading after liquefaction. The tested sand was Hostun RF sand, fine clear sand that has been widely used in laboratory testing in France. The results show that there is a difference between the behavior of samples having saturation degrees higher and lower than 95%. Besides, the development of pore water pressure increment is affected significantly by the value of cyclic stress ratio (CSR) and saturation degree. The Saturation degree also affects the appearance of criteria of liquefaction as Excess Pore-water Pressure (EPP) and Double Amplitude of axial strain (DA). The analysis of the effect of saturation degree on sample volumetric strain exhibits nonlinear liquefaction criteria for a given maximum value of CSR. The monotonic loading tests after liquefaction showed a small difference between the monotonic compressed behavior of the samples before and after liquefaction. [ABSTRACT FROM AUTHOR]

Electro-osmotic consolidation is taken as a valid and practical approach for soft ground improvement, and it is of vital significance to study the consolidation behaviour of electro-osmotic consolidation for the application purpose. This paper presents an analytical solution for two-dimensional (2D) electro-osmosis-enhanced preloading consolidation of unsaturated soil. The governing equations are first expanded using the eigenfunction expansion method in the y-direction, and then, the auxiliary function is constructed to rearrange the boundary conditions for mathematical convenience. The coupled partial differential equations are subsequently transformed into a system of ordinary differential equations (ODEs) containing only time t by adopting the eigenfunction expansion method in the x-direction. The exact solution is finally found by using the Laplace transform techniques to solve the ODEs system. Working example conducts a series of numerical simulations to validate the newly proposed solutions, and verification results demonstrate excellent agreement. Using the proposed solutions, this paper conducts parametric analyses to study the influences of coupled electro-osmosis and preloading on the consolidation behaviour. [ABSTRACT FROM AUTHOR]

Soils above the groundwater table and near the ground surface are often partially saturated, and the changes in the degree of saturation and matric suction caused by infiltration may have substantial effects on the stability of geotechnical systems. This work evaluates the significant effect of transient flow on the ultimate bearing capacity of strip footings founded on unsaturated soils under the simultaneous actions of vertical and horizontal loads and moment using the lower-bound theorems of the finite-element limit analysis and the incorporation of second-order cone programming. The unified effective stress theory is incorporated into the soil yield function to model the partially saturated state, and the closed-form solution for one-dimensional transient flow is adopted so as to evaluate the tempo-spatial variations of suction stress. For both sand and clay underlying soils, it is found that the increase of infiltration time leads to a notable decrease of the size of the normalized failure envelopes for either obliquely or eccentrically loaded shallow foundations. The impact of transient flow on the ultimate bearing capacity of surface footings is also observed to be greater for clay soils as compared with sand deposits due to the significant contribution of higher induced suction stress. [ABSTRACT FROM AUTHOR]

Rainfall infiltration can alter the pore-water pressure and decrease matric suction, which can lead to slope instability and landslides. Most of the previous numerical studies conducted on rain infiltration have assumed a two-dimensional (2D) failure mechanism, which neglects the three-dimensional (3D) characteristics of slopes. An analytical assessment framework was developed using the upper-bound theorem of limit analysis to determine the factor of safety and failure pattern in the case of a 3D vertical cut slope under diverse rainfall situations combined with the unsaturated soil mechanics. The Green–Ampt model was adopted to depict the infiltration process and the progression of the wetting front into the soil. The slope factor of safety was determined using the energy balance equation by adopting the strength reduction approach. The validity of the framework was verified using the numerical simulation method. The effects of duration, intensity, and patterns of rainfall and the 3D traits of slope on the slope failure pattern and stability were determined. A set of design charts was included for the preliminary design. The present study quantitatively determines the stability of slopes susceptible to rainfall infiltration. [ABSTRACT FROM AUTHOR]

This paper discusses the second-order reliability-based design optimization (SORBDO) of unsaturated finite slopes. An efficient search algorithm is proposed to locate the most probable slip surfaces using the Morgenstern–Price method of slices. The variability of fitting parameters of the soil water characteristic curve (SWCC) and shear strength parameters have been taken into account for evaluating the reliability of unsaturated finite slopes. Six unsaturated shear strength (USS) models are considered for the analysis of unsaturated soils. The most conservative model for the optimum design of finite slopes is recommended. Moreover, the results indicate that the mean and standard deviation of the fitting parameters of SWCC have a significant effect on the critical centers, critical slip surfaces, and reliability index of the unsaturated finite slopes. The results demonstrate a significant change in the critical centers associated with critical slip surfaces due to the change in the mean and coefficient of variation of the fitting parameters of SWCC. Statistical analysis of the experimental data shows that a positive correlation exists between fitting parameters of SWCC related to air entry value (af) and residual water content (mf). Negative correlations are found between the fitting parameters of SWCC related to air entry value (af) and slope (nf). Similarly, the SWCC fitting parameters related to residual water content (mf) and slope (nf) have negative correlations for clayey soils. It is observed that the correlated value of the reliability index is overestimated with reference to the uncorrelated value for clayey soil slopes. The proposed design charts are beneficial for engineers and practitioners for the sustainable design of unsaturated finite slopes. Overall, the developed rigorous SORBDO framework offers a more rational approach for assessing critical failure surfaces and critical centers, considering the interdependency of random variables. [ABSTRACT FROM AUTHOR]

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Ignoring the role of soil suction and implementing unsaturated soil mechanics when assessing slope stability in Indonesia is a common practice. One of the main reasons is due to the precognition that incorporating soil suction means using higher shear strength, which leads to less conservative analysis, while it is difficult to practically obtain accurate input parameters for unsaturated soil analysis. However, a number of slope failures occur all over the world due to rainfall, which becomes evidence that ignoring the role of soil suction may not necessarily lead to a conservative result. On 9 January 2021, rainfall-induced slope failure occurred at Cimanggung after four hours of heavy rainfall and killed 32 people. Many of them were injured, and houses were destroyed. This event shows the significance of considering the interaction between infiltration and soil suction when conducting slope stability analysis. Difficulties in obtaining input parameters for unsaturated soil analysis experimentally hindered practitioners in applying unsaturated soil mechanics. While the parameters can be estimated, it is always of question whether the estimated parameters are sufficiently accurate for practical purposes. In this paper, conventional site investigations were carried out while unsaturated soil parameters were estimated to study the mechanism which triggers the landslide that occurred at Cimanggung. It will be shown that estimating unsaturated soil parameters can be practically accurate and manage to capture the failure mechanism such as critical rainfall duration and critical slip surface. [ABSTRACT FROM AUTHOR]

The pseudo-static method was adopted to expand the application field of classical Rankine soil pressure theory to the circumstance of infinite inclined backfill acting on the retaining wall under earthquakes. The calculation formula of the active earth pressure for unsaturated soil was deduced by combining the shear strength model of unsaturated soil and the unified strength theory. The effects of matric suction, matric-suction-related friction angle and seismic coefficients on the intensity distribution of Rankine active earth pressure were evaluated, and the resultant action position was discussed. This research shows that rising seismic coefficients lead to a growth of the active earth pressure and an upward movement of the resultant action point; on the other hand, an increasing matric suction and its related friction angle causes a reduction of the active earth pressure and a downward movement of the resultant action. Examples indicate that the suggested formulas in this paper are in good agreement with those of the previous literature subjected to certain conditions. However, compared to the classic Rankine theory, the proposed method extends its application range by considering earthquake, unsaturated soil, inclined rough wall back, non-horizontal backfill surface, and the intermediate principal stress. [ABSTRACT FROM AUTHOR]

In general, the ultimate bearing capacity (UBC) of shallow foundations on unsaturated soils is characterized by the conventional shear strength (SS) parameters in which saturated theories are applied. However, in this case, it is clear that the foundations designed using the obtained values from the saturated cases not be economical. In recent years, procedures have been developed to estimate the UBC of foundations on unsaturated soils, that take into account drained and undrained loading conditions. However, these studies generally concentrate on sandy soils. The validity of the results proposed in the literature should be tested for other soils. Therefore, this paper includes a conventional direct shear box (DSB) test to determine the unsaturated SS of statically compacted silty soil, and a series of model tests were performed to determine the foundation's UBC. In the experimental model setup, the UBC values of different types and sizes of model foundations on silty soil layers with a different soil saturation degrees (SSDs)/matric suctions (MSs) and different void ratio values were measured. In addition, the soil-water characteristic curves (SWCCs) and SS parameters of unsaturated silt were obtained. Using the experimental results, a new equation is proposed for the characterization of the UBC of shallow foundations on unsaturated silty soils. Using this equation, the UBC of unsaturated soils can be determined based on the results of unconfined compressive strength tests (UC) measured on unsaturated soil samples and based on the degree of saturation and the fitting parameter. The results indicate that the measured bearing capacity values obtained via the model footing test, shows a good consistency with those obtained by the proposed equation. [ABSTRACT FROM AUTHOR]

To understand the behavior of unsaturated soil, considerable research has already been done. However, there is limited research to model the behavior of unsaturated soil in a generalized state boundary surface (SBS). SBS is the surface outside which a soil stress state can never exist. It is an envelope surface of all the possible soil stress paths in the p'-q-e space. To investigate the SBS of unsaturated soils, a series of triaxial compression tests were performed in this study under constant water content conditions using a doublecell triaxial test apparatus. The samples were prepared with 80%, 83%, and 86% degrees of compaction and water contents of 20% (Optimum water content) and 25%. All the soil specimens were consolidated isotropically under 500kPa confining pressure before being sheared with a constant void ratio under monotonic loading. The results show that constant void ratio tests depict contour-like lines, which can help in defining the SBS of unsaturated soil. [ABSTRACT FROM AUTHOR]

The soil–water characteristic curve is an important tool to evaluate the water-holding capacity of unsaturated soil. Plant roots can affect the matric suction of soil and the water-holding capacity and permeability of the soil. Therefore, the morphological characteristics of plant roots will lead to the difference in soil–water characteristics between soil slope and plant-covered slope. This study aims to investigate the effect of Vetiver root morphology on soil–water characteristic curves of plant-covered slope soil. The hydrological effect of the root distribution on the root–soil system was also discussed. The results showed that: (1) The root surface area index (RAI) and root volume ratio (Rv) of each soil section of the vetiver root system varied with depth in accordance with the Gaussian function distribution; (2) In the process of natural drying, the matric suction generated within the root system is significantly higher than that generated by evaporation of bare soil in the same soil layer. The ability of vegetation soil to enhance soil matrix suction increases with the increase of soil root surface area index; and (3) The α and n values of the SWCC model decreased with the increase of Rv (root volume ratio of soil), while the air entry value increased. Under the same water content, the matric suction corresponding to vegetation soil is significantly greater than bare soil. In addition, the soil–water characteristic curve can be effectively predicted by combining the Rv of vegetated soils. [ABSTRACT FROM AUTHOR]

In the current state of the study regarding passive earth pressure, the majority of literature was limited in scope to dry or saturated cases. However, most soils in practical engineering are unsaturated. Based on the log-spiral failure mechanism, this paper develops a limit equilibrium (LE) analysis to evaluate the passive earth pressure under the effect of steady unsaturated seepage conditions. For assessing the effect of suction stress on passive earth pressure, a suction stress-based effective stress representation is incorporated into a limit equilibrium analysis. The analytical formula of passive earth pressure coefficient is derived using the moment equilibrium equation for the backfill defined by a log-spiral slip surface. An optimization procedure is developed to search for the minimum passive earth pressure coefficient and the critical failure surface via MATLAB. A parametric analysis is performed to investigate the effects of some key parameters on passive thrust. Besides, examples of two different cases are given to verify the effectiveness of theoretical results. Results indicate that suction stress can increase the magnitude of the passive earth pressure coefficient and the suction-induced effect highly depends on the seepage conditions and soil types. [ABSTRACT FROM AUTHOR]

A simplified analytical method is proposed for one-dimensional unsaturated soil consolidation theory. First, in the one-dimensional compression test of unsaturated soil, the effective stress principle of saturated soil under compression is introduced to replace the constitutive relation expressed by the state of double stress in Fredlund's theory. Based on this, the governing equations composed of two bi-variable partial differential equations are obtained. In the process of derivation, the dissipation law of pore pressure during the consolidation process is analyzed and the theoretical rationality is demonstrated. When solving the system of equations, considering that the initial excess pore water and the gas pressure are caused by instantaneous loading, Hilf's theory is improved to calculate the change of pore pressure. This improvement is more in line with the actual situation and simplifies the approximate calculation method to obtain the analytical solution when the coupling effect of water and gas is considered. This method is verified by comparing with Terzaghi's theory and Fredlund's theory. It is shown that the solution of Terzaghi's theory is a special case of this method for saturated soil, and the results of the dissipative process obtained with the method is close to Fredlund's theory. [ABSTRACT FROM AUTHOR]

Under the action of Rayleigh waves, pile head is easy to rotate with a concrete pile cap, and pure fixed‐head condition is rarely achieved, which is a common phenomenon for it usually occurs on the precast piles with insufficient anchorage. In addition, the propagation characteristics of Rayleigh wave have been changed significantly due to the existence of capillary pressure and the coupling between phases in unsaturated soil, which significantly affects the pile‐soil interaction. In order to study the above problems, a coupled vibration model of unsaturated soil–pile system subjected to Rayleigh waves is established on the basis that the pile cap is equivalent to a rigid mass block. Meanwhile, the soil constitution is simplified to linear‐elastic and small deformations are assumed to occur during the vibration phase of soil–pile system. Then, the horizontal dynamic response of a homogeneous free‐field unsaturated soil caused by propagating Rayleigh waves is obtained by using operator decomposition theory and variable separation method. The dynamic equilibrium equation of a pile is established by using the dynamic Winkler model and the Timoshenko beam theory, and the analytical solutions of the horizontal displacement, rotation angle, bending moment and shear force of pile body are derived according to the boundary conditions of flexible constraint of pile top. Based on the present solutions, the rationality of the proposed model is verified by comparing with the previous research results. Through parametric study, the influence of rotational stiffness and yield bending moment of pile top on the horizontal dynamic characteristics of Rayleigh waves induced pile is investigated in detailed. The analysis results can be utilized for the seismic design of pile foundation under Rayleigh waves. [ABSTRACT FROM AUTHOR]

Purpose: In the ground vibration control, most of the previous studies focused on the vibration isolation performance of the wave impeding block (WIB) in elastic foundation or saturated soil foundation, while there are few reports on the vibration isolation performance of the WIB in unsaturated soil foundation which is more universal in nature. Therefore, we study the propagation characteristics of S-wave passing through an elastic wave impeding block in the unsaturated soil. Methods: Based on the propagation theory of elastic waves in unsaturated porous medium and single-phase elastic medium, considering that an elastic wave impeding block with a certain thickness is set in the unsaturated soil foundation, and using the potential function and continuous boundary conditions, the analytical solution of the reflected/transmitted amplitude ratio of S-wave passing through the WIB in the unsaturated soil are derived in this paper. The variation of the reflected/transmitted amplitude ratio with the shear modulus and density of the WIB, incident angle and incident frequency is investigated by parametric studies. Conclusion: The results showed that incident angle, the shear modulus and density of the WIB have a significant influence on the vibration isolation performance of the WIB in the unsaturated soil foundation. It is verified that the better vibration isolation performance can be obtained by reasonably controlling the shear modulus and density of the WIB. [ABSTRACT FROM AUTHOR]

The shear strength of soil is a key input parameter required for the satisfactory geotechnical design of any structure. The existence of most residual soils in unsaturated conditions highlights the importance of studying the shear strength of unsaturated soils. In this study, a modified triaxial apparatus was used to examine the mechanical behavior of 5 types of silica sands with mean grain sizes varying from 0.15 to 1.5 mm. The experimental test results of soil shear strength determination were compared with the shear strength estimated through analytical methods. To study the mechanical behavior of test soils at optimal and residual moisture contents, the test samples were initially prepared at the optimal moisture content. To study the effect of the initial moisture content on the mechanical behavior of test soils at the residual moisture content, the test samples were initially prepared at the optimal and residual moisture contents. Residual moisture content samples exhibited higher shear strength compared to optimal moisture content samples. Furthermore, the residual moisture content samples prepared at the optimal moisture content exhibited stronger mechanical behavior compared to the sample prepared at the residual moisture content. The analytical methods employed in this study were not able to accurately predict the shear strength of test soils at residual moisture content. [ABSTRACT FROM AUTHOR]

Firstly, in this paper, based on the theory of the porous elastic medium and combined with the effective stress principle of unsaturated soil, a set of governing equations is established to describe consolidation of the unsaturated soil. Secondly, an analytical expression under any dynamic loads is obtained with the help of Laplace integral transformation. Finally, analysis of numerical examples under specific boundary conditions is made to discuss one--dimensional consolidation characteristics under harmonic loads and the influence of factors on the consolidation characteristics of unsaturated soil, such as excitation frequency and initial saturation. [ABSTRACT FROM AUTHOR]

In unsaturated soil mechanics, the soil–water retention curve (SWRC) continues to play an important role, since it provides the necessary links between the properties and behaviour of unsaturated soils with a variety of engineering challenges. The temperature has been identified as the main factor influencing SWRC as compared to a variety of other parameters. The goal of this research is to describe theoretical and experimental aspects of the temperature effect on unsaturated soil water retention phenomena. Theoretically, a brief review of the constitutive laws governing the thermal-hydro-mechanical (THM) behaviour of unsaturated soils is presented, along with links between variations in suction with water content, temperature, and void ratio. It also provides a broad framework that would to be well adapted to describing many specific circumstances. Through a closed-form predictive relationship that is developed in this framework, the effect of temperature is examined. By using this relationship, the soil–water retention curve at arbitrary temperature could be determined from one at a reference temperature, therefore significantly decreasing the number of tests necessary to describe the thermo-hydro-mechanical behaviour of a soil. Besides, the SWRC of kaolinite clay was also measured at three different temperatures in an experimental program. The test findings reveal that when the temperature rises, the SWRC decreases significantly. The experimental results were then integrated with sixteen other available data sets covering a wide range of soil types, densities, and suction to create a complete verification program for analytical models. The proposed model has a good performance and reliability in forecasting the fluctuation of non-isothermal SWRC than any existing model, according to statistical assessment results. The analytical model can be used to examine the thermo-hydro-mechanical characteristics of unsaturated soils in numerical simulations.

Vegetation has been commonly used in sponge city to remediate problems related to rainstorm events. Unlike uniform rainfall which has been widely studied, effects of early-peak rainfall on hydrological responses in vegetated soils are unclear. Besides, there is a lack of quantitative method of accurately measuring wetting front (WF). This study aims to propose a new WF tracing method, and explore the hydrological responses to early-peak rainfall in unsaturated soils vegetated with dwarf mondo grass. During soil column tests, WF position, matric suction, volumetric water content, surface ponding and overflow drainage were measured. The new WF tracing method works reasonably well for all cases. As compared to uniform rainfalls, early-peak rainfalls caused (1) earlier onsets of ponding (by 20 minutes for vegetation case and by 5 minutes for bare soil) and overflow (by 52 minutes for vegetation case and by 37 minutes for bare soil), (2) greater overflow velocity (by 28% for vegetation case and by 41% for bare soil), and (3) slightly more total overflow amount. Vegetation delayed the ponding/overflow generations, and decreased total overflow drainage, due to enhanced infiltration of surface soil. At 5 cm depth, high-density mixture of fine and coarse roots caused an increase in the saturated water content (θs) and a reduction in the residual water content (θr), because of root-induced changes in soil structure. At 10 cm depth, low-density fine roots caused reductions in both θs and θr, and increased air-entry value, as roots occupy the pores.

Abstract Wildfire records demonstrate worsening patterns coupled with the spread to higher altitudes in several regions, raising the risk of post‐wildfire ground failures. This study investigates the post‐wildfire stability of unsaturated hillslopes against rainfall‐triggered shallow landslides. We developed a new physics‐based analytical framework incorporating wildfire‐induced changes in soil properties and near‐surface processes affecting the hillslope stability. A coupled hydromechanical infiltration model is integrated into an infinite slope stability analysis to simulate temporal changes in the depth profiles of soil water content, pressure head, and the resulting factor of safety (F.S.) of a vegetated slope. We consider the antecedent conditions of soil and vegetation cover, including the recovery phase after the fire, wildfire‐induced alterations in transpiration, and time‐varying infiltration rates. The model is verified against numerical simulations and employed in parametric studies evaluating the effects of wildfire severity and rainfall intensity‐duration. For the cases examined, it was shown that wildfire could reduce the F.S. of slopes by 25%. As a case study, the model successfully captured shallow rainfall‐triggered landslides that occurred in the Las Lomas watershed in California, USA, in 2019, 3 years after the Fish Fire burned the area. The proposed model uses measurable hillslope and wildfire characteristics and can be employed to evaluate the risk of shallow landslides in wildfire‐prone areas.

In current study, a test program is designed to observe the influence of wetting stress paths on behavior and instability of unsaturated soil in stress state space. The mechanical behavior was studied by performing three test series in constant matric suction plane. In first series, water infiltration was performed together with shearing starting from zero (0 kPa) deviatoric stress. In second series, first water was infiltrated at zero (0 kPa) deviatoric stress then samples were sheared in drained pore water pressure conditions. In third series, water was first infiltrated at zero deviatoric stress then specimens were sheared in undrained pore water pressure conditions. Instability was studied in constant shear stress plane by first shearing soil up to predefined level of deviatoric stress then infiltrating water keeping deviatoric stress constant on the specimen. This research also describes how unsaturated soil behavior and instability is varied with variation in net confining stresses when suction is decreased from initial measured value to start water infiltration with different wetting stress paths and their effect on shear state and failure surface in stress state space. The test results revealed that the movement of stress paths was independent of net confining stress, matric suction and shearing conditions employed in the study. [ABSTRACT FROM AUTHOR]

The Richards' equation describes the flow phenomenon in unsaturated porous media and is essential to hydrology and environmental science. This study evaluated the numerical stability of two different forms of the Richards' equation. Sensitivity analyses were performed to investigate the control parameters of the equation. The results show that the h-form Richards' equation has better applicability for calculating variable saturation flows than the θ-form Richards' equation. For the h-form Richards' equation, the hydraulic conductivity of the soil in the low-suction range and the specific moisture capacity in the high-suction range primarily influenced the solution. In addition, sensitivity analyses indicated that the saturated hydraulic conductivity, initial condition, and air-entry pressure have a higher sensitivity to the simulation results than the saturated water content, rainfall intensity, and decline rate of hydraulic conductivity. Moreover, their correctness needs to be guaranteed first in numerical simulations. The research findings can provide a helpful reference for improving the reliability of numerical simulations of unsaturated flows. [ABSTRACT FROM AUTHOR]