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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.

Damage caused by frost heave leads to costly maintenance in cold regions, like Hokkaido, Japan. Therefore, the study of the frost mechanism with experimental and numerical methods has been of great interest. Numerous models have been developed to describe the freezing process of saturated soil, which differs from the partially saturated conditions in the field. In fact, most subsurface soils are unsaturated. The freezing process of partially saturated soils is more complex than saturated soils, as the governing equations show strongly nonlinear characteristics. This study proposes a thermo-hydro-mechanical coupled model considering the heat transfer, water infiltration, and deformation of partially saturated soil to reproduce the freezing process of partially saturated frost susceptible soils distributed in Hokkaido. This model better considers the water-ice phase change and soil freezing characteristic curve (SFCC) during freezing under field conditions. The results from the multiphysics simulations agree well with the frost heave and water migration data from frost heave tests of Touryo soil and Fujinomori soil. In addition, this study discussed the influence of the various factors on frost heave amount, including temperature gradients, overburden pressures, water supply conditions, cooling rates, and initial saturation. The simulation results indicate that the frost heave ratio is proportional to the initial degree of saturation and is inversely proportional to the cooling rate and overburden pressure. Moreover, simulation under the open system generates much more frost heave than under the closed system. Finally, the main features of the proposed model are revealed by simulating a closed-system frost heave test. The simulation results indicate that the proposed model adequately captures the coupling characteristics of water and ice redistribution, temperature development, hydraulic conductivity, and suction in the freezing process. Together with the decreased hydraulic conductivity, the increased suction controls the water flow in the freezing zone. The inflow water driven by cryogenic suction gradient feeds the ice formation, leads to a rapid increase in total water content, expanding the voids that exceed the initial porosity and contributing to the frost heave.

This study aimed to evaluate the influence of the addition of coconut (coir) fibers on the microstructural, hydraulic and mechanical behavior of an unsaturated compacted soil. Specimens were molded and compacted, forming composites with 0%, 0.1%, 0.5% and 1% fiber in relation to their dry mass. The characterization of pores from the soil and fiber soil mixtures was performed by the Mercury Intrusion Porosimetry tests. Suction values were obtained through the filter paper method and soil water retention curves were adjusted with the Durner model due to the bimodal behavior. Tensile strength values were obtained from the indirect tensile strength test (Brazilian tensile test) for specimens with different suction values. It was found that the increase in fiber content in the material lead to a non-linear increase in macropores, which affected both the hydraulic and mechanical behavior of the soil. Furthermore, the shape of the soil water retention curve was preserved, but there were changes in the values of first and second air entry and residual suction. The tensile strength was negatively influenced, reaching a reduction of about 30% in the situation with higher fiber content. However, for higher levels, the behavior of the soil changed from brittle to ductile, increasing the supported deformations.

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.

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.

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.

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.

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., El objetivo de este estudio es evaluar la coherencia de la modelización de los problemas termohidráulicos en las barreras de arcilla. en barreras de ingeniería a base de arcilla. Este estudio se centra en dos aspectos: la modelización de presión de vapor en función de la temperatura, y las capacidades caloríficas específicas del agua líquida y del vapor de agua en relación con la entalpía del vapor de agua. vapor de agua en relación con la entalpía de vaporización y la energía interna del agua líquida y del vapor de agua. vapor de agua. En cuanto al primer aspecto, se han examinado varias formulaciones de la presión de vapor saturado inspeccionado, evaluando su precisión y la información proporcionada en el intervalo de temperaturas de 0 a 150 ◦C. En cuanto al segundo aspecto, la entalpía de vaporización y la energía interna del agua se utilizaron para evaluar la coherencia de los pares de valores de capacidad calorífica específica en el mismo rango de temperaturas. temperatura. También se inspeccionaron los valores de la bibliografía. Una expresión suficientemente precisa y sencilla para la presión de vapor de agua saturada con la temperatura se ha identificado como la opción óptima para la modelización. Se proponen recomendaciones sobre los valores constantes de la capacidad calorífica específica para el agua líquida y el vapor para maximizar la coherencia en el intervalo de temperaturas estudiado. Sin embargo, la pérdida Sin embargo, la pérdida de precisión en la entalpía o la energía interna de vaporización asociada a los pares de capacidad calorífica específica inspeccionados es limitada. inspeccionados es limitada.

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.

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. Published version This research was supported by the Nazarbayev University Research Fund under Social Policy Grant and Faculty Development Competitive Research Grants Program (FDCRGP) Grant No. 20122022FD4133.

Polynomial chaos expansion (PCE) is widely adopted in geotechnical engineering as a surrogate model for probabilistic analysis. However, the traditional low-order PCE may be unfeasible for unsaturated transient-state models due to the high nonlinearity. In this study, a temporal-spatial surrogate model of adaptive sparse polynomial chaos expansions (AS-PCE) is established based on hyperbolic truncation with stepwise regression as surrogate models to improve computational efficiency. The uncertainty of pore water pressure of an unsaturated slope under transient-state rainfall infiltration considering hydraulic spatial variability is studied. The saturated coefficient of permeability [Formula: see text] is chosen to be spatial variability to account for the soil hydraulic uncertainty. The effects of location and time and the performances of AS-PCE are investigated. As rainfall goes on, the range of the pore pressure head becomes larger and the spatial variability of [Formula: see text] has little influence in the unsaturated zone with high matric suction. The pore pressure head under the water table suffers more uncertainty than it in the unsaturated zone. The [Formula: see text] in the high matric suction zone has a trend of rising first and then falling. Except for the high matric suction zone, the [Formula: see text] rise over time and they are almost 1 at the end of the time. It can be concluded that the AS-PCE performs better for low matric suction and positive pore pressure head and the fitting effect gradually increases as the rainfall progresses. The quartiles and at least up to second statistical moments can be characterized by the AS-PCE for transient infiltration in unsaturated soil slopes under rainfall.

Slope instability triggered by rainfall is common in tropical countries like India, where the soil of hill slopes are generally in unsaturated condition, having higher strength than in the saturated condition due to matric suction. This study aims to investigate the change in pore water pressure (PWP) with rainfall infiltration into the slope and the corresponding effect of the slope stability on different hill slopes from different regions of India. By applying rainfall on the surface of slope as a flux boundary condition, a transient seepage analysis is performed using SEEP/W, and the corresponding results in the form of PWP distribution within the slope are used as input for SLOPE/W program of Geostudio 2018 R2, to check the stability of slope. This study involves the use of CMIP5 (Coupled model intercomparision project phase 5) simulations for rainfall data of different regions for the duration of 2015–2050, 2051–2075 and 2076–2100; which is completely based on non-stationary approach (i.e., the statistics of extremes changes with time) rather than the conventional methods of extracting rainfall data that are based on stationary approach. The stability of different slopes is checked for 95 percentile rainfall intensity as well as for maximum rainfall intensity for a period of 2015–2100, and the analysis is performed for 5 days for which the duration of rainfall is considered to be 24 h. It is concluded that due to climate change, slopes which may be marginally stable under historical and present rainfall events can fail under projected precipitation estimates, and this can be understood by the results of Haflong slope stability analysis as the 30° slope, which is marginally stable under maximum rainfall intensity for the period 2015–2050 having factor of safety (FOS) value 1.0013, will fail under the maximum rainfall intensity for the period 2051–2075 and 2076–2100 having FOS value 0.9935 and 0.9989, respectively. A continuous decrease in FOS with increasing rainfall intensity is observed for all the study areas under consideration.

Since unsaturated soil conditions are normally experienced above the groundwater table, most treated or stabilised soils for roadworks, earth dams’ embankments, landfill sites, hydraulic barriers etc. could be regarded as existing in this region. The soil–water characteristic (or retention) curve (SWCC) is a useful conceptual tool by which an evaluation of unsaturated soil's property functions and corresponding macro-scale behaviour (strength, volume change, hydraulic conductivity, fluid flow, diffusivity, etc.) can be carried out. Hence, an examination of some of the various factors that could affect the hydraulic or water retention property of the stabilised soil is very vital both for laboratory studies and field practice. However, a thorough assessment of the water retention behaviour of stabilised soils can be understandably limited sometimes. This could be partly due to some of the perculiar conditions associated with soil preparation methods, soil type, soil-stabiliser mix proportion used, curing conditions, method of compaction, durability assessment modalities and other logistical issues surrounding either laboratory instrumentation or in-situ application. This article presents a critical and comprehensive review of these factors on the stabilised soil’s water retention behaviour and also provides a systematic understanding of the mechanisms of stabilisation occurring at the micro- and macro-mechanical levels. Recommendations are also made to stimulate further discussions on the synthesis of SWCC of stabilised soils vis-a-vis factors influencing them with possible interpreted engineering behaviours such as shear strength and soil consolidation.

International audience; The Barcelona Basic Model is a mechanical model for unsaturated soils which uses the net stress and suction as independent stress variables. This paper presents a thorough study of its thermodynamically consistent coupling with a hydraulic model. A general framework for poroelastoplasticity formulated in terms of the net stress and suction is developed and then applied to derivation of a consistent hydro-mechanical coupling for the Barcelona Basic Model. The resulting restrictions for description of the hydraulic behaviour are analysed and they are found to be too strict and not completely physically acceptable. It is demonstrated on a model example that the applicability of the resulting hydro-mechanical model is limited. It is concluded that poromechanical models using the net stress and suction as stress variables generally (at least what poroelasticity concerns) should be developed and utilised carefully with respect to the thermodynamical consistency. Otherwise different types of models should be employed for coupled hydro-mechanical modelling.

The seasonal variability of geotechnical parameters in the unsaturated zone is typically neglected in the design of geotechnical works. In most of the geotechnical projects the parameters are determined only for the saturated condition. Although it is known that this condition is the most critical to soil strength and deformability, this conservative approach may neglect a possible important contribution of the unsaturated condition, resulting in an increase in the cost of the geotechnical solution. This paper presents and discusses the site characterization of the active zone of an unsaturated sandy soil profile under different suction conditions. Laboratory tests with controlled suction (retention curves, triaxial compression with bender elements and oedometer tests) were carried out on undisturbed samples collected from 1.0 to 5.0 m depth. The results show that strength and deformability parameters are strongly affected by soil suction and are less influenced by confinement stress up to 5.0 m depth. All the investigated subsoil profile shows a collapsible behavior, more pronounced closer to the ground surface and under the effect of higher suction values. The findings highlight the importance of incorporating the suction influence in the site investigation, parameter determination, and geotechnical design for more economical, reliable, and environmentally sustainable solutions.

In addition to the intensity of precipitation, the different hydraulic and mechanical properties of a soil hill can also be responsible for landslides. More specifically, the same rainfall amount can trigger or not trigger a mass movement depending on its characteristics. This issue represents a great geotechnical hazard in mountainous regions such as Brazil, Italy, South Africa, Japan and Hong Kong. The understanding of each of the key factors involved in a rainfall-triggered landslide can be deepened and also quantified. Thus, this research defines, through a numerical model, which parameters are the key factors involved in slope stability. A homogeneous and unsaturated hill was studied. Its different hydraulic and mechanical properties were varied. Geometry and mechanical parameters were shown to exert the greatest influence on stability. Hydraulic parameters, for the same amount of rain, showed a lower influence. The fitting parameters of the soil–water characteristic curve of the materials had a low impact on stability when compared to other parameters assessed. Our conclusions can help future laboratory and field studies to focus more on the accuracy and confidentiality of the key parameters. The results are also important as they give direction to studies related to precipitation threshold definition.

A soil’s physical properties, mineral types, and pore structure significantly influence the shape and properties of the soil-water characteristic curve (SWCC). This study investigated the effects of the soil’s physical properties and mineral types on the SWCC and pore-size distribution (PSD). Eight different soils from an alluvial deposit in Istanbul and Adapazarı/Türkiye were used in the study. The test samples were prepared by compaction at optimum water content (OWC) and wet side of optimum water content (wet of OWC). The samples were prepared by consolidation from the slurry. The PSDs of the samples were calculated using the SWCCs and evaluated with scanning electron microscope (SEM) analysis. In addition, the mineral types of all soils were determined by X-ray diffraction analysis. The soil which contains illite-type minerals has higher matric suction than containing kaolin-type. The effect of the clay percentage is more pronounced in silty soils than in plasticity and activity. Soil suction increased with decreasing compaction water content in clayey soils. The air entry water contents rose as the void ratio, liquid limit, clay content, and plasticity increased. The compaction conditions affected the macropore structure more than the micropore structure. In addition, the ratio of macro-micro pore sizes increased with the rise of the compaction water content.

In order to assess slope stability owing to rainfall, the availability of an effective and simple-to-use methodology, relating directly rain to eventual landslide triggering, is undoubtedly useful. To this purpose, a simplified method aimed to the prediction of rainfall-induced shallow landslides in unsaturated soils is proposed in the present study. This method takes advantage of some closed-form solutions to evaluate the change in pore pressure due to infiltration of a rainfall characterized by a given intensity and duration, and the simple scheme of infinite slope to calculate a threshold for the change in pore pressure when the slope is under limit conditions. Particularly, using the present approach, a critical curve can be defined to establish the rainfall events that can trigger a failure process at a given depth, where suction before rainfall is known. The proposed method appears promising from an engineering viewpoint, since it is simple to use and requires few parameters as input data. In addition, these parameters can be determined from conventional geotechnical tests. The validity of the proposed approach is corroborated by some comparisons with the results of well-documented case studies.

Rainfall-induced slope failures commonly occur in residual soil slopes. Slope failures are triggered by the reduction in soil strength. This is attributed to the decrease in soil suction due to rainwater infiltration. Slope covers like capillary barrier system and vegetative cover are effective methods that can be used to prevent rainfall-induced slope failures. The capillary barrier system is able to limit the rainwater infiltration, and vegetation can contribute to the increase in soil strength. Vetiver grass is widely planted in tropical and subtropical areas of the world for soil and water conservation. This study investigates the characteristics of unsaturated soil slope covered with capillary barrier system and Vetiver grass in comparison with the original slope through numerical analyses and field measurements. The analyses were carried out under the advanced, normal, and delayed rainfall patterns. The results of the analyses indicated that the capillary barrier system played a more significant role than the Vetiver grass in maintaining slope stability, although both the capillary barrier system and Vetiver grass contributed to the slope stability. In addition, both numerical analyses and field measurements showed that under the delayed and normal rainfall patterns, when antecedent rainwater infiltration could increase the soil moisture, the capillary barrier system performed much better compared to Vetiver grass in maintaining soil matric suctions and slope stability. Housing & Development Board Published version This study was supported by the Housing and Development Board and Nanyang Technological University, Singapore.

Soil-atmosphere interaction has implications in different scientific research contexts and is increasingly investigated through field measurements. This paper reports a detailed description of interaction between shallow soil and atmosphere at two test sites in Oltrepò Pavese area (Northern Italy). The two test sites are in the same climatic area but are characterised by different geological features. In fact, the first objective is to compare the behaviour of two different soils, namely a clayey-sandy silt (CL) and a silty clay (CH), under similar meteorological events. Soil-atmosphere interaction is studied on the basis of long-term (about 87 and 42 months for the two test sites, respectively) monitoring data of both volumetric water content and soil water potential, recorded at different depths along two vertical soil profiles in the first two metres from ground level. Field measurements, together with meteorological data such as precipitation and air temperature, allow for clear identification of the seasonal fluctuations of unsaturated soil hydraulic properties. To infer detailed information, the recorded data were processed and relationships between soil water potential and water content were investigated. Different time spans, from several months to a few days, even including single rainy events, are considered to show the hydraulic soil behaviour. The hysteretic cycles of water content with respect to soil water potential and non-equilibrium flow are highlighted. In particular, the measured soil water potential is in the range of 0–800 kPa and of 0–1500 kPa for the CL and CH soil, respectively. At both sites, the observed hysteretic cycles are more frequent in the hot season (summer) than in the cold season (winter) and tend to reduce with depth. The experimental results are compared with the soil water characteristic curves (SWCCs) to assess whether and to what extent the SWCCs are reliable in modelling the hydraulic behaviour of partially saturated soils, under atmospheric forcing, at least in the considered climatic contexts.

Preferential flow is a non-equilibrium flow in unsaturated soil through which water infiltrates deep into the ground quickly. It has been studied in many fields, such as environment, agriculture, and hydrology. However, researchers from different disciplines have a different understanding of preferential flow, and it is difficult to grasp its development trends and research frontiers through qualitative analysis in a single field, while they can be quantitatively and objectively analyzed through bibliometrics with scientific knowledge map tools. This paper collects 3315 research studies on preferential flow in soil from the Web of Science (WoS) core collection database within 30 years, conducts a statistical analysis on keywords, countries, and research institutions of these studies based on CiteSpace, draws visualized scientific knowledge maps, and presents the development trends and research frontiers of preferential flow. Results showed that preferential flow is a multi-scale coexistence phenomenon, and researchers from different disciplines study preferential water flow movement and pollution at different research scales. New techniques and ideas are research hotspots and directions. Moreover, the difference between bibliometrics methods and review methods is analyzed. This paper is presented to provide a referable knowledge structure and new ideas for research in related fields and to help promote cross-integration between disciplines.

The small-strain stiffness of soil is significant in the accurate prediction of the deformation caused by interactions between foundation soil and structures. Considering the whole range of small strain (10−6~10−3), a bending element-resonant column (BE-RC) combined test system was developed to conduct continuous tests on the shear modulus of unsaturated soil. Under the dehydration path, it was used to investigate the small-strain shear modulus of unsaturated silty-fine sand in Hangzhou Bay, China. The results show that the shear modulus under different net stresses and matrix suctions appeared to non-linearly decay with the increase in strain until stable values were reached at a large strain. At the beginning from the saturated state, the Gmax value increased slowly with decreasing saturation and reached its maximum value at the optimum saturation (Sr)opt; then, it rapidly decayed to the level in the saturated, once the saturation degree decreased to a level lower than (Sr)opt. Additionally, an improved prediction model was proposed for the Gmax of unsaturated sand, considering different saturations. Based on the mesoscopic evolution of internal pore water morphology and the variation in intergranular stress caused by capillary action, the variation in the Gmax could be divided into three segments of saturation: the boundary effect stage, the transition stage and the unsaturated residual stage.

Considering unsaturated triaxial tests including a ceramic disk as initial and boundary value problems, a series of suction change, isotropic consolidation, and drained shearing processes was simulated using a soil-water-air coupled elastoplastic finite deformation analysis code. As a result, it was demonstrated that the delayed behavior of a wetting-induced collapse during the suction change and isotropic consolidation processes, as well as water-absorption behavior during the subsequent drained shearing process, were attributed not to the characteristics of the soil specimen, but to the permeability of the disk. Thus, it is important to take the ceramic disk into account in unsaturated triaxial tests regarded as initial and boundary value problems in order to understand the temporal change behavior of unsaturated soil specimens. Otherwise, there is a risk that the behavior appearing as the solution of an initial and boundary value problem may be modeled as a constitutive relation.

Recently, climate change has been causing severe droughts and water scarcity in drylands. A proper understanding of the soil surface-atmosphere boundary fluxes is essential to combatting such environmental problems. In those regions, evaporation is considered to be the dominant flux. It is divided into three stages that follow different mechanisms and are highly dependent on the soil properties. Most of the existing studies focus on the evaporation process from a macroscale perspective and estimate the evaporation rate based on the atmospheric conditions, while only a few of them consider the soil properties. This paper studied the influence of the soil pore structure on the actual evaporation and formation of the unsaturated layer. Homogeneous drying sandy soil column tests were conducted under a unified low atmospheric demand. The pore structure was investigated by varying the texture and relative density. The significance of Stage 2 in the evaporation process was asserted. Additionally, it was affirmed that vapor diffusion is dominant during Stage 2, where the actual evaporation reduction rate depends on the receding rate of the vaporization plane. Finally, a comprehensive, robust, and easily determined index, that correlates well with the duration and evaporated water during Stage 1 and Stage 2, was proposed. The Pore Size Distribution Index (IPSD) was found to reflect the variations in the pore structure and to consider the capillary and diffusion flow mechanisms. It serves as a fundamental step towards evaluating the actual evaporation and drying front depth during the evaporation process.

Tailing dams and waste dumps formed by the accumulation of mine wastes are usually at long-term unsaturated state under evaporation and consolidated drainage conditions. Soil-water retention relation is one of the key constitutive relations to analyze the seepage processes in tailing dams and waste dumps. In this study, typical coarse- and fine-grained tailings from a metallurgical mine were chosen to study the soil-water retention characteristics of the tailing samples. To begin with, the relationship between volumetric water content and suction of the tailing samples was experimentally measured, and typical soil-water characteristic curves (SWCCs) (i.e., Gardner, van Genuchten and Fredlund–Xing curves) were applied to fit the experimental data. After that, four empirical models to estimate the parameters in SWCCs (i.e., Aubertin-1998 model, Aubertin-2003 model, Vanapalli-2005 model and Chin-2010 model) were tested, and the Vanapalli-2005 model was the best-fit model for the tailing samples. Furthermore, this study proposes a generalized emergency risk assessment soil-water retention characteristics model for tailing dams and waste dumps, and a framework for the quick estimation of parameters in the SWCC is proposed as well. The recommended soil-water retention characteristics model and the related parameters can be used to predict water levels in tailing dams and waste dumps, which are very helpful for emergency risk assessment under rainstorm or flooding conditions. HIGHLIGHTS This study is the first study to prove the feasibility of using the Vanapalli-2005 model to estimate parameters in soil-water characteristic curves (SWCCSs) for mine wastes.; This study proposes a framework for the fast determination of SWCCs for mine wastes, which can be used to predict water levels in mine wastes.; The framework is helpful for the emergency risk assessment of mine wastes under rainstorm and flooding conditions.

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.

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.

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.

The importance of soil water retention characteristics in modelling the hydro-mechanical response of unsaturated soils has been well recognised by many investigators in recent years. Determination of strain-dependent soil water retention curve (SWRC) is likely to be extraordinarily difficult. The first two authors have recently shown that SWRC can be computed from the gradation curve and the calculation result is consistent with the experimental results obtained from pressure plate tests. In this paper, based on a hypothesis related to change in the pore size distribution (POSD) due to volumetric strain of soil skeleton, a method to compute strain-dependent SWRC is presented. It is found that at initial degrees of saturation higher than 0.8, the influence of volumetric strain may be marginal whilst at initial degrees of saturation lower than 0.8, its influence is likely to be substantial. In all cases, the gradation curve of the soil affects the SWRC.

Arsenic is a metalloid frequently found in contaminated sites due to its easy mobilization in the environment. This has attracted the researchers who have studied this phenomenon from an environmental engineering perspective. Although there is evidence indicating that soil contamination impacts its geotechnical behavior, there is no available information about the changes in the soil’s suction after its contamination. The objective of this paper was to evaluate the soil–water retention curve of arsenic-contaminated soil. An unsaturated soil sample was contaminated with arsenic at two different concentrations and a filter paper calibration curve was developed for each contaminant concentration. Soil contamination decreased the BET area and increased its pore sizes. At a saturation rate of 0.8, the matric suction of the contaminated soils decreased (from 1681 kPa to 260 kPa), while the osmotic suction increased (from 23 kPa to 530 kPa), compared with the natural soil’s condition. Furthermore, the soil’s suction values obtained with the ASTM D5298 calibration curve were higher than those obtained using the calibration curve developed for the contaminated state. Thus, to avoid misunderstanding the mechanical behavior of the unsaturated soils under contaminated conditions, the osmotic suction should be considered and evaluated with the corresponding calibration curve.

Support vector machine techniques (polynomial and radial basis function) and random forest regression techniques have been used to predict erodibility of an unsaturated soil collected from an erosion watershed located at Amuzukwu, Nigeria. This location has been on the spotlight due to the level destruction resulting from erosion gullies. The soil, which was found to be of “Benin formation”, has been treated with composites of waste-based binders in a cleaner and greener technology to avoid the conventional emissions emanating from the use of ordinary cement. These materials were applied in a nanotextured scale, which improved the reactive surface and nucleating ability of the supplementary cementitious additives. This work is focused on proposing more reliable intelligent models, which can be used in future designs, construction and monitoring of the performance of the facilities at the erosion site without relying on repeated laboratory experiments. The performance indices, which included r, R2, MSE, RMSE, MAE and MAPE were used to test the validity and accuracy of the models. The three models showed agreement with the performance accuracy though SVM-radial basis function (SVM-RBF) outclassed both the previously used ANN and the SVM-p and RFR with R2 of 0.999 and minimal errors. The additives also proved to be good greener and cleaner cementing additives in the stabilization of soils with substantial improvements.

In many geotechnical applications, especially in the study of weather-induced landslides, a reliable soil hydraulic characterization in unsaturated conditions is required. Currently, the experimental techniques that neglect the hydraulic hysteresis represent the greatest limitation to landslide forecasting. In this paper, a procedure to obtain an unsaturated soil hydraulic characterization on natural pyroclastic samples is proposed and verified. The approach enables the evaluation of the soil hydraulic properties along the main drying path and wetting/drying cycles to fully quantify the effects of the hydraulic hysteresis. Pyroclastic soil samples collected at a test site at Mount Faito in the Campania region (southern Italy) were tested. The experimental investigation consisted of a sequence of testing phases: a constant-head hydraulic conductivity test, a forced evaporation test followed by several wetting–drying cycles, and a drying test in a pressure plate apparatus. The hysteretic model proposed by Parker and Lenhard (1987) was adopted to fit the data, while inverse modelling of the forced evaporation tests allowed to derive the model parameters. Therefore, the main drying and wetting branches and the soil response to drying and wetting cycles from any reversal point were reproduced with the model, which suitably described the hysteretic behaviour of the pyroclastic soil under all conditions and along all paths.

The capillary barrier system is a widely researched method used to control rainfall infiltration into soil slopes for mitigating rainfall-induced landslides. Conventional capillary barrier systems, however, are subjected to several weaknesses, such as the inability of the upper fine layer to function effectively under intense or prolonged rainfall, and sliding failure or erosion may occur in the fine layer as a result of excessive lateral seepage. This study aims to investigate the feasibility of using biomediated soil cover in a capillary barrier system to minimize rainfall infiltration into a residual soil slope. Firstly, the engineering properties of the original and biomediated residual soils were investigated. Secondly, an instrumented one-dimensional physical soil column was set up to investigate the infiltration behaviour of the tropical residual soil with and without biomediated soil cover. A numerical seepage model was simulated to compare the experimental and numerical results, as well as to verify the input parameters of the numerical simulation. Lastly, a two-dimensional slope model was simulated to investigate the effectiveness of the biomediated soil cover in minimizing infiltration under both intense (1-h, 4-h, 8-h, 24-h extreme rainfalls) and prolonged (72-h extreme rainfall) rainfall conditions. The results showed that the soil column with biomediated soil cover could effectively maintain the soil in an unsaturated state for a longer period of infiltration (i.e., 60 min) as compared with the original residual soil (i.e., 10 min only). The numerical simulation results agreed reasonably well with the experimental findings. The two-dimensional seepage analysis results indicated that the slopes with biomediated soil cover could reduce the infiltration of water into the underlying soil slope, and hence resulted in a shallower wetting front, particularly under short and intense extreme rainfall conditions.

The seepage calculation of unsaturated soil has always been an important subject in the field of geotechnical engineering, and it greatly influences the stability of geotechnical engineering. Currently, most of the unsaturated soil seepage calculation formulas consider only the saturation. In fact, the seepage of the soil is also affected by other factors, such as pore size. Therefore, considering the influence of void ratio in the seepage calculation of unsaturated soil is worthy of further investigation. In this study, a seepage function of unsaturated soil, which considers the influence of dry density and saturation, was constructed and introduced into ABAQUS program. An engineering case was calculated, and the difference between the results considering two variables and that only considering saturation was compared and analyzed. The analysis of the results indicates that this function can better characterize unsaturated seepage, and its subprogram is applicable.

Excavated geo-relics are vulnerable to damage by natural processes. The aim of this study is to contribute to the establishment of a technical framework for the preservation of openly exhibited geo-relics. This study also examines the preservation of an openly exhibited geo-relic in Japan, which has experienced surface deformation in the soft soil layer due to water infiltration. The surface deformation is numerically investigated by performing seepage-deformation analyses based on unsaturated soil mechanics in order to understand its mechanism and to obtain effective countermeasures. The results show that deformation develops in the surface layer of the slope as the bonding between soil particles, represented by skeleton stress, and decreases when water infiltrates the slope. Although the calculation considers the influence of groundwater, as well as precipitation, the results show that the deformation of the slope is primarily controlled by precipitation, not by groundwater. Furthermore, the elevation of the groundwater does not contribute to the development of surface deformation. Based on the mechanism of the surface deformation, replacing the surface layer with a well-compacted, highly permeable soil is proposed to improve slope stability. It is predicted that this proposed method will be effective because the replaced zone retains sufficient strength and stiffness when it is wet, despite a decrease in the skeleton stress due to rainfall infiltration. This countermeasure has been adopted for the actual restoration of a damaged slope.

Slope failures are one of the most vital problems in geotechnical engineering applications which may cause damage both in human life and financial. Slope failures are frequently observed especially in regions where rainfall is high. The most important factor affecting slope stabilities is rainfall infiltration. It is known that infilled soils are generally unsaturated soil. Therefore, the analysis of infill slopes should be performed within the scope of unsaturated soil mechanics. Rainfall infiltration causes the groundwater level to rise, which reduces the shear strength of the soil causing slope instability. In this study, the effect of rainfall intensity, rainfall duration, soil permeability, groundwater level, cohesion of soil, internal friction angle of the soil, and slope angle of the slope on the slope stability were analyzed parametrically by using GEOSTUDIO software. The infiltration analyses were performed using Seep/w module and factor of safety number of unsaturated soil slope were determined using Morgenstern method of Slope/w module of GEOSTUDIO software. Furthermore, different factor of safety numbers was found for each analysis along with the interpretation of the parameters effecting the slope stability. The results obtained in the present investigation showed that the rainfall intensity affects the stability of the slope. It was determined that there existed a non-linear relationship between rainfall intensity and safety number. As the rainfall intensity increases, the negative pore water pressure in the soil decreases nonlinearly, and the factor of safety number of the slope similarly decreases by 10% relative to the initial condition. When the rainfall duration change was taken into account, it was found that short time-heavy rainfalls affected the slope stability more than long time-low density rainfalls. Moreover, slope angle significantly influenced slope stability. It was observed that when the slope angle increased from 40° to 50°, factor of safety number decreased by 18.34%.

The unsaturated soil mechanics has been studied extensively in the past few decades. Determining the shear strength properties requires direct pore-air and pore-water pressure measurements. However, pore pressures of unsaturated soil can only be measured at advanced geotechnical laboratories with specialty setups. The confined undrained triaxial compression (CUTC) test without pore pressure measurement, a simpler test that can be done at most geotechnical laboratories, was used in this study. Regina clay samples were prepared at three moisture contents (22%, 26%, and 30%) and compacted using the same effort. Three confining pressures (10, 50, and 100 kPa) and three strain rates (0.0084%, 0.084%, and 0.84% per minute) were tested. The moisture contents of 22% and 26% samples had comparable results because the optimum water content (26%) samples had lower initial void ratios after compaction. The moisture content of 30% samples had the lowest strengths. For samples with the same moisture content, the undrained shear strength was positively related to confining pressure and strain rate. A methodology was developed to estimate the pore pressure parameters in CUTC tests without pore pressure measurements. Simplified deviator stress versus isotropic confining pressure relationship was derived and assumed to be linear in the stress ranges examined. The sensitivity of the pore pressure parameters to each input variable was analyzed. The tangent pore-air and pore-water pressure parameters for uniaxial loading (Da and Dw) were sensitive to matric suction, the angle of shear strength change rate related to matric suction, and the effective angle of internal friction. Da and Dw were less sensitive to effective cohesion and the ratio of pore-air and pore-water pressure parameters. The tangent pore-air and pore-water pressure parameters related to isotropic loading (Ba and Bw) were sensitive to Da, Dw, and effective angle of internal friction. Ba and Bw were also sensitive to each other. The pore pressure parameters were used to back-calculate the pore pressures and the effective stress during the test. Using CUTC tests without pore pressure measurements with estimations of pore pressure parameters was deemed a feasible method to estimate unsaturated soil properties.

Argillaceous swelling rocks are generally unsaturated. Partial rock desaturation due to tunnel ventilation is a phenomenon that inevitably occurs during construction of underground structures. The characteristics of the unsaturated zone must be known in order to predict the swelling behavior of natural expansive rock. The laboratory tests described here provide information for determining the necessary parameters. The numerical model used to predict moisture distribution within the desiccating sample provides a realistic description of the process. The presented results contribute to a better understanding of the swelling rock behavior and provide a rational framework for the analysis of the desiccation process and its influence on the swelling. The influence of swelling rock desaturation on its behavior in tunneling is described. It has been demonstrated that the same rock may abide by different swelling laws, depending on the saturation level. The consequence of this is the nonlinear characteristic curve in semilogarithmic scale. The nonlinearity of the characteristic curve is of great significance in the design and construction of tunnels. This new nonlinear characteristic curve emphasizes, more than other curves, the known fact that small stresses significantly reduce swelling deformation, i.e. that a small allowable displacement significantly contributes to the reduction of swelling stress. In other words, this curve emphasizes the danger of installing resistant tunnel supports, while pointing to advantages of yielding supports.

In this work, we describe soil moisture profiles related to typical colluvial slopes that were involved in rainfall-induced shallow failures occurring in alpine and pre-alpine areas of the Friuli Venezia Giulia Region (NE Italy). The trend of the volumetric water content (θw) showed a general increase from the ground surface to the bottom soil layer, with two or three marked moisture peaks. The saturation degree (S) varied from 65–70% (topsoil horizon) to nearly saturated basal colluvium (S = 95–100%). Soil moisture data demonstrates that, for a very humid climate, colluvial covers are often close to the saturation condition for most of the year. The calculated suction profiles indicated that maximum values ranging from 40 to 55 kPa often occur in the slope surficial soil (depth < 0.2–0.5 m). This negative pore-water pressure greatly decreases after a heavy rainfall event because of the infiltration process. Complete saturation of colluvial cover in the alpine and pre-alpine regions generally requires rainfall exceeding 150–200 mm for a 24-h storm duration. This results in a recurrence time of Tr ≅ 5–10 years for critical rainfall episodes involving colluvial slopes in the Friuli Venezia Giulia Region. The case histories analyzed demonstrate the importance of performing a detailed lithostratigraphic analysis of the colluvial deposit in order to properly define the suction measurement points, which there should be more of than the three-point determinations usually reported in the literature (for example, z = 0.5, 1.0 and 1.5 m).

Soil water content measurement is critical in practical engineering. The actively heated fiber Bragg grating optic sensor (FBGS) has great potential of multi-point measurement for soil water content measurement in field. In this study, the effect of heating time on the measurement accuracy is discussed, and modifications are made for actively heated fiber optic (AHFO) sensors. The results demonstrate that if an integration data analysis method is used, the accuracy and reliability of soil water content measurement with AHFO sensors will be improved. Both a short fiber length and a short-term heating pattern are effective and can help to reduce soil disturbance. With the proposed integration method, a short heating time is guaranteed for measuring the soil water content. Such improvements will reduce the thermal disturbance to soil sample and improve the reliability of measurement.

This study focuses on the simulation of transient ground temperatures in a field-scale soil-borehole thermal energy storage (SBTES) system in San Diego, California. The SBTES system consists of an array of thirteen 15 m-deep borehole heat exchangers installed in conglomerate bedrock at a spacing of approximately 1.5 m. Heat collected from solar thermal panels was injected into the SBTES system over a 4-month period, after which the subsurface was monitored during a 5-month ambient cooling period. The SBTES system is located in the vadose zone above the water table with relatively dry subsurface conditions, so a coupled heat transfer and water flow model was used to simulate the ground response using thermo-hydraulic constitutive relationships and parameters governing vapor diffusion and water phase change calibrated using soil collected from the site. The simulated ground temperatures from the model match well with measurements from thermistors installed at different radial locations and depths in the SBTES system and are greater than those simulated using a conduction-only model for saturated conditions. Significant overlap between the effects of the borehole heat exchangers was observed in terms of the ground temperature. Although the numerical simulations indicate that permanent decreases in degree of saturation and thermal conductivity occurred at the borehole heat exchanger locations, the zone of influence of these changes was relatively small for the particular site conditions.

博士の専攻分野の名称 : 博士(学術)学位授与年月日 : 令和2年9月23日

The purpose of this research is to investigate the performance and efficiency of reinforced slope in the stability of geocell layers in unsaturated soil conditions. Slope reinforced with geocell acts like a beam in the soil due to the geocell having a height (three-dimensional). Due to its flexural properties, it has moment of inertia as well as bending strength, which reduces the displacement and increases the safety factor of the slope. Taking into consideration unsaturated conditions of soil contributes a lot to making results close to reality. One of the well-known models among elastoplastic models for modeling unsaturated soils is Barcelona Basic Model, which has been added to the FLAC2D software by codification. Changes in thickness, length and number of geocell layers are remarkably effective on slope stability. The results show that the geocell's reinforcing efficiency depends on the number of layers and depth of its placement. As the depth of the geocell's first layer increases, the lateral and vertical side elevation of the upper part of the slope increases with respect to the elevation. Load capacity increases with increasing geocell length. By increasing the length of the geocell layer, the joint strength, the mobilized tensile strength, and the bending moment are increased. At u/H = 0.2, an increase in the bending momentum of about 20% occurs with increasing geocell thickness. In u/H = 1, the increase in bending momentum is 10.4%. In addition, by increasing the thickness of the geocell, the Value of moment of the inertia increases and, as a result, the amount of geocell reinforcement bending moment increases.

The soil water characteristic curve and microstructure evolution of unsaturated expansive soil improved by microorganisms in Nanning, Guangxi were studied by means of filter paper method and scanning electron microscope imaging (SEM). Based on Fredlung & Xing model, the influence law of different cement content on the soil water characteristic curve of improved expansive soil is proved. According to the analysis of SEM test results, the influence mechanism of MICP method on the engineering characteristics of improved expansive soil is revealed. The results show that with the increase of cement content, the saturated water content and residual water content of the improved expansive soil gradually increased. At the same time, the water stability gradually increased while the air inlet value gradually decreased. The improved expansive soil changes from the superposition of flat particles and flake particles to the contact between spherical particles and flake particles, which indicates that the aggregate increases significantly. With the increase of the content of cement solution, the contact between particles tends to be smooth and the soil pores gradually tend to be evenly distributed. The particle size and microstructure of soil particles was changed and the connection between particles was enhanced in the improved expansive soil. Eventually the strength and water stability of expansive soil were improved. The conclusions above not only provide a theoretical basis for the in-depth study of engineering characteristics of unsaturated expansive soil improved by MICP method, but also offer theoretical evidence for perfecting engineering technology of expansive soil improved by MICP method.

Support vector machine (SVM) with its feature known as the statistical risk minimization (SRM) has been employed in the prediction of coefficient of curvature and uniformity on unsaturated lateritic soil treated with composites of hybrid cement and nanostructured quarry fines. This feature utilized by SVM is the advantage it exercises over other intelligent learning techniques. This prediction has become necessary due to the time and equipment needs required to regularly conduct laboratory experiments prior to earthwork designs and construction. It is important to note that earthwork projects involving unsaturated soils pose threats of failure due to volume changes during seasonal cycles of wetting and drying especially for hydraulically bound environments and substructures. With an intelligent prediction, these design and construction worries are overcome. The soil used in the current work has been classified as an A-7-6 group soil with highly plastic consistency. Multiple experiments were conducted to generate multitude of datasets for the hybrid cement, nanostructured quarry fines, clay content and activity and frictional angle, which were selected as the independent variables for the model to predict coefficients of curvature and uniformity as the dependent variables. In order to correlate the relationship between the input and output parameters and as well validate the SVM model, detailed statistical analysis including Pearson's coefficient of correlation (R) and determination (R2) and error analysis were conducted. Based upon the statistical analysis, the parameters were observed to have good correlation and determination ranging between 0.97 and 0.99. It was also observed that SVM outclassed MLR more in predicting Cu then it did in predicting Cc. Finally, sensitivity analysis was carried out and it was found that the Cc value is dependent mostly on frictional angle while Cu is dependent most on the NQF.