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The main objective of this paper is to reduce the dimensionality of unknown coefficient vectors of finite-element (FE) solutions in two-grid (CN) FE (TGCNFE) format for the nonlinear unsaturated soil water flow problem by using a proper orthogonal decomposition (POD) and to design a new reduced-dimension iteration TGCNFE (RDITGCNFE). For this objective, a new time semi-discrete CN (TSDCN) scheme for the nonlinear unsaturated soil water flow problem is first designed and the existence, stability, and error estimates of TSDCN solutions are demonstrated. Subsequently, a new TGCNFE format for the nonlinear unsaturated soil water flow problem is designed and the existence, unconditional stability, and error estimates of TGCNFE solutions are demonstrated. Next, a new RDITGCNFE format with the same FE basis functions as the TGCNFE format is built by the POD method and the existence, unconditional stability, and error estimates of RDITGCNFE solutions are discussed. Ultimately, the rightness of theory results and the superiority of the RDITGCNFE format are verified by two sets of numerical tests. It is worth noting that the RDITGCNFE format differs completely from all previous reduced-dimension methods, including the authors' previous works. Therefore, the study of this paper can not only provide a new theoretical method for the dimensionality reduction of numerical models for nonlinear problems but also provide an algorithm implementation technology for the numerical simulation of practical engineering problems. [ABSTRACT FROM AUTHOR]

Due to cyclic wetting and drying, the hydro-mechanical behavior of unsaturated soil is impacted significantly. In order to assess the soil strength parameters, knowing the unsaturated behavior is important. Soil moisture content is an important parameter that can define the shear strength of the soil. Most of the highway slopes of Mississippi are built on highly expansive clay. During summer, the evaporation of moisture in the soil leads to shrinkage and the formation of desiccation cracks, while during rainfall, the soil swells due to the infiltration of water. In addition to this, the rainwater gets trapped in these cracks and creates perched conditions, leading to the increased moisture content and reduced shear strength of slope soil. The increased precipitation due to climate change is causing failure conditions on many highway slopes of Mississippi. Vetiver, a perennial grass, can be a transformative solution to reduce the highway slope failure challenges of highly plastic clay. The grass has deep and fibrous roots, which provide additional shear strength to the soil. The root can uptake a significant amount of water from the soil, keeping the moisture balance of the slope. The objective of the current study is to assess the changes in moisture contents of a highway slope in Mississippi after the Vetiver plantation. Monitoring equipment, such as rain gauges and moisture sensors, were installed to monitor the rainfall of the area and the moisture content of the soil. The data showed that the moisture content conditions were improved with the aging of the grass. The light detection and ranging (LiDAR) analysis was performed to validate the field data obtained from different sensors, and it was found that there was no significant slope movement after the Vetiver plantation. The study proves the performance of the Vetiver grass in improving the unsaturated soil behavior and stability of highway slopes built on highly expansive clay. [ABSTRACT FROM AUTHOR]

Adding different materials to soil can improve its engineering properties, but traditional materials such as cement, lime, fly ash, etc., have caused pollution to the environment. Recently, biopolymers have shown many advantages, such as economy and environmental protection, which make them applicable to geotechnical engineering. This study summarizes the effects of biopolymers on soil's engineering properties and the main directions of current research. Firstly, the advantages and disadvantages of a variety of widely used biopolymer materials and their effects on the specific engineering characteristics of soil (i.e., water retention characteristics, strength characteristics, permeability characteristics, microstructure) are introduced, as well as the source, viscosity, pH, and cost of these biopolymers. Then, based on the theory of unsaturated soil, the current research progress on the water retention characteristics of improved soil is summarized. The key factors affecting the strength of biopolymer-treated soil are introduced. Due to the actual environmental conditions, such as rainfall, the permeability and durability of biopolymer-treated soil are also worthy of attention. In summary, it is necessary to study the variation laws of the engineering properties of biopolymer-treated soil in the full suction range, and to predict such laws reasonably. The relevant results are conducive to the safer and more scientific application of biopolymers in geotechnical engineering practice. [ABSTRACT FROM AUTHOR]

An interlayer existed between the ballast layer and subgrade in the conventional railway substructure. Considering that the shear strength τ of the interlayer soil was influenced by the changes in the ballast grain content and water content, this aspect was explored in the present study. Monotonic triaxial tests were fulfilled, which considered five coarse grain contents fv and three water contents of fine soil wf. The results showed that the growth in fv contributed to an increment in τ of the soil mixture under both saturation and unsaturation. Conversely, in previous studies, the growth of fv induced an increment in τ under saturation, but a decline in that under unsaturation. This was explained by the competing influences of fv and suction ψ: in previous studies, increasing fv induced a decline in the dry density of the fine soil fraction ρd–f, which contributed to a decline in ψ. When the negative influence of declining ψ outweighed the positive influence of the incrementing fv, the τ of the soil mixture decreased. Meanwhile, modelling of the τ–ψ relationship in the soil mixture with varying fv was performed. This proposed model was examined using the test results from both the present and previous studies, which shows its reasonably good performance. [ABSTRACT FROM AUTHOR]

Unsaturated soil is a three-phase medium with three interfaces, and the mathematical equations that represent its behavior must be developed in a fully coupled manner for accurately predicting its hydromechanical behavior. In this paper, a set of fully coupled governing equations was developed for the dynamics of unsaturated soil, considering the interaction among the bulk phases and interfaces. In addition to implementing the complete governing equations, a simplified formulation was developed for practical applications. The derivation of the finite element formulation considering all the terms in the partial differential equations resulted in a formulation called complete formulation and was solved for the first time in this paper. Another formulation called reduced formulation was derived by neglecting the relative accelerations and velocities of water and air in the governing equations. In addition, small and large deformation theories were developed and implemented for both formulations. To show the applicability of the proposed models, the dynamic behavior of an unsaturated soil embankment was simulated using both small and large deformation formulations by applying minor and severe earthquakes. The reduced formulation was found to be computationally efficient and numerically stable. The smaller displacements predicted by large deformation theories show that the results are consistent with the expected behavior. Large deformation theories are considered suitable when the geotechnical system undergoes large deformation and may lead to accurate prediction. [ABSTRACT FROM AUTHOR]

The design, energetic performance, and thermal impact of large-scale geothermal collector systems (LSCs) are dependent on the thermal conductivity of unsaturated soils (λ). The aim of this study was to investigate the benefits of two different λ measurement methods using single-needle sensor measuring devices on a laboratory scale. Since large-scale determinations are required in the context of LSCs, the potential for deriving λ from electrical resistivity tomography measurements (ERTs) was also examined. Using two approaches—the continuous evaporation method and the punctual method—thermal conductivities of soil samples from Bad Nauheim (Germany) were measured. The results were compared with averaged λ derived from three ERT sections. With the evaporation method, significant bulk density changes were observed during the experimental procedure, which were caused by the clay content and the use of repacked samples. The punctual method ensures a sufficiently constant bulk density during the measurements, but only provides a small number of measurement points. The thermal conductivities derived from ERTs show largely minor deviations from the laboratory measurements on average. If further research confirms the results of this study, ERTs could provide a non-invasive and unelaborate thermal exploration of the subsurface in the context of large-scale infrastructure projects such as LSCs. [ABSTRACT FROM AUTHOR]

Although many unsaturated soil experiments have successfully delivered positive outcomes, most studies just concisely illustrated sensor techniques, because their main objectives focused on bridging research gaps. Inexperienced research fellows might rarely follow up those techniques, so they could encounter very trivial and skill-demanding difficulties, undermining the quality of experimental outcomes. With a motivation to avoid those, this work introduces technical challenges in applying three sensor techniques: high precision tensiometer, spatial time-domain reflectometry (spatial TDR) and digital bench scales, which were utilized to measure three fundamental variables: soil suction, moisture content and accumulative outflow. The technical challenges are comprehensively elaborated from five aspects: the functional mechanism, assembling/manufacturing approaches, installation procedure, simultaneous data-logging configurations and post data/signal processing. The conclusions drawn in this work provide sufficient technical details of three sensors in terms of the aforementioned five aspects. This work aims to facilitate any new research fellows who carry out laboratory-scale soil column tests using the three sensors mentioned above. It is also expected that this work will salvage any experimenters having troubleshooting issues with those sensors and help researchers bypass those issues to focus more on their primary research interests. [ABSTRACT FROM AUTHOR]

Rammed earth has the potential to reduce the carbon footprint and limit the energy consumption in the building sector due to its sustainable characteristics. Still, its use is not generalized due to a lack of understanding of the material behavior, notably its sensitivity to water. The coupled hydro-mechanical behavior has been recently studied in the framework of unsaturated soil mechanics, using suction as the parameter to represent the hydric state. This dependency of the mechanical behavior on the hydric state leads to uncertainty of the drying period required to progress in the construction process. Notably, the drying period before building the next floor is unknown. To determine the drying period, thermo-hydro-mechanical coupled finite element method simulations were carried out on a single wall by using the unsaturated soil mechanics approach and safety criterion recommendations from the practical guide for rammed earth construction in France. It was determined that it takes significant time for the construction of additional floor both in 'summer-like' and 'winter-like' environmental conditions, whereas the walls were far away from the ultimate failure state. Thus the drying periods were overestimated. It was concluded that the safety criterion from the practical guide is very conservative and drying periods can be reduced without significantly compromising the safety factor. [ABSTRACT FROM AUTHOR]

Estimation of soil moisture distribution in desert regions is challenged by the deep unsaturated zone and the extreme natural environment. In this study, an entropy-based method, consisting of information entropy, principle of maximum entropy (PME), solutions to PME with constraints, and the determination of parameters, is used to estimate the soil moisture distribution in the 10 m deep vadose zone of a desert region. Firstly, the soil moisture distribution is described as a scaled probability density function (PDF), which is solved by PME with the constraints of normalization, known arithmetic mean and geometric mean, and the solution is the general form of gamma distribution. A constant arithmetic mean is determined by considering the stable average recharge rate at thousand year scale, and an approximate constant geometric mean is determined by the low flow rate (about 1 cm a year). Followed, the parameters of the scaled PDF of gamma distribution are determined by local environmental factors like terrain and vegetation: the multivariate linear equations are established to qualify the relationship between the parameters and the environmental factors on the basis of nineteen random soil moisture profiles about depth through the application of fuzzy mathematics. Finally, the accuracy is tested using correlation coefficient (CC) and relative error. This method performs with CC larger than 0.9 in more than a half profiles and most larger than 0.8, the relative errors are less than 30% in most of soil moisture profiles and can be as low as less than 15% when parameters fitted appropriately. Therefore, this study provides an alternative method to estimate soil moisture distribution in top 0-10 m layers of the Badain Jaran Desert based on local terrain and vegetation factors instead of drilling sand samples, this method would be useful in desert regions with extreme natural conditions since these environmental factors can be obtained by remote sensing data. Meanwhile, we should bear in mind that this method is challenged in humid regions since more intensive and frequent precipitation, and more vegetation cover make the system much more complex. [ABSTRACT FROM AUTHOR]

Various electrochemical methods were used to understand the behavior of steel buried in unsaturated artificial soil in the presence of cathodic protection (CP) applied at polarization levels corresponding to correct CP or overprotection. Carbon steel coupons were buried for 90 days, and the steel/electrolyte interface was studied at various exposure times. The coupons remained at open circuit potential (OCP) for the first seven days before CP was applied at potentials of −1.0 and −1.2 V vs. Cu/CuSO4 for the remaining 83 days. Voltammetry revealed that the corrosion rate decreased from ~330 µm yr−1 at OCP to ~7 µm yr−1 for an applied potential of −1.0 V vs. Cu/CuSO4. CP effectiveness increased with time due to the formation of a protective layer on the steel surface. Raman spectroscopy revealed that this layer mainly consisted of magnetite. EIS confirmed the progressive increase of the protective ability of the magnetite-rich layer. At −1.2 V vs. Cu/CuSO4, the residual corrosion rate of steel fluctuated between 8 and 15 µm yr−1. EIS indicated that the protective ability of the magnetite-rich layer deteriorated after day 63. As water reduction proved significant at this potential, it is proposed that the released H2 bubbles damage the protective layer. [ABSTRACT FROM AUTHOR]

The application of unsaturated soil mechanics in routine geotechnical engineering applications requires the determination of unsaturated soil properties. Unfortunately, the cost of direct measurement of unsaturated soil properties goes beyond the financial budget of most clients. A solution has arisen, however, that involves the measurement of two less costly soil properties functions that can be used in conjunction with a series of assumptions and estimation methodologies. The two laboratory tests involve measurement of the: (i) gravimetric water content versus soil suction, referred to as the soil-water characteristic curve (w-SWCC) and (ii) water content versus void ratio, referred to as the shrinkage curve (SC). These two unsaturated soil property relationships can be used along with saturated soil properties to extend unsaturated soil properties over the full range of soil suctions. "Estimation procedures" have been developed and verified for all physical properties of interest in unsaturated soil mechanics. The use of estimation procedures has meant that the geotechnical engineer must operate within a new paradigm. The new paradigm provides sufficient accuracy for most geotechnical engineering applications. The net result is an increased decision-making capability for geotechnical engineers. [ABSTRACT FROM AUTHOR]

Fast-moving landslides (i.e., debris/earth flows) are often caused by heavy rainfall occurring in small areas, and are not predictable. On the other hand, innovative methods for geomechanical characterization, numerical analysis, and modeling are required to attempt to reproduce a given debris/earth flow event. As our capability to reproduce very complex phenomena increases, we can improve our prevention approaches. In this paper, a debris flow event that occurred in the Enna area (Sicily) is described. Starting from the study of the geological framework and the historical background, this research focused on the causes that triggered the landslide. In situ and laboratory tests, including geophysical investigations and triaxial tests in unsaturated conditions, were carried out to investigate the factors affecting the dynamics of the event. This study gives us better knowledge of the mechanical and hydraulic properties that can be used to model these events, to assess the most appropriate strategies for the prevention and mitigation of related risks. [ABSTRACT FROM AUTHOR]

Climate change is known to cause alterations in weather patterns and disturb the natural equilibrium. Changes in climatic conditions lead to increased environmental stress on embankments, which can result in slope failures. Due to wetting–drying cycles, expansive clayey soil often swells and shrinks, and matric suction is a major factor that controls the behavior. Increased temperature accelerates soil evaporation and drying, which can cause desiccation cracks, while precipitation can rapidly reduce soil shear strength. Desiccated slopes on embankments built with such soils can cause surficial slope failures after intense precipitation. This study used slope stability analysis to quantify how climate-change-induced extreme weather affects embankments. Historic extreme climatic events were used as a baseline to estimate future extremes. CMIP6 provided historical and future climatic data for the study area. An embankment was numerically modeled to evaluate the effect on slope stability due to the precipitation change induced by climate change. Coupled hydro-mechanical finite element analyses used a two-dimensional transient unsaturated seepage model and a limit equilibrium slope stability model. The study found that extreme climatic interactions like precipitation and temperature due to climate change may reduce embankment slope safety. The reduction in the stability of the embankment due to increased precipitation resulting from different greenhouse gas emission scenarios was investigated. The use of unsaturated soil strength and variation of permeability with suction, along with the phase transition of these earthen embankments from near-dry to near-saturated, shows how unsaturated soil mechanics and the hydro-mechanical model can identify climate change issues on critical geotechnical infrastructure. [ABSTRACT FROM AUTHOR]

The hydrological and mechanical behavior of soil is determined by the moisture content, soil water (matric) potential, fines content, and plasticity. However, these parameters are often difficult or impractical to determine in the field. Remote characterization of soil parameters is a non-destructive data collection process well suited to large or otherwise inaccessible areas. A ground-based, field-deployable remote sensor, called the soil observation laser absorption spectrometer (SOLAS), was developed to collect measurements from the surface of bare soils and to assess the in-situ condition and essential parameters of the soil. The SOLAS instrument transmits coherent light at two wavelengths using two, continuous-wave, near-infrared diode lasers and the instrument receives backscattered light through a co-axial 203-mm diameter telescope aperture. The received light is split into a hyperspectral sensing channel and a laser absorption spectrometry (LAS) channel via a multi-channel optical receiver. The hyperspectral channel detects light in the visible to shortwave infrared wavelengths, while the LAS channel filters and directs near-infrared light into a pair of photodetectors. Atmospheric water vapor is inferred using the differential absorption of the on- and off-line laser wavelengths (823.20 nm and 847.00 nm, respectively). Range measurement is determined using a frequency-modulated, self-chirped, coherent, homodyne detection scheme. The development of the instrument (transmitter, receiver, data acquisition components) is described herein. The potential for rapid characterization of physical and hydro-mechanical soil properties, including volumetric water content, matric potential, fines content, and plasticity, using the SOLAS remote sensor is discussed. The envisioned applications for the instrument include assessing soils on unstable slopes, such as wildfire burn sites, or stacked mine tailings. Through the combination of spectroradiometry, differential absorption, and range altimetry methodologies, the SOLAS instrument is a novel approach to ground-based remote sensing of the natural environment. [ABSTRACT FROM AUTHOR]

Broad mountainous areas in the western Campania (southern Italy), where young pyroclastic deposits extensively outcrop, frequently experience rainfall-induced slope movements of different degrees of mobility, causing heavy damage and fatalities. Such landslides cannot be easily mitigated, and the implementation of physically based early warning systems is still not able to predict the post-failure evolution of slope movements and the exposed areas at risk. This paper is devoted to overcoming this limit. To this end, the mechanical characterization of pyroclastic soil, carried out through an extensive laboratory testing program, is presented and compared with those of two other ashy soils of different depositional mechanisms. The results show that the depositional mode influences soil properties; to begin with, it affects the unsaturated shear strength, whose intercept of cohesion is up to 5 kPa higher in ashes of flow deposition than in airfall ash deposits. The saturated undrained soil response allowed for the identification of different levels of susceptibility to the liquefaction of pyroclastic deposits, which is one of the main factors governing the post-failure evolution of landslides. Gathering all the acquired information, including saturated and unsaturated soil shear strength, permeability function, and water retention curves, into a soil database, it was possible to present all data under a unitary framework. Finally, the implementation of the proposed flowchart for a simplified assessment of post-failure evolution to be employed in regional early warning systems can enhance our knowledge of the areas at risk. [ABSTRACT FROM AUTHOR]

Natural and artificial slopes are frequently constituted, at least in part, by soils in unsaturated conditions. The authors discuss possible strategies to determine the hydro-mechanical parameters that can be used for a numerical analysis of such phenomena and the role of matric suction, which reflects soil infiltration, in the investigation of rainfall-induced landslides. Castelli et al. [[3]] report experimental data derived from laboratory and site tests, carried out with the aim of evaluating soil parameters for debris-flow run-out simulations. [Extracted from the article]

The analysis of the behavior of soil and foundations when the piles in offshore areas are subjected to long-term lateral loading (wind) is one of the major problems associated with the smooth operation of superstructure. The strength of the pile-soil system is influenced by variations in the water content of the soil. At present, there are no studies carried out analyzing the mechanical and deformational behavior of both the material of the laterally loaded piles and soil with groundwater level as a variable. In this paper, a series of 1-g model tests were conducted to explore the lateral behavior of both soil and monopile under unidirectional cyclic loading, based on the foundation of an offshore wind turbine near the island. The influence of underground water level and cyclic load magnitude on the performance of the pile–soil system was analyzed. To visualize the movements of soil particles during the experimental process, particle image velocimetry (PIV) was used to record the soil displacement field under various cyclic loading conditions. The relationship curves between pile top displacement and cyclic steps, as well as the relationship curves between cyclic stiffness and cyclic steps, were displayed. Combined with fractal theory, the fractal dimension of each curve was calculated to evaluate the sensitivity of the pile–soil interaction system. The results showed that cyclic loading conditions and groundwater depth are the main factors affecting the pile–soil interaction. The cyclic stiffness of the soil increased in all test groups as loading progressed; however, an increase in the cyclic load magnitude decreased the initial and cyclic stiffness. The initial and cyclic stiffness of dry soil was higher than that of saturated soil, but less than that of unsaturated soil. The ability of the unsaturated soil to limit the lateral displacement of the pile decreased as the depth of the groundwater level dropped. The greater the fluctuation of the pile top displacement, the larger the fractal dimension of each relationship curve, with a variation interval of roughly 1.24–1.38. The average increment of the cumulative pile top displacement between each cycle step following the cyclic loading was positively correlated with fractal dimension. Based on the PIV results, the changes in the pile–soil system were predominantly focused in the early stages of the experiment, and the short-term effects of lateral cyclic loading are greater than the long-term effects. In addition, this research was limited to a single soil layer. The pile–soil interaction under layered soil is investigated, and the results will be used in more complex ground conditions in the future. [ABSTRACT FROM AUTHOR]

A new wicking geotextile is proposed to control the water content of fine-grained soil subgrade. By comparing the spatial distribution of volumetric water content and matric suction before and after the installation of the wicking geotextile, the effectiveness of the geotextile in controlling the subgrade humidity is evaluated. Firstly, the hydraulic parameters of the wicking geotextile are obtained through laboratory tests using a pressure plate apparatus. Then, a numerical model for water flow in the subgrade is established using COMSOL to obtain the spatial distribution characteristics of humidity in the subgrade under different groundwater levels (2~8 m). The results show the wicking geotextile exhibits strong hydrophilicity, low water retention, and high horizontal permeability. Compared to the subgrade without geotextile, the water content of the soil above the geotextile decreases significantly by 7.6~9.6% at groundwater levels of 4~8m, while the saturation decreases by 18.3~23.0%, and the matric suction increases by 2~2.3 times. The wicking fabric functions as an effective drainage material to serve as a capillary barrier in the cross-plane direction and an effective drainage tunnel to transport water in the in-plane direction. The dynamic resilient modulus of the subgrade increases by 23.2~43.6%. The wicking geotextile effectively absorbs and drains weakly bound water in unsaturated soil due to the matric suction difference and its horizontal drainage capacity to improve the bearing capacity of the subgrade. It suggests that using wicking geotextile for drainage and reinforcement in fine-grained soil subgrades with groundwater levels ranging from 4 to 8 m is beneficial. [ABSTRACT FROM AUTHOR]

The study of analytical solutions for the bearing capacity of reinforced soil foundations is a very important topic in engineering mathematics. Existing evaluations of the foundation-bearing capacity on reinforced soils are based on dry conditions, while many foundations are located on unsaturated soils in real engineering. In this paper, a new formula for the bearing capacity of reinforced strip footings on unsaturated soils is presented. Two sliding failure mechanisms are constructed based on the position of the reinforcement layer relative to the sliding surface. The distribution of apparent cohesion in the depth direction is calculated by considering the effect of matrix suction. By additionally considering the work conducted by the reinforcement and the contribution of the apparent cohesion, the bearing capacity formula is obtained using the upper bound theorem of limit analysis. The bearing capacity solution is obtained by adopting the sequential quadratic programming (SQP) algorithm. Comparing the results under two failure mechanisms, the optimal bearing capacity and the optimal embedment depth of reinforcement are obtained. The results of this paper are consistent with those of the existing literature. Finally, the effects of reinforcement embedment depth, effective internal friction angle, uniform load, and unsaturated soil parameters on the optimal bearing capacity are investigated through parametric analysis. This paper provides useful recommendations for the engineering application of reinforced strip footings on unsaturated soils. [ABSTRACT FROM AUTHOR]

Investigation of the swelling pressure of buffer/backfill materials is a critical aspect in the design of high-level radioactive waste (HLW) disposal repositories. In this study, to clarify the swelling pressure–suction relation for compacted bentonite upon the hydration path, constant-volume swelling pressure tests with suction control were conducted. The swelling pressure–suction curves indicated that the swelling pressure of the specimens increased significantly with increasing dry density, while the shape of the curves during hydration depended on the dry density. Moreover, the swelling pressure–suction curves exhibited a distinction between unsaturated and saturated segments divided by the critical saturated state (CSS) curve, which proves the unique existence of a CSS curve in the stress space independent of the stress path. With the introduction of the CSS curve into the s–p space, the conventional stress space of unsaturated soil could expand to that of unsaturated expansive soil. The results obtained in this study could provide the mechanical parameters for the construction of disposal repositories. In addition, the stress space with CSS curve proposed in this study provides a new approach to building constitutive models of bentonite materials. [ABSTRACT FROM AUTHOR]

This case study focuses on the area of El Plateado near the city of Loja, Ecuador, where landslides with a high impact on infrastructures require monitoring and control. The main objectives of this work are the characterization of the landslide and the monitoring of its kinematics. Four flights were conducted using a remotely piloted aerial vehicle (RPAS) to capture aerial images that were processed with SfM techniques to generate digital elevation models (DEMs) and orthoimages of high resolution (0.05 m) and sufficient accuracy (below 0.05 m) for subsequent analyses. Thus, the DEM of differences (DoD) and profiles are obtained, but a morphometric analysis is conducted to quantitatively characterize the landslide's elements and study its evolution. Parameters such as slope, aspect, topographic position index (TPI), terrain roughness index (TRI), and topographic wetness index (TWI) are analyzed. The results show a higher slope and roughness for scarps compared to stable areas and other elements. From TPI, slope break lines have been extracted, which allow the identification of landslide features such as scarps and toe tip. The landslide shows important changes in the landslide body surface, the retraction of the main scarp, and advances of the foot. A general decrease in average slope and TRI and an increase in TWI are also observed due to the landslide evolution and stabilization. The presence of fissures and the infiltration of rainfall water in the unsaturated soil layers, which consist of high-plasticity clays and silts, contribute to the instability. Thus, the study provides insights into the measurement accuracy, identification and characterization of landslide elements, morphometric analysis, landslide evolution, and the relationship with geotechnical factors that contribute to a better understanding of landslides. A higher frequency of the RPAS surveys and quality of geotechnical and meteorological data are required to improve the instability analysis together with a major automation of the GIS procedures. [ABSTRACT FROM AUTHOR]

The number of random fields required to capture the spatial variability of soil properties and their impact on the performance of geotechnical systems is often varied. However, the number of random fields required to obtain higher-order statistical moments of model outputs has not yet been studied. This research aims to investigate the number of Monte Carlo simulations needed to achieve stationary higher-order statistics of a pore pressure head in an unsaturated soil slope under steady-state infiltration. The study recommends using at least 500 Monte Carlo samples for the probabilistic analysis of geotechnical engineering models. A more conservative choice for up to second-moment analysis is 1000 samples. The analysis reveals significant variations in skewness, which become stationary for all mesh grids when the number of samples exceeds 15,000. Kurtosis stabilizes only when the number of samples reaches 25,000. The pore pressure head in the unsaturated zone is less uncertain. Additionally, the probability density function of the pore pressure head follows a leptokurtic distribution. [ABSTRACT FROM AUTHOR]

The traditional Tempe cell can be used to adequately determine the soil water characteristic curve (SWCC) for soils that do not undergo significant volume change as matric suction is increased, such as coarse-grained material such as sand with a low air entry value (AEV) (<500 kPa). When soils undergo substantial volume change as soil suction increases, such as fine-grained silts, clays, and oil sands tailings material, the soils need to be tested with distinctly different methods involving two apparatuses when using the Tempe cell. A single-step Tempe cell technique was developed and tested to measure the geotechnical and unsaturated properties of oil sands tailings samples. A series of nine Tempe cells were simultaneously used to measure the geotechnical and unsaturated soil properties of untreated fluid fine tailings (FFT) and treated flocculated centrifuged tailings (FCT). The results of the single-step Tempe cell technique provide several useful engineering functions relating matric suction to water content (SWCC), void ratio (volume change), solids content, and undrained shear strength. Both the traditional and single-step Tempe cell techniques yield comparable SWCC results, but the single-step Tempe cell yields result about three times faster than the traditional Tempe cell. The geotechnical results indicate that both the solids content and undrained shear strength of the FCT are greater than those of the untreated FFT and this indicates that flocculation and centrifugation increase solids content and undrained shear strength of the treated samples. Furthermore, the results indicate that the FFT starts at higher fine void ratio than the FCT and loses more water (volume change) at matric suctions lower than 7 kPa. Beyond 7 kPa, the compressions of both samples become the same. The single-step Tempe cell technique is, however, labor-intensive. The number of Tempe cell can be reduced to six depending on the starting load of the test. The use of the single-step Tempe cell technique in providing fast estimates of SWCC and geotechnical properties for oil sands tailings will be attractive to practitioners who intend to incorporate matric suction in oil sands geotechnical engineering problems. [ABSTRACT FROM AUTHOR]

Rainfall-induced landslides represent a severe hazard around the world due to their sudden occurrence, as well as their widespread influence and runout distance. Considering the spatial variability of soil, stochastic analysis is often conducted to give a probability description of the runout. However, rainfall-induced landslides are complex and time-consuming for brute-force Monte Carlo analyses. Therefore, new methods are required to improve the efficiency of stochastic analysis. This paper presents a framework to investigate the influence and runout distance of rainfall-induced landslides with a two-step simulation approach. The complete process, from the initialization of instability to the post-failure flow, is simulated. The rainfall infiltration process and initialization of instability are first solved with a coupled hydro-mechanical finite element model. The post-failure flow is simulated using the coupled Eulerian–Lagrangian method, wherein the soil can flow freely in fixed Eulerian meshes. An equivalent-strength method is used to connect two steps by considering the effective stress of unsaturated soil. A rigorous method has been developed to accurately quantify the influence and runout distance via Eulerian analyses. Several simulations have been produced, using three-dimensional analyses to study the shapes of slopes and using stochastic analysis to consider uncertainty and the spatial variability of soils. It was found that a two-dimensional analysis assuming plain strain is generally conservative and safe in design, but care must be taken to interpret 2D results when the slope is convex in the longitudinal direction. The uncertainty and spatial variability of soils can lead to the statistic of influence and runout distance. The framework of using machine-learning models as surrogate models is effective in stochastic analysis of this problem and can greatly reduce computational effort. [ABSTRACT FROM AUTHOR]

Expansive soil is a kind of unsaturated soil that is rich in hydrophilic clay minerals. The shallow slope stability of expansive soil is one of the important research topics in geotechnical engineering. However, there are no suitable methods for analyzing the shallow slope stability of expansive soil. Hence, this paper proposes a new method based on a coupling effect of saturation and expansion for analyzing the shallow slope stability. Especially, the coupling effect of saturation and expansion is introduced in detail, and used to further study the shallow slope stability. With the described coupling effect and the infinite slope, a formula calculating the overlying load of the shallow soil is established by the symmetrical limited expansion along the slope and perpendicular to the plane. Moreover, a calculation model for the factor of safety is presented according to the limit equilibrium method. The experiments are designed to demonstrate the feasibility and effectiveness of the proposed analysis method for the shallow stability of newly excavated and newly filled expansive soil slopes by rainfall. In the present study, the moisture content and shear strength of the shallow expansive soil slope are investigated, and the factor of safety is calculated. The results also show that the initial moisture content has an important influence on the shallow stability in terms of the two expansive slopes previously mentioned. [ABSTRACT FROM AUTHOR]

Clayey sand is widely distributed and commonly encountered in geotechnical engineering practice. To understand its bearing capacity behavior under unsaturated conditions, plate load tests are performed on sand–kaolin mixture samples with varying water tables. The distributions of suction and volumetric water content with depth are measured by vibrating wire piezometers and soil moisture sensors, respectively. It is shown by the test results that the bearing capacity increases when the water table in the soil sample drops. The influence of suction on the bearing capacity is found to be dependent on the height of the water table and the hydraulic loading history of the soil sample. The plate load test results are interpreted using bearing capacity equations. Good agreement is obtained between measured and calculated bearing capacities. This study provides a simple method to estimate the bearing capacity of in situ unsaturated soil foundations. [ABSTRACT FROM AUTHOR]

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

There has been a proliferation of equations proposed to describe the unsaturated shear strength envelope going back to the 1970s. However, there have been limited studies to verify the suitability of one unsaturated shear strength equation over another. Most proposed shear strength equations have attempted to relate the shear strength of an unsaturated soil to some aspect(s) of the soil–water characteristic curve (SWCC). Estimation procedures have generally focused on using that of air-entry value (AEV) as defined by the drying (or desorption) branch of the degree of saturation SWCC (S-SWCC). This paper studies the suitability of using two "anchor points" (or reference points) along the drying S-SWCC to estimate the unsaturated soil shear strength function. The anchor points referred to are the air-entry value (AEV) of the soil and the "residual suction point" of the soil defined in terms of the S-SWCC. Shear strength conditions associated with both so-called anchor points are used as "boundary conditions" that should be satisfied when estimating the shear strength function for unsaturated soils. Past research laboratory measurements published in the research literature are used as part of the verification process for this study. [ABSTRACT FROM AUTHOR]

The interaction between a laterally loaded pile and the surrounding soil is typically limited to the shallower soil layer. Often, this zone is above the water table and therefore the interaction takes place under unsaturated conditions. The available evidence is scarce but suggests that unsaturated conditions play a major role on the pile's response. The actual mechanisms governing the soil–pile interaction under unsaturated soil conditions are not understood entirely, and this paper provides a useful insight on this topic. The analysis is carried out with a fully coupled three-dimensional numerical model, the soil behaviour is simulated with a Modified Cam Clay Model extended to unsaturated conditions. The model accounts for the increase in stiffness and strength of unsaturated soils as well as the volumetric collapse upon wetting. The constitutive model is calibrated on the laboratory data and validated against centrifuge data with satisfying agreement. The results highlight the substantial differences in the soil reaction against the pile depending on different water saturation profiles. The study also shows that the influence of unsaturated conditions on the pile response increases as the pile's flexibility increases. Comparing the findings with currently available design methods such as the p-y curves, it is found that these do not adequately describe the unsaturated soil reaction against the pile, which opens the door for new research in the field. The proposed numerical model is a promising tool to further investigate the mechanisms underlying the soil–pile interaction under unsaturated soils. [ABSTRACT FROM AUTHOR]

At present, there are only a few existing models that can be used to predict the relative permeability of unsaturated soil under deformations, and the calculation process is relatively complex. In order to fit the measured value of the relative permeability coefficient of unsaturated soil before deformation, this work employs the simplified unified model of the relative permeability coefficient of unsaturated soil, and it obtains the index λ before deformation of the soil. In addition, the value of index λ remains unchanged before and after deformation. Based on the actual measured value of the soil–water characteristic curve before deformation, the air-entry value prediction model is used to predict the air-entry value of soil with different initial void ratios. The relative permeability coefficient of unsaturated soil is then conveniently predicted using the graphical method in combination with the simplified unified model. The method is validated by using the test data of silt loam, sandy loam, and unconsoildated sand. The results show that the predicted results are consistent with the measured values. The prediction method in this paper is simple and overcomes the limitations associated with the determination of the index λ. It expands the application range of the unsaturated relative permeability coefficient model while improving the accuracy of predictions. [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]

Definition: This entry presents the theoretical fundamentals, the mathematical formulation, and the numerical solution for the problem of desiccation cracks in clayey soils. The formulation uses two stress state variables (total stress and suction) and results in a non-symmetric and nonlinear system of transient partial differential equations. A release node algorithm technique is proposed to simulate cracking, and the strategy to implement it in the hydromechanical framework is explained in detail. This general framework was validated with experimental results, and several numerical examples were published at international conferences and in journal papers. [ABSTRACT FROM AUTHOR]

Seasonally frozen soil where uneven freeze–thaw damage is a major cause of highway deterioration has attracted increased attention in China with the rapid development of infrastructure projects. Based on Darcy's law of unsaturated soil seepage and heat conduction, the thermal–hydraulic–mechanical (THM) coupling model is established considering a variety of effects (i.e., ice–water phase transition, convective heat transfer, and ice blocking effect), and then the numerical solution of thermal–hydraulic fields of subgrade can be obtained. Then, a new concept, namely degree of freeze–thaw damage, is proposed by using the standard deviation of the ice content of subgrade during the annual freeze–thaw cycle. To analyze the freeze–thaw characteristics of highway subgrade, the model is applied in the monitored section of the Golmud to Nagqu portion of China National Highway G109. The results show that: (1) The hydrothermal field of subgrade has an obvious sunny–shady slopes effect, and its transverse distribution is not symmetrical; (2) the freeze–thaw damage area of subgrade obviously decreased under the insulation board measure; (3) under the combined anti-frost measures, the maximum frost heave amount of subgrade is significantly reduced. This study will provide references for the design of highway subgrades in seasonally frozen soil areas. [ABSTRACT FROM AUTHOR]

This study describes the development of a landslide monitoring system for the purpose of reducing damages caused by landslides in natural terrain. The system was developed to analyze the effects of landslide-inducing rainfall and the behavior of slopes through 12 monitoring stations that are distributed across eight national parks in Korea. Several sensors and a data acquisition equipment to monitor landslide were installed in each station. The composition of the system and its operating program were designed to efficiently manage the sizeable amounts of real-time monitoring data that are collected from the various stations. To test the potential of the developed system for reliable landslide hazard evaluations, data measured over a five-year period by the two monitoring stations in Jirisan National Park were analyzed. Subsequently, the suction stress of the soil over the monitoring period was calculated by applying laboratory test result of the geotechnical and unsaturated soil properties in the analysis domain area. The infinite slope stability analysis combined with an effective stress concept based on the suction stress was applied to calculate the factor of safety. This method also enabled the temporal and quantitative evaluation of slope stability in natural terrain. In addition, based on the monitoring and slope stability analysis results, an analysis for the spatial classification of landslide hazards was conducted. The analysis results quantitatively and statistically demonstrated that 98% of historical landslide initiation areas were classified as high hazard levels. [ABSTRACT FROM AUTHOR]

The evaluation of riverbank stability often represents an underrated problem in engineering practice, but is also a topical geotechnical research issue. In fact, it is certainly true that soil water content and pore water pressure distributions in the riverbank materials vary with time, due to the changeable effects of hydrometric and climatic boundary conditions, strongly influencing the bank stability conditions. Nonetheless, the assessment of hydraulic and mechanical behavior of embankments are currently performed under the simplified hypothesis of steady-state seepage, generally neglecting the unsaturated soil related issues. In this paper, a comprehensive procedure for properly defining the key aspects of the problem is presented and, in particular, the soil characterization in partially saturated conditions of a suitably compacted mixture of sand and finer material, typical of flood embankments of the main river Po tributaries (Italy), is reported. The laboratory results have then been considered for modelling the embankment performance under transient seepage and following a set of possible hydrometric peaks. The outcome of the present contribution may provide meaningful geotechnical insights, for practitioners and researchers, in the flood risk assessment of river embankments. [ABSTRACT FROM AUTHOR]

Deep slope movements and, eventually, slope failure, have been often interpreted to be due to slope-vegetation-atmosphere interaction on slopes formed of clayey materials in the Italian Southern-Eastern Apennines, as reported in the literature. Such slopes are generally formed of flysch, within which clay is the main lithotype. Such clays are characterized by a disturbed meso-fabric, as an effect of the intense tectonics. The paper presents the results of coupled hydromechanical numerical analyses of the slope-vegetation-atmosphere interaction for a clay slope representative for the geomechanical scenario where such climate-induced deep slope movements have been repeatedly recorded. In the analyses, different model initialization procedures and parameter values were adopted. The comparison of the numerical results with the site data is aimed at assessing the effects of the soil-vegetation-atmosphere interaction taking place in the top strata of the slope, on the stress-strain conditions across the whole slope, and on the slope stability. The comparison between the numerical results of the analyses carried out entailing different initialization stages are intended to evaluate the influence of such a stage on the model predictions. It is found that only when the slope model initialization accounts for the slope loading history, developed over geological time, the numerical predictions get close to the site observations. In such case, the numerical results confirm that deep movements consequent to progressive failure may take place in clay slopes due to the slope-vegetation-atmosphere interaction. [ABSTRACT FROM AUTHOR]

Soil-water characteristic curve (SWCC) is a significant prerequisite for slope stability analysis involving unsaturated soils. However, it is difficult to measure an entire SWCC over a wide suction range using in-situ or laboratory tests. As an alternative, the Arya and Paris (AP) model provides a feasible way to predict SWCC from the routinely available particle-size distribution (PSD) data by introducing a scaling parameter. The accuracy of AP model is generally dependent on the calibrated database which contains test data collected from other sites. How to use the available test data to determine the scaling parameter and to predict the SWCC remains an unresolved problem. This paper develops a Bayesian approach to predict SWCC from PSD. The proposed approach not only determines the scaling parameter, but also identifies fitting parameters of the parametric SWCC model. Finally, the proposed approach is illustrated using real data in Unsaturated Soil Database (UNSODA). Results show that the proposed approach provides a proper prediction of SWCC by making use of the available test data. Additionally, the proposed approach is capable of predicting SWCC in the high suction range, allowing engineers to obtain a complete SWCC in practice with reasonable accuracy. [ABSTRACT FROM AUTHOR]

The final disposal of high-level nuclear waste in many countries is preferred to be in deep geological repositories. Compacted bentonites are proposed for use as the buffer surrounding the waste canisters which may be subjected to both thermal and hydraulic loadings. A significant increase in the temperature is anticipated within the buffer, particularly during the early phase of the repository lifetime. In this study, several non-isothermal and non-isothermal hydraulic tests were carried on compacted MX80 bentonite. Compacted bentonite specimens (water content = 15.2%, dry density = 1.65 Mg/m³) were subjected to a temperature of either 85 or 150 °C at one end, whereas the temperature at the opposite end was maintained at 25 °C. During the non-isothermal hydraulic tests, water was supplied from the opposite end of the heat source. The temperature and relative humidity were monitored along predetermined depths of the specimens. The profiles of water content, dry density, and degree of saturation were established after termination of the tests. The test results showed that thermal gradients caused redistribution of the water content, whereas thermo-hydraulic gradients caused both redistribution and an increase in the water content within compacted bentonites, both leading to development of axial stress of various magnitudes. The applied water injection pressures (5 and 600 kPa) and temperature gradients appeared to have very minimal impact on the magnitude of axial stress developed. The thickness of thermal insulation layer surrounding the testing devices was found to influence the temperature and relative humidity profiles thereby impacting the redistribution of water content within compacted bentonites. Under the influence of both the applied thermal and thermo-hydraulic gradients, the dry density of the bentonite specimens increased near the heat source, whereas it decreased at the opposite end. The test results emphasized the influence of elevated temperatures (up to 150 °C) on the thermo-hydro-mechanical response of compacted bentonites in the nuclear waste repository settings. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

Proper soil water retention curves (SWRCs) are necessary for a fair analysis of groundwater flow in unsaturated slopes. The question is whether hydraulic parameters operating in situ can be reliably determined from laboratory tests or physical prototype models in order to interpret and predict soil water distributions in the field. In this paper, some results obtained by tests at different scales (testing on laboratory specimens and a physical prototype) are presented to explore the hydraulic behavior of pyroclastic soils. A theoretical interpretation of the observed behavior in the laboratory and using a physical prototype is proposed by adopting the hysteretic model of Lenhard and Parker. For each tested soil, the main hysteretic loop determined by interpreting experimental tests (at laboratory and prototype scales) overlaps with paths detected by coupling the field measurements of matric suction and water content collected at the site at the same depth. From these results, the physical prototype (medium scale) and the soil specimen (small scale) seem to be acceptable for determinations of SWRC, provided that the air entrapment value is well known. [ABSTRACT FROM AUTHOR]

This research aims to propose the use of spectral analysis of surface wave (SASW) tests along with in-situ suction measurements for non-destructive determination of shrinkage cracks. The underlying principle behind this proposed method is that, while suction and the small-strain shear modulus are positively correlated for intact samples, this is not the case for cracked ground. A series of SASW tests were performed on a clay embankment at different periods, during which the suction, modulus, and shrinkage crack depth varied seasonally. The soil water retention curve (SWRC) of the undisturbed sample collected from the cracked zone was determined, which related the suction-to-moisture content and void ratio of the soil. A free-free resonant frequency (FFR) test in the lab was conducted to determine the small-strain shear modulus (G0) at various moisture contents. The small-strain moduli from the SASW tests on the intact ground were generally higher than those from the FFR tests due to the effect of confining stress. A drop in the small-strain modulus determined using the SASW test was observed as an increase in suction-induced cracks and it relieved the horizontal stress. The crack depth measured in the field was then modelled using a semi-empirical procedure that can be used to predict crack depth relative to suction. [ABSTRACT FROM AUTHOR]

Air-fall pyroclastic deposits on steep slopes in Campania (Southern Italy) are periodically subjected to rainfall-induced landslides that may evolve into catastrophic flowslides. To protect built-up areas, early warning systems (EWSs) have been implemented which are essentially based on pluviometric thresholds or models unable to accurately monitor the physical phenomena responsible for flowslide generation in pyroclastic deposits. Over the last 20 years, landslides with no evolution in flows occurred in this area and the alarms generated by existing EWSs in the cases of rainfall were both false and highly costly, thus eroding public trust in EWSs. To improve existing EWSs, two complex models for pyroclastic soils (Cervinara and Sarno) are proposed in this paper. These two models allow correct simulation of the physical processes, such as saturation increase due to rainwater infiltration and mechanical degradation as far as undrained instability, which govern postfailure evolution. The paper concludes with the presentation of a framework proposal to be used in defining a soil database, as well as a framework for flowslide generation forecast to be used for implementation within EWSs. [ABSTRACT FROM AUTHOR]