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Torrential rainfall is one of the significant factors contributing to shallow slope failures. This study discusses the effect of the different filter gabion sizes against slope instability due to torrential rainfall numerically. The experimental investigation of effects of different filter gabion sizes was extensively investigated. The investigations were carried out in a two-dimensional domain using the saturated–unsaturated consolidation analysis method with an elastoplastic model for unsaturated soils considering two suction effects. Four different sizes of filter gabion were used in this investigation. The results of numerical simulations of slope model tests showed that, as rainwater accumulated at the slope toe, the pore water pressures (PWPs) near the slope toe increased to be positive or larger than the surface soil’s air entry PWP. The phreatic surface that appeared on the slope surface as the positive PWPs increased with the elapsed time. In the simulations, large displacement, considered surface failure, started once the phreatic surface reached the slope surface. The simulated phreatic surface appearance time on the slope surface extended as the filter gabion size became larger. Nevertheless, the results showed that the applied analysis method is powerful and reliable for simulating slope failure behaviours and their proper countermeasures under torrential rainfall. [ABSTRACT FROM AUTHOR]

It is important to understand the shear behavior of oil-contaminated soils in many geotechnical problems, such as the analysis of pipelines and storage tanks affected by oil leakage. In this study, the shear behavior of a silt permeated with silicone oil/water (denoted by OS and WS, respectively) was investigated through direct shear tests. The residual shear strength was measured at various degrees of liquid saturation (65–100%) and net normal stresses (50 to 400 kPa). The oil and water retention curves were measured and used to explain the shear behavior. The results show that the residual friction angle of OS is 30% larger than that of WS. This is likely because OS and WS show aggregated and matrix structures, respectively, as evidenced by the Scanning Electron Microscope (SEM) test. The former structure includes more particle contacts than the latter one. These structural differences can be attributed to varying fluid properties, such as wettability, viscosity, and dielectric constant. When the volumetric degree of water saturation decreases, the total cohesion of WS increases substantially because of the strengthening effects of air-water interfaces on the soil skeleton. In contrast, the degree of oil saturation has minuscule effects on these variables, mainly because OS has a low oil retention ability and the air-oil suction remains very low (i.e., less than 2 kPa) during the desaturation process. [ABSTRACT FROM AUTHOR]

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

To explore the effect of suction stress on effective stress, shear strength, and the relationship with the soil–water characteristic curve of unsaturated reticulated red clay, a series of triaxial compression tests, soil–water retention tests, and particle flow code 3D (PFC3D) simulation under different matric suction and net confining pressure was conducted. The suction stress characteristic curve (SSCC) and soil–water characteristic curve (SWCC) of reticulated red clay was obtained. These test results show that the effective stress based on suction stress converged to the unique failure envelope and that there is an internal relationship between SSCC and SWCC. The simulation results of PFC3D were very close to the laboratory measurements in the observation peak value and variation laws of the stress–strain curve. Based on the discrete element method, PFC3D takes rigid spheres or disks as basic units and simulates practical problems through the motion and interaction of the basic units. The parameters obtained by inversion of the stress–strain curve of PFC3D simulation were basically the same as laboratory measurements. Therefore, PFC3D simulation can be used to obtain SSCC and SWCC. These results are expected to promote the development of unsaturated soil mechanics. [ABSTRACT FROM AUTHOR]

Shear strength of unsaturated soils is an important engineering property that is required for addressing geotechnical problems, the prediction of which remains to be a challenging task for design engineers due to the complex interaction problem. This study presents a new shear strength equation based on the micromechanical model and the disturbed state concept for unsaturated soils. The original point of this study is considering the solid contact area ratio which was neglected in most of the existing equations. Using the proposed model, the non-linear relationship between the matric suction, saturation degree, and the shear strength of unsaturated soils are described. Validation of the shear strength model was verified against the experimental data and several current models on six different types of soils. The results indicate that the proposed model has a good performance in predicting the shear strength of unsaturated soils, and generally is better than other existing models. In response to varying climatic conditions, the analytical model was then extended to consider the effect of temperature on the shear strength of unsaturated soils. The comparison between predicted and measured results was carried out on compacted silt for three different temperatures. The results show that the proposed model is capable of accurately predicting changes in unsaturated shear strength as a function of temperature. [ABSTRACT FROM AUTHOR]

Masado is a completely decomposed granite soil widely distributed in western Japan. One of the major causes of the failure of a slope, either made of natural Masado (cut slope) or manmade Masado (reclaimed slope), is heavy rainfall. It is therefore essential to investigate the hydraulic/mechanical behaviour of saturated/unsaturated Masado and provide a fundamental database for constitutive modelling. In this paper, a series of water retention, oedometer and triaxial tests on unsaturated Masado were conducted. The water retention properties of Masado were clarified first, and then, oedometer and triaxial compression tests were conducted under various hydraulic/mechanical conditions. Different from normal loading patterns, i.e., constant suction or constant net confining stress under different drainage/ventilation condition in triaxial tests, a very special loading pattern named as constant degree of saturation was designed and conducted for the triaxial tests, in order to determine if it could be a suitable state variable for a constitutive model. From the tests, it is found that the strength and deformation of Masado are greatly dependent on the interaction among the suction, the degree of saturation and the hydraulic/mechanical loading conditions. Based on the test results, it is possible for us to propose a constitutive model based on the rational state variables obtained in the tests conducted in the research. [ABSTRACT FROM AUTHOR]

The stability of unsaturated slopes under seismic loading has become an important issue over the past few years which is the indication of its practical significance in geoengineering. This study aims at exploring the seismic stability of unsaturated sandy clay and silty clayey sand slopes using 2D limit equilibrium analysis for different slope geometries and ground water levels. To this end, the pseudo-static and the displacement-based sliding block Newmark's approaches are employed for parametric studies by considering the records of the 2003 earthquake that devastated the city of Bam, in the southeast of Iran. In all analyses, the unsaturated state of the soil materials is taken into account in accordance with the changes in the matric suction profile within the soil body using the effective stress approach of unsaturated soil mechanics. The research outcomes, in terms of the safety factor, are then analyzed and compared for different examined conditions. The parametric study in this research considered the method of analysis, slope geometry, ground water conditions, and soil type as the most influential parameters on the factor of safety. The results revealed that considering the unsaturated parameters for the examined soils results in higher factor of safety in comparison with the corresponding cases in which unsaturated parameters are not adopted. A comparative study was also carried out based on a threshold safety factor, in which the safety factors obtained from all analyses, undergoing various conditions and loadings, provide a preliminary insight into the prospective seismic microzonation of unsaturated slopes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

During rainfall infiltration and fluctuations in the water level, the seepage process of a landslide changes between a saturated and unsaturated state. The resulting landslide deformations do not occur immediately, but experience a time development with typical rheological properties. Slip-surface soil is a special kind of soil formed during the sliding process of a landslide, and its creep properties reflect the landslide dynamics process. The author conducted an unsaturated creep test on the slip-surface soil of a large landslide to study the unsaturated creep properties of slip-surface soil under different stress levels and matric suction conditions. The results showed that the unsaturated creep process of slip-surface soil includes an instantaneous deformation and attenuation deformation stage, which are mainly affected by the matric suction and net confining pressure. The extended Burgers creep equation (M-2 K) for the triaxial test of unsaturated soil was derived based on the test results, and the rationality of the model was verified by comparing its predicted values with experimental data. [ABSTRACT FROM AUTHOR]

The stability of excavation in unsaturated soil is closely related to the variation of soil properties and matric suction. Determination of different failure modes and identifying the contribution of each one in an unsaturated soldier-piled excavation are the vital aspects of system reliability analysis. To address these issues, this paper provided a stochastic framework with a random elasto-plastic finite element–based program coded in MATLAB to evaluate the reliability indices of individual failure modes with considering the inherent uncertainty of real site soil properties and unsaturated state. In the next step, the sequential compounding method (SCM) was utilized to obtain the system reliability index by compounding the reliability indices of individual failure modes. Numerical results of a case study showed that in all failure modes, considering unsaturated state not only increases the mean value of factor of safety (FS) but also decreases the related standard deviation, which can be counted as a goal of reliability analysis. Among the reliability indices of the components, the most critical one is attributed to the lateral displacement. Furthermore, the safety ratio concerning the shear force has the maximum reliability index compared to the others. Moreover, based on the coefficient of variation (COV) of the components, it was found that the uncertainty of the soil parameters has the most significant effect on the global safety factor of the excavation. [ABSTRACT FROM AUTHOR]

Rainfall is one of the most significant natural factors for inducing landslides. The shallow saturated zone formed in slopes during rainfall is an important factor in inducing shallow landslides. In this paper, the characteristics and influencing factors of the saturated zone in soil slopes, and the relationship between the soil surface permeability coefficient and the rainfall intensity are analyzed. In this research, the equation that characterizes the rainfall intensity by the soil saturation is built based on the Mualem-van Genuchten model, which mathematically describes the soil water characteristics of unsaturated soils. Using this model, the target saturation of the slope surface during rainfall can be determined accurately, with an average deviation of − 0.008. Based on the equations, the innovative critical curve model of the rainfall intensity−time required to form the shallow saturated zone is established and calculated. The influence of the soil material, slope angle, rainfall intensity and antecedent rainfall on the critical curve for the shallow saturated zone is analyzed. The results indicate that for slopes of 30°~40°, it is easy for the shallow saturated zone to form, and the previous rainfall after more than 16 days has little impact on the critical curve. Furthermore, an innovative landslide prediction model, based on the critical curve for the shallow saturated zone and I-D modeling, is proposed to provide benchmarks for the prediction-specific characteristics of deep or shallow landslides, which is verified by the case of the Madiwan landslide. [ABSTRACT FROM AUTHOR]

Mechanical behavior of residual soils are studied by performing constant water content direct shear tests on reconstituted specimens using total stress analysis. The testing program involves initial degree of saturation (Sr) and applied normal stress as the control parameters. For three different soil samples, a total of 55 direct shear tests are conducted at Sr = 60%, 80%, and saturated conditions, in the normal stress range of 15–60 kPa. The results showed that shear strength increases with decreasing Sr and the relationship between shear strength and Sr is nonlinear. More dilative response is observed with decreasing Sr and decreasing applied normal stress. Maximum dilatancy and Sr relationship is nonlinear. At large displacements, samples prepared at different Sr levels showed similar ultimate shear resistances under similar applied normal stresses. It is concluded that Sr and applied normal stress dramatically influence the mechanical behavior of compacted residual soils studied. [ABSTRACT FROM AUTHOR]

In the tropical area, most shallow foundations and embankments are involved in dealing with unsaturated soils. Any collapse or settlement in unsaturated soils is determined by two independent stress variables: the applied stress and suction. The massive collapse related to suction changes may lead to unrecoverable damages to compacted earth fills both economic and human loss while this collapse has not been seen in the design. The aim of this study was to investigate the influence of stress states and matric suction on collapse behavior of an unsaturated Malaysian granitic residual soil grade V. A suction-controlled double wall triaxial test was carried out to obtain the axial strain of the compacted specimen during loading and wetting collapse. In addition, there has been an attempt to predict the collapse behavior of the soil by using Rotational Multiple Yield Surface Framework (RMYSF). During loading in imposed suction, the axial strain increased as deviator stress increased. When the soil was inundated and the imposed suction was decreased step by step down to zero, the axial strain was very small at suction close to residual suction. It gradually increased as suction decreased. A massive settlement around 1.92% of the initial height was observed while the soil was nearly saturated. The findings also proved that the RMYSF model is able to predict the collapse behavior of the tested soil sufficiently. [ABSTRACT FROM AUTHOR]

Liquefaction has been the major source of damage for structures and infrastructures in most recent earthquakes, as it induces loss of strength and stiffness in soils, resulting in settlement of buildings, landslides, and failure of pipelines and earth dams. Such a phenomenon is primarily associated with saturated cohesionless soils and strong-motion seismic events able to induce pore water pressure build-up. Even though the degree of soil saturation is strictly related to the oscillation of groundwater table and to the interaction flows (namely precipitation and evaporation) between the exposed surface and the atmosphere, the initial distribution of pore water pressure is commonly simplified in liquefaction potential analysis and the presence of unsaturated soil layers is almost always neglected. To try to fill this gap, this work investigates the effect of the initial distribution of pore water pressure on the liquefaction strength evaluation, considering the potential presence of unsaturated soil layers. The work uses as case-study a well-investigated inhabited levee damaged by the 2012 Emilia earthquake. The initial distribution of pore water pressure within the dyke is carried out by solving the Richards equation in steady-state and transient conditions, assuming as boundary conditions the evolution of potential flows recorded at the site in the time period including the earthquake event. The cyclic resistance of unsaturated soils is considered during the execution of dynamic analyses in effective stress conditions. Results of the analyses show that initial conditions influence the liquefaction strength of earth structure and that neglecting the cyclic resistance of unsaturated soils is not a conservative assumption. [ABSTRACT FROM AUTHOR]

Rainfall-induced slope failures in natural terrains are destructive natural disasters. Transport of fine particles may be induced by the rainwater seepage in a natural terrain slope comprising mixed coarse and fine particles. In this study, the interaction of internal erosion and infiltration in a soil slope is investigated. A coupled model of unsaturated flow and internal erosion is established. The effects of internal erosion on pore water pressure profiles and slope stability are studied. Parametric studies on erosion parameters and hydraulic parameters are conducted. The results of the numerical example show that internal erosion occurs mainly in the zone within the wetting front, which accelerates the advance of the wetting front and decreases the slope stability. The coefficient of erosion flux rate, βer of the erosion law, is the main factor that affects the internal erosion. The effect of erosion on the wetting front movement is more significant with large values of βer. The effects of parameters i* and αer are less significant when compared with βer. When the rainfall flux is equal or greater than the saturated coefficient of permeability, the influence of internal erosion on water infiltration and slope stability is significant. The effect of internal erosion can be neglected as long as the rainfall flux is less than the saturated coefficient of permeability. When the air-entry value of the soil is greater, the influence of internal erosion on infiltration and slope stability becomes less significant. [ABSTRACT FROM AUTHOR]

Water retention, unsaturated permeability, and deformation-failure characteristics induced by infiltration were studied by analyzing stress-dependent soil–water characteristics and the wetting stress path of unsaturated intact clayey loess in Baoji, China. Test results in Baoji and Heifangtai were compared. The results show the following: (1) The soil–water characteristic shape is related to stress, and suction decreases with water content; (2) the van Genuchten model is applicable to the clayey loess in Baoji, (3) the unsaturated permeability coefficient is related to stress, which increases with axial net stress; (4) under certain confining stress, hydraulic properties of loess vary considerably by region and, compared with loess in Heifangtai, the water retention capacity of clayey loess in Baoji is greater; (5) brittle deformation is dominant in Baoji but plastic deformation is dominant in Heifangtai; and (6) deformation behavior and the strength of loess is related to the confining stress, and the combined action of that stress and suction cause breakage of loess cementation bonds. The decrease of matric suction is not necessary for loess deformation, but the breakage of cementation bonds is the essence of the decreasing strength and final deformation failure of loess. [ABSTRACT FROM AUTHOR]

The effects of temperature on the behaviour of soils are of great concern in many geotechnical applications. This paper reports an experimental study that focused on the effect of temperature (between 20 and 50 °C) on the structural strength of a clayey soil using a modified micropenetrometer. Laboratory penetration tests using two penetration rates were conducted on highly overconsolidated specimens for which the degree of saturation ( S) and dry density ( ρ) were controlled. The results indicated that the penetration resistance (the total resistance, the friction and the end resistance) decreased with increasing temperature, suggesting that the structural strength of soil was softened by increasing the temperature. In addition, the rate of decrease in structural strength with increasing temperature was greater when the degrees of saturation was low and the dry density was high. Tests performed under cyclic heating and cooling revealed that subjecting the soil to this thermal cycling improves its structural strength. Moreover, increasing the rate of penetration caused higher penetration resistance, while its influence on the thermomechanical behaviour of the soil was only very slight. [ABSTRACT FROM AUTHOR]

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