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Precipitation is one of the most important factors inducing shallow slope failures, and the shallow slope covering bedrocks is prone to instability after heavy rainfall. In one-dimensional (1D) seepage–deformation coupling issues, permeability coefficient and moisture vary with matric suction in unsaturated soil. Combining mass conservation, Darcy's law, and elastic theory, an analytical solution for coupled seepage–deformation in unsaturated soil slopes during rainfall infiltration is derived using the Fourier integral transformation method. The analytical solution can be applied to a 1D seepage problem in a soil slope with flux at the top and impervious bedrock in the base under heavy precipitation, and is conducive to study infiltration into the slope under rainfall conditions. To validate the accuracy of the proposed analytical solution in this study, it is compared with monitored pore-water pressure data from the Gufenping Landslide in the red-bed region located in Nanjiang, Sichuan, China. The compared result shows a good consistency between the analytical solution and the measured results, with a minor relative error. Investigation of the parameters demonstrates that the water-level rise is closely related to the coupling, which is influenced by precipitation duration, precipitation intensity, soil properties, and slope angle. The bottom boundary of the slope is considered to be impermeable in this study, which leads to rainfall accumulation at the base over time, and the coupled effect becomes more pronounced at the bottom boundary. [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]

Landslides due to catastrophic weather events, especially heavy rainfall, have risen significantly over the last several decades, causing significant damage and affecting the health and livelihoods of millions of people. Using tree roots to bio-engineer shallow slopes has been proven to be a cost-effective, sustainable measure and thus has gained increasing popularity. As slope failure often occurs under heavy precipitation, it is important to understand the mechanical interactions in the soil matrix surrounding a root to better estimate the reinforcement capacity of a root system, especially as the soil undergoes wetting from drier conditions. However, very few studies of root reinforcements have considered the effects of degree of saturation on behaviour. In this study, steel wires are used as a root analogue to explore the impact of root geometry, soil dilation and soil saturation on the pull-out behaviour of a root and three commonly used unsaturated soil strength models have been used to interpret the pull-out results. It was found that roots with larger diameter did not contribute to additional resistance. Also, a linear relationship between degree of saturation and pull-out strength was identified over a large range of suctions and one of the unsaturated soil strength models seemed to provide a more reasonable interpretation. The results will help future bioengineering slope design by improving the understanding of soil-root interface behaviour, including the effect of root diameter in slippage failure and greater emphasis on the importance of taking degree of saturation into account in unsaturated soil strength models. [ABSTRACT FROM AUTHOR]

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

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

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

Geosynthetic-reinforced soil (GRS) walls using marginal soils can operate under unsaturated conditions depending on climate conditions and drainage inside the reinforced zone. Geocomposite reinforcements have been suggested to act as internal drainage layers, but their hydraulic behavior can also be strongly affected by climate conditions. Numerical analyses were conducted to observe the impact of four distinct tropical climate conditions (arid, semi-arid, humid subtropical and humid tropical) on suction profiles and stability of reinforced soil walls constructed using geogrid and geocomposite reinforcements. The climate simulation involved the incorporation of a soil-atmosphere interaction on water balance and on the unsaturated transient infiltration. Results indicate the GRS walls can operate under relatively high suction levels under arid climates in which cumulative evaporation overcomes infiltration. Any climate that has rainy seasons with consecutive rainfalls with intensities close to the infiltration capacity of soil and/or monthly cumulative precipitation higher than 200 mm/day led to critical conditions in terms of soil water saturation and stability. Under unsaturated conditions of soil, the drainage effectiveness of geocomposites is significantly reduced and adverse capillary break effects become critical. • Evaporation has a significant effect on stability of geosynthetic reinforced soil structures. • Unsaturated condition was kept in climates which the monthly precipitation does not overcome the evaporation. • The critical FOS was reached in climates with monthly cumulative precipitation of 200 mm. • The pattern of rainy season has a significant effect on the FOS. • Under the real climate conditions, the effectiveness of draining geocomposite reinforcements is significantly reduced. [ABSTRACT FROM AUTHOR]

Post-wildfire mass wasting is a major problem throughout many regions worldwide. Recent dramatic increases in global wildfire activities coupled with a shift in wildfire-prone elevation to higher altitudes raise the need to better predict post-fire rainfall-triggered landslides. Despite its importance, only a limited number of studies have investigated landslide susceptibility in areas hit by wildfires using hydromechanical models. However, most of these studies follow either qualitative or semi-quantitative approaches without explicitly considering the fire's effects on the impacted area's physical behavior. This study aims to develop and employ a physics-based framework to generate susceptibility maps of rainfall-triggered shallow landslides in areas disturbed by wildfire. A coupled hydromechanical model considering unsaturated flow and root reinforcement is integrated into an infinite slope stability model to simulate the triggering of shallow landslides from rainfall. The impact of fire is considered through its effects on soil and land cover properties, near-surface processes, and canopy interception. The developed model is then integrated into a geographic information system (GIS) to characterize the regional distribution of landslide potential and its variability considering topography, geology, land cover, and burn severity. The proposed framework was tested for a study site in Southern California. The site was burned in the San Gabriel Complex Fire in June 2016 and experienced widespread landsliding almost three years later following an extreme rainstorm in January 2019. The proposed framework could successfully model the location of observed shallow landslides. The model also revealed a significantly higher likelihood for slope failure in areas burned at moderate to high severities as opposed to unburned and low-burn severity areas. The findings of this study can be employed to predict the timing and general locations of rainfall-triggered shallow landslides following wildfires. • Novel physics-based framework developed to predict post-fire rainfall-triggered landslides. • Integration of hydromechanical and infinite slope stability models considering fire effects. • Successful modeling of observed shallow landslides in a Southern California study site. • Higher likelihood of slope failure in areas burned at moderate to high severities revealed. [ABSTRACT FROM AUTHOR]

Rainfall-induced soil slope failures are among the most frequent and widespread landslides in the world. The infiltration of water plays a critical role in the instability of slopes under rainfall conditions. By employing the physical models, field monitoring data becomes instrumental for the back analysis of soil parameters, providing critical insights into the mechanisms of slope instability. In this review, the initial section investigates the hydraulic characteristics of unsaturated soils, encompassing the soil-water characteristic curve, permeability function, and their variability nature. Subsequently, a comprehensive overview of infiltration models for unsaturated soil, coupled with their numerical and analytical solutions, is provided. The subsequent section systematically introduces physical model-based Bayesian parameter inference, with a particular emphasis on the formulations of likelihood functions. The exploration then shifts towards investigating the impacts of prior knowledge, likelihood, model structure, and assimilation approach on an unsaturated soil slope monitoring case. Finally, the limitations of the current approaches and future outlooks in this field are presented. [ABSTRACT FROM AUTHOR]

Pre-existing (open) cracks at slope crests, especially tension cracks, are a significant factor in the transition from shallow to deep-seated landslides in unsaturated soil slopes induced by rainfall. This paper presents an analytical framework to assess the stability of unsaturated soil slopes for deep-seated rotational failure considering arbitrary crack depths and unsteady infiltration, and its effectiveness is verified by previous literature and finite element analysis. A closed-form formula is proposed for the maximum depth of cracks in unsaturated soils by the stability of the vertical crack boundary. A parametric study is performed to investigate the impact of some key parameters on the stability of unsaturated soil slopes with cracks. Dimensionless stability charts are developed, which can calculate the factor of safety for the slopes with cracksof any depth and can reflect that the potential slip surfaces through or without passing through the crack tip. In addition, given the existence of a critical crack depth and maximum crack depth, with a ratio ranging from 0.5 to 0.6 based on the stability charts, deep-seated rotational failure is more likely to occur than failure caused by a crack boundary. [ABSTRACT FROM AUTHOR]

A two-layer hydro-mechanically coupled material point method has been formulated in the framework of unsaturated soil mechanics. The method employs two layers of material points to represent coupling effects between the solid and fluid phases. Several numerical examples demonstrate unique features of the method, including free water infiltration into the unsaturated soil, dynamic soil–water interaction in an example of underwater soil collapse and initially saturated soil column collapse. Furthermore, rainfall-induced failure in unsaturated soil slopes is analyzed. The proposed method can well capture change in matric suction, development of shallow and deep-seated shear bands, and finally, large-deformation post-failure behavior. Parametric studies also demonstrate soil cohesion, dilatancy and friction angle play significant roles on the slope failure mechanisms. [ABSTRACT FROM AUTHOR]

The landslide in Xingye County of Guangxi is studied as the research object. The seepage field, stress field and deformation field of unsaturated soil slope are simulated and analyzed by finite element analysis software of geostudio. The stability of unsaturated soil slope and the evolution rule of slip surface are studied. The results show that the response of shallow soil mass to rainfall is intense. Pore water pressure, volume water content and rainfall intensity are positively correlated. Matrix suction and effective stress are sharply reduced. The decrease of shear strength is not conducive to slope stability. Under the influence of rainfall, the maximum shear stress of the shallow soil increases. The displacement at the top and foot of the slope changes greatly, and easily causes failure and instability. The minimum value of stability coefficient appears after rainfall, not during rainfall, showing an obvious. The result of numerical simulation is similar to that of field investigation. The study has certain reference value for landslide disaster prevention, mitigation work and stability evaluation. [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]

Some slopes in Yogyakarta, Indonesia, were covered by volcanic soil of the quaternary deposit of Merapi volcano. Typically, the soil layers of the slope consist of fine sand or silty sand. During the wet season (November - February), the slope is prone to instability due to rainfall infiltration. This paper presents the field suction monitoring of silty-sand slope. A 2.57 m high slope at Universitas Muhammadiyah Yogyakarta campus was instrumented with a mini tensiometer, moisture probe, and rain gauge to study the effect of suction on the slope stability during rainfall in wet season. The investigation shows that the rainfall decreases the suction and increase the moisture of soil. This situation leads to slope instability. The factor of safety fluctuates corresponding to the rainfall intensity. The lowest factor of safety was attained on 30 December, in which the rainfall intensity is the highest, i.e. 48 mm/day. [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 diffuse shallow landslides are one of the most critical natural hazards as they often evolve into highly destructive flow slides and debris flows. Vegetation is recognised to play a key role in landslide occurrence and is frequently invoked as a potential remedial measure for slope stabilisation at the catchment scale. The beneficial action of vegetation is generally associated with mechanical (root anchoring) and hydrological (suction generated by root water uptake) effects. There is indeed a third effect that has thus far been little explored. The rhizosphere, the portion of soil directly affected by plant roots, is characterised by hydraulic conductivity higher than the underlying soil horizons. This significantly affects hillslope hydrology by promoting lateral diversion of rainwater. This paper presents a case study in Scotland where the rhizosphere is demonstrated to play a major role in controlling shallow landslides. Field investigation and laboratory testing were carried out to characterise the hydraulic conductivity of the rhizosphere and deeper horizons. In turn, this formed the basis for the development of a physically based model for the slope. The model was first validated against its capability to simulate failure of two historical landslides and then exploited to demonstrate the beneficial effect of the rhizosphere. The lesson learned from this study is that shallow landslide hazard can be mitigated by enhancing the capacity of the rhizosphere to act as a natural lateral drainage. This implies that plants with root-system architecture that enhances lateral subsurface flow should be privileged when designing vegetation-based remedial measures. [ABSTRACT FROM AUTHOR]

In order to study the lawof water flowmigration in multi-layer soil slope, the homogeneous soil slope model and the multi-layer soil slope model were established. The temporal and spatial characteristics and influencing factors of water flowmigration were explored, with the development of saturated-zone taken into account, based on the governing equation of moisture content and matrix suction in multi-layer soil slope derived from the VG model. The relationship between water flowmigration and effective stress was also studied. It is found that the difference in the matrix suction in the horizontal direction decides the change of water flowmigration mode and the rate of change of matrix suction along the horizontal direction determines the rate increment of water flowhorizontal direction. The formation of the saturated-zone is a necessary condition for forming the vertical water flow. After the rainfall stops, the down-slope water flowis rapidly generated only in the shallowlayer of 0 ~ 1. 3 m, with the forming of down-slope water flowin deep area lagging behind. The horizontal water flowat the interface of two layers of soil with large differences in permeability changes sharply, and the forming of up-slope water flowis always accompanied with the decrease of effective stress. [ABSTRACT FROM AUTHOR]

Microplastics (MPs) defined as smaller 5 mm plastic particles have received increasing attention due to their global occurrence and potential toxicity. This study investigated the effects of environmental factors (rainfall intensity, 13 and 29 mm/h) and MP characteristics (morphology (fiber, flake, and film), polymer type (polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS)) and size (100–300, 300–500, and 500–1000 μm)) on the vertical transport of MP in unsaturated soil conditions using lab-scale column experiments. Additionally, the occurrence and characteristics of MP detected in soil/sediment (total 13 samples) and groundwater samples (total 6 samples) were explored in the field study. Laboratory-scale column experiments revealed that heavy rainfall intensity (29 mm/h) increased the degree of MP vertical transport in unsaturated soil conditions and MP fibers showed the greatest vertical mobility among the various morphologies of MPs assessed. For the polymer type and size, the lighter PP polymer or the larger size of MP (500–1000 μm) showed higher mobility. In the field study, a statistical difference in MP abundance was observed depending on the population density and degree of urban development in both soil and groundwater samples. Comparing to the two different types of environmental media samples obtained from the same site, there was a significant difference in the composition of polymer types present while statistically no difference in MP abundance was observed between the two media samples (i.e., soil or sediment and groundwater). In addition, MP fibers and polyethylene (PE) were predominantly detected in our two study areas. These results suggest that various types of MP can pass through the unsaturated zone by water infiltration, even if it takes a long time to reach groundwater. Overall, we found that the degree of vertical transport of the MPs was highly sensitive to environmental conditions and MP characteristics. [Display omitted] • Increasing rainfall intensity accelerated the microplastics (MP) vertical transport • Fiber and polypyrene MP showed the greatest vertical transport among the MP tested • Greater MP vertical mobility was observed with greater MP size • No statistical difference MP abundance was observed between soil and groundwater • Fiber and polyethylene MPs were predominantly found in soil and groundwater [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]

Abstract Hysteresis is a common feature in the hydraulic properties of unsaturated soils. At a given matric suction, the volumetric water content on a wetting curve is always lower than that on a drying curve. The hysteresis may affect the mechanical behavior and transient process in unsaturated soils. This study focuses on the hysteresis observed in soil-water retention curve (SWRC) tests in the laboratory and inferred from a semi-empirical equation for numerical implementations. Coupled hydro-mechanical finite element analyses, employing an initial drying, main drying, and main wetting curve as hydraulic properties of the soil, were performed to numerically study the effect of hysteresis on the hydro-mechanical behavior of unsaturated soil subjected to water infiltration. Numerical results show that the unsaturated soil exhibits lower soil strength and matric suction distributions when applying the main wetting curve as compared with other SWRCs. The hysteresis appears to affect the stress-strain behavior of the unsaturated soil during prolonged rainfall. Therefore, to predict the instability of the rainfall-induced soil slope more precisely, it is necessary to apply the wetting curve of the SWRC, which shows the hydraulic characteristics of the slope surface. Highlights • Estimation of hysteresis of soil-water retention curve (SWRC) • Coupled hydro-mechanical finite element analyses • Role of hysteretic SWRCs in the transient condition • Emphasis on the hysteresis effect [ABSTRACT FROM AUTHOR]

Based on the basic theory of unsaturated soil mechanics， a solid-liquid-gas three-phase coupling control equation for two-dimensional unsaturated soil slopes was established by considering the compressibility of porous media. The numerical simulation results of the established coupling control equations based on the PDE platform of COMSOL multiphysics software was compared with the experimental results of the classical Liakopoulos sand column. The established three-phase coupling control equation was verified， and the influences of different factors on the seepage field and displacement field were discussed. The simulation results showed that the permeability can slightly retard the rainfall seepage process and delay the deformation of unsaturated soil slope. The air intake coefficient has a slight impact on the settling process of unsaturated soil slope. [ABSTRACT FROM AUTHOR]

Rainfall-infiltration-induced fines migration within soil slopes may alter the local porosity and hydraulic properties of soils, and is known to be a possible cause of the failure of slopes. To investigate the intrinsic mechanisms, a mathematical formulation capable of capturing the main features of the coupled unsaturated seepage and fines migration process has been presented. Within the formulation, an unsaturated erodible soil is treated as a three-phase multi-species porous medium based on mixture theory; mass conservation equations with mass exchange terms together with the rate equations controlling fines erosion and deposition processes are formulated as the governing equations and are solved by the FEM method. The influences of both the fines detachment and deposition on the stability of slopes under rainfall infiltration have been investigated numerically. The results show that depending on whether the fines move out or get captured at pore constrictions, both desired and undesired consequences may arise out of the fines migration phenomenon. It is suggested that more attention should be paid to those slopes susceptible to internal erosion whose safety analysis cannot be predicted by traditional methods. [ABSTRACT FROM AUTHOR]

This paper presents a coupled air-water flow analysis to evaluate the performance of a three-layer capillary barrier for controlling water infiltration into and gas emission from a waste containment system in a high precipitation environment, and to optimize combinations of local soil layers for barrier construction. A multi-phase flow model is proposed considering the movements of the gas and water phases simultaneously. The governing partial differential equations are solved in COMSOL Multiphysics software. Several combinations of lean clay with sand (CL), clayey sand with gravel (SC), silty sand with gravel (SM), sandy silt (ML), and well-graded gravel with silt (GW-GM) are examined. The rates of percolation water and gas emission are used as indicators to compare the performance of different combinations. A fine-grained surface soil layer reduces both water infiltration and gas emission due to its low desaturation rate and high water-retention capacity. The coarse middle layer plays a critical role, promoting capillary effects and hindering water infiltration during rainfall as well as draining any infiltrated water or percolated gas. [ABSTRACT FROM AUTHOR]

In many areas of the world, slopes covered by shallow unsaturated non-plastic soil layers experience rainfall-induced landslides causing heavy damage and casualties every year. Landslide occurrence depends on the amount of water infiltrated and stored. Among the contributing factors are the hydraulic conditions at the lowermost boundary, a feature that is often disregarded. The paper focuses on this topic, presenting the results of some laboratory and numerical experiments on ash-pumice interfaces. A strategy is then proposed for selecting the lowermost boundary condition, and some studies are carried out to compare the results obtained with the proposed solution and other more popular ones. [ABSTRACT FROM AUTHOR]

Rainfall-induced landslides are a common occurrence in terrain with steep topography and soils that have degradable strength. Rainfall infiltration into a partially saturated slope of infinite extent can lead to either a decrease or complete elimination of soil suction, compromising the slopes' stability. In this research the rainfall infiltration coupled with deformation of a partially saturated soil slope during rainfall infiltration is analyzed. The limit equilibrium conditions and the shear strength relationship of a partially saturated soil are employed to develop an analytical solution for calculating the stability of an infinite partially saturated slope due to rainfall infiltration. The analytical solutions are able to consider the influence of the coupled effects on the stability of the slope. The factors that affect the safety of a partially saturated slope of infinite extent are discussed. The results indicate that the poro-mechanical coupling of water infiltration and deformation has an important effect on the stability of the infinite unsaturated slope. [ABSTRACT FROM AUTHOR]

The 2004 Niigata-ken Chuetsu Earthquake in Japan caused serious damage to embankments. Considerable damage occurred in mountainous regions, and the recorded rainfall before this earthquake exceeded the annual rainfall. In this study, numerical simulations were performed to reproduce the damage and to investigate the effect of rainfall on seismic behavior. In the simulations, a three-phase (soil, water, and air) coupled analysis was adopted to consider the behavior of unsaturated soil, and a conventional elastoplastic constitutive model was used for unsaturated soil. The deformation of the embankments was quantitatively reproduced by incorporating rainfall in the seepage analysis before the dynamic response analysis, and it was found that heavy rainfall significantly affected the dynamic behavior. Furthermore, parametric studies of countermeasures showed that the embankments would deform as a result of poor-quality materials even if sufficient drainage or impermeable roadbeds were constructed. [ABSTRACT FROM AUTHOR]

In this study, the influence of transient flow on the active and passive lateral earth pressures of variably saturated backfills is thoroughly evaluated employing the lower bound theorems of the finite element limit analysis with second-order cone programming. In order to account for the tempo-spatial variations of suction stress within the soil stratum during infiltration, a closed-form solution derived for one-dimensional transient flow through variably saturated porous media is adopted. The unsaturated ground condition is simulated by incorporating the well-established unified effective stress approach into the soil yield function. The soil medium is considered to be both isotropic and anisotropic, due primarily to the various distributions of suction stress along different directions within the partially saturated porous medium. In order to model the soil suction stress anisotropy, an iterative procedure is employed by differentiating between the mobilized suction stress within the horizontal and vertical planes. In general, the influence of the transient flow condition is magnified for cohesive soils compared with granular backfills. In either case, such an effect is more noticeable for the active lateral earth pressure as compared to the passive counterpart. In the anisotropic condition, while the suction stress anisotropy within the soil medium has fairly negligible influence on the contribution of transient flow to the lateral earth pressure coefficient in the active state, the influence of elapsed infiltration time on the passive earth pressure coefficient turns out to be more at play at lower anisotropy ratios. • Soil-environment-structure interaction is examined with emphasis on lateral earth pressures. • FELA analyses are performed incorporating transient flow of the unsaturated backfill. • The influence of anisotropic matric suction is accounted for based on grain-scale considerations. • The results highlight the important influence of transient-flow and suction-stress anisotropy. • The effect of elapsed infiltration time is magnified in the presence of higher suction anisotropy. [ABSTRACT FROM AUTHOR]

Assuming that the water content and hydraulic conductivity of unsaturated soil are exponential functions of the pressure head, Green's function is used to obtain analytical solutions to one-dimensional coupled rainfall infiltration and deformation in unsaturated soils under varying surface rainfall flux. The analytical solution considers varying flux and pressure head at the top boundary and arbitrary initial conditions. In particular, nonlinearly increasing rainfall intensity at the surface boundary is considered. The result indicates that the coupling of seepage and deformation has a significant effect on the pressure-head profiles for the transient unsaturated seepage. The coupling effect is closely related to the boundary conditions. The coupling effect almost disappears if ponding at the ground surface occurs. The ponding time is different under the coupled and uncoupled conditions. There is a quick change in the pressure-head profiles near the time of ponding or peak or discontinuous rain intensity. [ABSTRACT FROM AUTHOR]

Understanding root-water uptake in soil slopes is essential because any changes in soil moisture would lead to changes in soil suction and in turn changes in soil shear strength and slope stability. Uncertainties associated with plant characteristics are present in the root-water uptaking process. The objective of this paper is to evaluate the suction profile in a vegetated slope with a grass cover and the stability of the slope during rainfall infiltration. Random field theory is applied to model the spatial variation of the maximum transpiration rate of Bermuda Grass, which is a native grass species in Hong Kong. The slope is subjected to 12 h, 24 h, and 48 h of drying processes before rainfall. A modified Richards equation governing water flow in unsaturated–saturated media is employed to incorporate a sink term of Bermuda Grass. The effects of initial conditions induced by the drying processes of different periods prior to rainfall on the suctions retained in the vegetated slope after rainfall are investigated. Uniform and triangular root distributions are considered to simulate likely root architectures and their influence on the suctions retained. Slope stability analysis is further carried out to investigate how plant transpiration contributes to slope stability. The results demonstrate that, given the same maximum transpiration rate, the root distribution has little influence on inducing or retaining suctions. At a low infiltration flux, the higher the coefficient of variation of the maximum transpiration rate and the longer the pre-drying time, the wider the range of the suctions that a vegetated slope can retain. A pre-drying process of 48 h results in a significant increase in the slope factor of safety. [ABSTRACT FROM AUTHOR]

A new simulation chain for early prediction of rainfall-induced landslides in unsaturated soils is presented. It includes a special computational weather code for forecasting the evolution of the synoptic weather and its changes due to interaction with the Earth's surface (rainfall pattern), and a hydro-mechanical code to analyse rainfall effects on slope stability by computing degree of saturation and pore pressure changes due to rainwater infiltration. The linkage between these two numerical codes is ensured by an interface with the aim of bringing the data provided by the first code, which operates at basin or slope scale. The simulation chain can work in computational times that may be considered suitable for civil protection operations. [ABSTRACT FROM AUTHOR]

Abstract: To examine the relationship between the increase of the shear strain and the volumetric water content, the suction, and the pore pressure in a slope under rainfall, an analysis of the measurement of the soil–water and the deformation of the sandy model slope due to rainfall infiltration is conducted. At first, the surface displacement of the slope mostly occurs due to the increase of the pore pressure. The surface displacement increases slightly under unsaturated conditions at first and later increases significantly with the increase of the pore pressure. The surface displacement increases slightly immediately after the generation of the pore pressure at first, and the ratio of the increase of the surface displacement to the increase of the pore pressure subsequently grows larger. The shear strain in the soil layer of the model slope also increases to a small amount with the increase of the volumetric water content or the decrease of the suction in unsaturated conditions at first and later increases significantly with the increment of the positive pore pressure. The relationship between the shear strain and the positive pore pressure is hyperbolic, and the maximum pore pressure is greater at a deeper layer. [Copyright &y& Elsevier]

The authors discuss the effect of anisotropic conductivity on the suction and reliability index of an unsaturated embankment for uniform rainfall whose intensity is lower than the saturated hydraulic conductivity. The analysis is performed on good-drainage soil of a specific horizontal conductivity and various anisotropic ratios such that the vertical conductivity is reduced. The initial suction profile for the unsaturated zone is 15 kPa. The effect of the shear strength caused by matric suction in the unsaturated zone is explained by the modified Mohr-Coulomb failure criteria. The uncoupled approach of finite element method and limit equilibrium method is used to evaluate the reliability index at different elapsed time of an antecedent rainfall. The anisotropic conductivity ratios affect the volumetric water content and the suction profile. The reliability index remains practically unchanged at high anisotropic ratios but decreases noticeably at low ratios when the wetting front reaches and elevates the water table level. [ABSTRACT FROM AUTHOR]

In 1996, a portion of a highly instrumented experimental catchment in the Oregon coast range failed as a large debris flow from heavy rain. For the first time, we quantify the 3-D multiphysical aspects that triggered this event, including the coupled sediment deformation-fluid flow processes responsible for mobilizing the slope failure. Our analysis is based on a hydromechanical continuum model that accounts for the loss of sediment strength due to increased saturation as well as the frictional drag exerted by the moving fluid. Our studies highlight the dominant role that bedrock topography and rainfall history played in defining the failure mechanism, as indicated by the location of the scarp zone that was accurately predicted by our 3-D continuum model. [ABSTRACT FROM AUTHOR]

Abstract: Analytical solutions to 1-D horizontal and vertical water infiltration in saturated/unsaturated soils are developed that can consider the variation of rainfall with time. In this model, water content and the permeability coefficient are assumed to be exponential functions of the pressure head, and diffusivity is constant. By means of Fourier integral transformation, the analytical solutions are expressed as infinite series. The steady-state solutions for horizontal and vertical infiltration in unsaturated soils are then derived. The solutions can consider both flux and pressure head boundaries. The solutions are easy to implement compared with numerical solutions, although the restricted assumptions may limit their applicability in some ways. The analytical solutions provide a reliable tool for checking the accuracy of various numerical methods under the condition of constant diffusivity. Finally, the analysis carried out in a case study indicates that the pressure head differences caused by the transient infiltration in both the horizontal and vertical directions can be estimated using the steady-state solutions, and the effect of gravity on water infiltration mainly depends on the boundary conditions. [Copyright &y& Elsevier]

This paper presents an application for the analysis of structures formed by unsaturated layered soils subjected to rainfall and seepage flow; the results are part of a current research project on rainfall infiltration. A three - phase coupled infiltration - deformation method for unsaturated soil was used for the numerical analysis. The effects of the water permeability and horizontal drainage on the distribution of seepage flow velocities, saturation, and generation of deformations for an unsaturated layered embankment were investigated. The results show that the generation of deformation on the embankment surface highly depends on the water permeability of the soil. In addition, through horizontal drainage simulations, the advantage of this type of solution for decreasing the pore water pressures on the back of the slope embankment, thus avoiding local failure (erosion), is shown. [ABSTRACT FROM AUTHOR]

Abstract: Rainfall infiltration and suction variation in unsaturated soils must be taken into consideration in the analysis of most slope stability problems, particularly in the tropical regions where the annual precipitation is high. The process of rainfall infiltration into unsaturated soils is an extremely complex problem attributed to the non-linearity of the hydraulic property functions of the unsaturated soils. This paper describes in detail two instrumented laboratory models, i.e. one-dimensional soil column, and two-dimensional slope model used to provide experimental evidences for the transient suction variations in the unsaturated soils under certain rainfall conditions. The performances of the laboratory models were tested on four typical types of residual soils, i.e. sand-gravel, silty gravel, sandy silt, and silt (kaolin), and a two-layered soil system, i.e. sandy silt underlain by silty gravel. The results showed that the suction distributions for the single-layered homogeneous soils obtained from the simpler one-dimensional soil column were almost identical to that of two-dimensional slope model. However, the two-dimensional slope model should be employed for the two-layered soil system because of the dominant effect of the lateral flow mechanism. The capillary barrier effect was observed when a less permeable soil layer was underlain by a more permeable soil layer. The minimum suction value in soil is governed by the rainfall intensity, rainfall duration, and the saturated permeability of soil. The infiltration rate of the fine-grained soils that subjected to shrink and crack was independent of the soil permeability, but was significantly governed by the preferential flows developed in the soils. [Copyright &y& Elsevier]

In this paper, moisture migration in loess considering temperature effect is studied by tests on unsaturated loess samples with different densities and initial moisture contents. Test results reveal that obvious changes in moisture content distribution in a loess sample can be observed after temperature difference is exerted on the two ends of the sample. Moisture content at the cold end increases and that at the hot end decreases. Under the effect of temperature difference, moisture content difference at the two ends of a soil sample is related to the initial moisture content, soil density, and magnitude of the temperature difference. Generally speaking, larger temperature differences and smaller soil densities result in more obvious moisture migration and larger moisture content differences at the two ends of the soil sample. When the initial moisture content is large, the moisture content difference caused by a temperature difference is small; when the initial moisture content is small, the moisture content difference caused by a temperature difference is also small; when the initial moisture content is moderate, the moisture content difference caused by a temperature difference is large. After the analysis of test results, taking the soil density and moisture content into account, a formula is obtained to determine the moisture content gradient resulting from the temperature gradient. Reliability of the formula is verified by comparing the measured and calculated data. Because of the reverse migration of liquid water and water vapor at the end of the experiment, it is difficult to determine the thermal potential and matrix potential. Based on the experimental data, this paper probes into the water potential equation that can be used for stability analysis. The equation considers the comprehensive impact of soil density, temperature gradient, moisture content, and moisture content gradient on water potential. It only applies to analyze stable distributions of temperature and does not apply to unstable temperature distributions. [ABSTRACT FROM AUTHOR]

Abstract: Slope failures in the tropical regions, particularly Malaysia are commonly triggered by frequent rainfall. The tropical rainfall can be characterized as short and intense throughout the year, and prolonged and less intense during monsoon seasons. Under such circumstances, various rainfall patterns should be included in the analysis of rainfall-induced slope failure in the tropical regions. This paper is aimed to demonstrate a simple model for preliminary evaluation of rainfall-induced slope failure. The critical rainfall patterns for four typical types of soil were first determined. Seepage finite element analyses were conducted using the extreme rainfall of ten-year return period for Johor Bahru, Malaysia. The results showed that the ratio of rainfall intensity to soil saturated permeability (i.e., I/k sat) plays an important role in determining the critical rainfall pattern. Two critical combinations of antecedent rainfall and major rainfall, 1-day, 2-day, 3-day, 5-day, 7-day, 14-day, and 30-day antecedent rainfalls and the redistribution of the critical combination of antecedent rainfall and 1-day major rainfall were responsible for the formation of suction envelope in soil. The suction envelope, representing the worst suction distribution in soil, was used for the computation of factor of safety of soil slope through the modified infinite-slope–limit-equilibrium method. A model, PERISI, was developed based on the findings from numerical simulation. The suction envelope and factor of safety computed from the PERISI model showed good agreements with the results obtained from Seep/W and Slope/W computer programs and the results derived from the model of Rahardjo et al. developed in 1995. [Copyright &y& Elsevier]

Abstract: A full-scale field experiment involving artificial rainfall has been conducted in an instrumented loess cut slope in an expressway in a loess plateau, Northwest China. Instrumentations including soil moisture probes, tensiometers, piezometers, vacuum sensors, water level sensors, and rain gauge were installed in the slope. After monitoring for about 2 years, artificial rainfall was induced at the site in April 2007. This paper presents the results from soil moisture probes, tensiometers, and water levels to reveal the surface infiltration process in an unsaturated soil. The results show that the top 0.7 m in loess is an active zone under the process of infiltration and evaporation, when subjected to a maximum rainfall intensity of 40 mm/day. The depth of the “wetting front” during the wet season was limited to the top 2 m of soil. During high rainfall intensity of 120 mm/day, the depth of the “wetting front” is about 3 m. Based on the variation of volumetric water content and matric suction in soil, a method to analyze the rainfall infiltration is proposed. By means of Gravity-Predominant Flow (GPF) concept, the infiltration rate is analyzed in the deeper soil. The results explain well the observations that loess slopes could be stable in the wet season, but it may tend to slide about 3–6 months later in the dry season. The surface infiltration has a very limited effect on the permanent groundwater table, which is at a greater depth. [Copyright &y& Elsevier]

Abstract: Shallow slope failures in residual soil during periods of prolonged infiltration are common throughout the world. Using a one-dimensional infiltration model and an infinite slope analysis, this study examines an approximate method of determining how infiltration influences the surficial stability of two-layered slopes. The method extends Moore''s infiltration model, which is based on the Green–Ampt model, to cover more general situations, including those where water moves upward from a perched water table in decreasingly permeable soil. The method has also been used to evaluate the likelihood of a shallow slope failure being induced by a particular rainfall event. In making this evaluation, the method takes into accounts the rainfall intensity and duration of various return periods in a two-layered soil profile. A comparison of the results of the infiltration model with the results of numerical analyses shows that, with the use of properly estimated input parameters, the proposed model compares reasonably well with other model that rely on more rigorous finite element method. [Copyright &y& Elsevier]

A full-scale field study was conducted to investigate the effects of rainfall infiltration on a natural grassed expansive soil slope in China. A 16 m wide × 28 m long area was selected for instrumentation. The instrumentation included jet-filled tensiometers, moisture probes, a tipping bucket rain gauge, and a vee-notch flow meter. One artificial rainfall event amounting to about 370 mm rain depth in total was applied to the slope. The monitored results suggested that there was about a 3 day delay in the response of surface runoff, pore-water pressure, and water content to the commencement of the simulated rainfall. The depth of influence of the rainfall, depending on the elevation along the slope, ranged from 2.8 to 3.5 m. Positive pore-water pressures were measured within the influence depth, and there existed significant subsurface downslope flow at the end of the simulated rainfall, particularly near the lower part of the slope. A comparison of infiltration rates between the grassed area and a bare area nearby indicated that the presence of grass significantly increased the infiltration rate and reduced surface runoff. The cracks and fissures developed in the unsaturated expansive soil played an important role in the hydrological process. [ABSTRACT FROM AUTHOR]

Abstract: Geotechnical engineers and geologists are frequently concerned with the complex phenomenon of rainstorm-induced landslides in mountainous tropical regions. This paper analyses the hydrological conditions that contributed to a huge deep-seated slide of a slope in Rio de Janeiro, Brazil. The slope failure occurred after a short period of intense rainfall, buried cars, and caused structural damages to an adjacent building. On the following day, the failure surface was completely saturated, despite the fact that no groundwater was observed within the slope. Understanding a rain-induced landslide is a considerable challenge. In the present case, there was an extra question to be answered: why were no slope movements observed a few months earlier, when a more intense rainfall reached the slope? Geological and geotechnical surveys were carried out aiming at determining complete soil profile characterization, piezometric levels and hydraulic parameters. Field investigations indicated a variable thickness unsaturated residual soil layer overlying a gneissic rock. The transition between the saprolitic soil and the sound rock consisted of a highly fractured and weathered rock. 3D flow modelling has shown that the amount of rain infiltration was insufficient to produce positive pore-water pressures. It was likely that development of preferential infiltration paths could explain the hydrological conditions of the slope during landslide. Water infiltration through the fractured soil–rock transition induced a rapid development of positive pore water within the soil mass. This favourable scenario might have been created by the lack of maintenance of the crest drainage channel, adjacent to the slide area. It is a common engineering practice to try to associate rainfall amounts to the risk of slope failure. The role that bedrock plays in generating pore pressure is seldom considered. The numerical analyses presented in this paper pointed out at the complexity of flow modeling of rain infiltration through unsaturated residual soil slopes. It essentially revealed that flow mechanisms other than rain infiltration over the slope surface might play a major role on hydrological pattern of slopes. [Copyright &y& Elsevier]

• Constant shear stress test apparatus. • Deformation and failure characteristics. • Effect of cyclic wetting and drying. • Effect of dry density, overburden pressure, rainfall intensity, and slope angle. • Effects of various combinations of the above four factors. Slope failures caused by rainfall are common occurrences in many parts of the world and are severe in Japan where there are significant numbers of slopes susceptible to failure. To understand the mechanisms and conditions leading to such failures, constant shear stress tests that mimic the field stress path were conducted on a sandy soil taken from a natural sliding slope whose surface soils were moving. Water was infiltrated from the bottom and top of an initially unsaturated cuboid soil specimen at constant shear stress until failure. The effects of cyclic wetting and drying on the deformation characteristics and four parameters reflecting the initial conditions, i.e., the dry density, overburden pressure, rainfall intensity, and slope angle, as well as various parameter combinations, on the time and moisture content allowing for failure initiation were investigated. The experimental results exhibited a unique relationship between the deformation and moisture content and suggested a relationship for predicting the time and moisture content of onsets of landslides based on the four parameters. The results obtained can serve as guidelines for the design of landslide hazard detection systems and mitigation measures against rainfall. [ABSTRACT FROM AUTHOR]

As sudden catastrophic geological disasters, landslides always cause numerous casualties and much property damage. Landslides in reservoir areas are rarely caused by a single influential factor such as rainfall or fluctuation of reservoir water level (FRWL); rather, they are frequently caused by the combination of influential factors. Since rainfall and FRWL conditions vary by location in reservoir areas, it is necessary to discuss slope stability under various possible combinations of rainfall and FRWL while considering the specific characteristics of the region. In this paper, model tests on the slope stability of the Three Gorges Reservoir (TGR) area were conducted with S1 sand under three different rainfall and FRWL combinations. Additionally, numerical simulations on the model tests, in which an unsaturated soil constitutive model was selected to describe the mechanical–hydraulic behaviour of saturated/unsaturated soil, were conducted. In the simulation, to reflect the regional characteristics, a relationship between the rainfall infiltration rate and the unsaturated permeability of the sand was proposed. By carefully determining the parameters involved in the constitutive model, soil–water–air coupling finite element analyses were conducted to simulate the model tests on slope stability. By comparing the calculated results with the test results, the numerical method used in this paper offers satisfactory accuracy to describe the various failure mechanisms of the model slopes under different loading combinations. The results of both the model tests and the numerical simulation indicate that in the TGR area during one year of operation, a decrease in the reservoir water level combined with a long weak rainfall is more likely to trigger a landslide than the other combinations studied. Additionally, the combination of a short heavy rainfall with FRWL in the short term during the wet season can be dangerous; thus, this combination needs to be considered in the operation of the TGR. • Three combinations of rainfall and water level fluctuation are observed in TGR area • Landslides were different under three combination patterns of external loads • Saturated/unsaturated cycles can reduce slope stability and trigger landslide • Combination of water level decrease with weak rainfall is most dangerous to slope [ABSTRACT FROM AUTHOR]