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A rainfall-induced landslide occurred in the north highwall of the Fushun west open pit at 5:00 (UTC + 8) on July 26, 2016, in China. The landslide was about 3.1 × 106 m3 and caused considerable destruction of houses, roads, and railways. Field investigations, laboratory tests, and numerical analyses have been performed to explore the failure characteristics and formation mechanism of the landslide. The landslide was divided into three parts: the crack area, the sliding body area, and the accumulation area. The X-ray diffraction and scanning electron microscope techniques were used to reveal the mineral composition and microstructure of the landslide material. Then, a conceptual model of the landslide mechanism was constructed and the process of the landslide was divided into four stages: unloading and cracking stage; sliding and partial locking stage; shearing out and failure stage; and flowing and accumulating stage. A combined seepage and stability analysis was performed to explore the mechanism of the landslide by numerical simulation. The relationship of volumetric water content and hydraulic conductivity to matric suction was established to describe the hydraulic characteristics of the slope material under infiltration. The results show that the maximum matric suction in the slope decreases continually with the increase of rainfall intensity and duration, and the north highwall will not be stable after 7.2 h for 16 mm/h of rainfall. The original slope was stable and heavy rainfall triggered the landslide, and the obtained critical failure surface matched the field survey closely. The findings improved understanding of the failure mechanism and process of rainfall-induced landslides may be used for evaluating the stability of slopes and early identification for both active and inactive open-pit mines.

Developing rainfall thresholds is crucial for predicting rainfall-induced landslides. Typically, such thresholds are established by correlating past landslide events with corresponding rainfall records. However, this process particularly becomes challenging in regions with limited data. To address this issue, a methodology is designed in this study, which combines both transient seepage and stability analysis, followed by threshold development. This method first identifies the cumulative rainfall ( E ) and rainfall duration ( D ) parameters for the critical stability condition (i.e., Factor of Safety ≤ 1). Subsequently, this information is used to derive the E–D threshold line using the frequentist probabilistic approach. The methodology is applied to six shallow landslides located along the National Highway–10 in the Kalimpong region of Darjeeling Himalaya, India. At a 5% exceedance probability (T 5 ), the threshold is estimated as T 5  = 26.92 D 0.44 for a duration range of 1 <  D (hour) < 54. The comparison of the derived threshold has been found in good agreement with previous attempts, along with the reliability assessment for past recorded landslides. Hence, it is envisaged that the presented methodology will be applicable for developing rainfall thresholds in other data-scarce landslide-prone areas.

When analysing the slope stability of levees, the soil mechanical parameters of the saturated and unsaturated cross-sectional areas are the key influencing factors. Thus, prediction of the seepage line is necessary. In practice, steady-state boundary conditions are typically assumed, where the seepage line reaches the maximum elevation within the levee. This scenario leads to conservative, potentially oversimplified safety assessments, reflecting the awareness that the actual level of safety is not fully known and acknowledges the inherently dynamic rather than steady-state nature of flood events. This study proposes a novel approach to enhance the safety assessment of homogeneous levees by accounting for the dynamic evolution of phreatic lines over time. By considering transient seepage flow and incorporating key flood hydrograph parameters, such as rising or peak duration, and using an analytical estimate of the transient seepage line, this method allows the practical application of risk-based design in flood protection contexts. An equation grounded in numerical principles was developed to empirically derive the transient behaviour of the seepage line, significantly reducing the computational effort compared with numerical methods. This equation can be subjected to a probabilistically supported assessment of homogeneous levees, contributing to an improved understanding of transient geohydraulic conditions.

Rainfall infiltration can alter matric suction and significantly affect the active earth pressure acting on retaining structures with unsaturated backfills. This effect becomes more complex when hydraulic layering and finite-width failure mechanisms are involved. In this study, a coupled analytical–numerical framework is developed for a vertical retaining wall with a two-layer unsaturated backfill subjected to time-varying rainfall. A one-dimensional transient seepage formulation based on the Gardner-type hydraulic model is combined with a three-dimensional kinematic upper-bound mechanism generated by double-logarithmic-spiral curves. The transient pressure head and effective saturation are converted into a suction-induced resisting contribution and introduced into the work-rate balance. The effects of initial water-content distribution, bottom hydraulic boundary condition, normalized wall width, rainfall intensity, and layer thickness ratio are investigated. The results show that the calculated active earth pressure coefficient is controlled by the coupled influence of transient suction loss, hydraulic contrast between layers, and finite-width confinement. Clay-related profiles are more sensitive to the initial hydraulic state and finite-width effect than sand-dominated profiles. The bottom boundary condition may either increase or decrease the active pressure depending on the hydraulic contrast. For some rainfall intensities, an intermediate layer thickness ratio produces a larger calculated active pressure than the corresponding homogeneous limits. These findings should be interpreted within the assumptions of one-dimensional vertical seepage, hydraulically and mechanically continuous layer interfaces, a smooth wall-back contact, and the adopted admissible three-dimensional failure mechanism.

The stability of hill slopes is a critical concern in regions prone to landslides, affecting both human, nature and infrastructure. In tropical countries like Bangladesh, many soil slopes consist partially or entirely of unsaturated soils, where suction forces enhance the shear strength of slope materials. Rainfall precipitation leads to water infiltration into the soil, increasing its moisture content and diminishing suction forces, potentially resulting in slope failure. Conversely, hydraulic conductivity significantly affects slope stability and is particularly reliant on the soil–water characteristic curve of the slope materials. This paper examines the influence of hydraulic conductivity and rainfall infiltration on the stability of a hill slope situated in the Chattogram Metropolitan City region. Laboratory and field studies, coupled with numerical analysis were conducted for both steady-state and transient seepage condition across the year to simulate the rainfall infiltration through unsaturated soils. The study revealed that short-term, low-intensity rainfall results in a reduction of Factor of Safety (FoS) toward the end of each month. In contrast, long-term, high-intensity rainfall leads to a decrease in FoS until the middle of the month, followed by a subsequent increase until the end of each month. However, the FoS increases in the dry season when rainfall infiltration is less than hydraulic conductivity, whereas it decreases in the rainy season when rainfall infiltration is higher than hydraulic conductivity. The suction strength at the top of slope diminishes by 60% following rainfall.

Rainfall-induced landslide (RFIL) disasters have become a major concern within the Indian Himalayan region. On August 14, 2023, at around 7:20 AM (India Standard Time), an excessive rainfall event triggered a landslide in the Summer Hill area of Shimla, Himachal Pradesh, India, named locally as the Shiv Bawdi landslide. The abrupt occurrence of this landslide claimed 20 lives and caused damage to the Upper and Lower Summer Hill Roads and the Kalka-Shimla Railway track. Therefore, a preliminary analysis was carried out to understand the landslide characteristics and possible failure mechanism. This includes field investigation, Interferometric Synthetic Aperture Radar (InSAR) analysis, unmanned aerial vehicle (UAV)-based mapping, and pore water pressure (PWP) simulations. The obtained results indicate that the landslide affected an area of approximately 7.52 × 103 m2 and had a volume of about 3.71 × 104 m3. Similarly, the estimated maximum velocity of this landslide was approximately 9.27 m/s, with a maximum momentum of 3.44 × 105 kg m/s. The time-series InSAR analysis also revealed pre-failure deformation signatures at the landslide site. Further, using the recorded rainfall, transient seepage analysis was performed to assess the pre-failure PWP condition. The analysis suggests that continuous antecedent rainfall resulted in the formation of positive PWP, leading to the development of localized perched aquifers by saturating the slope material. Moreover, the high-intensity rainfall on the day of failure ultimately triggered the landslide in the form of a debris flow.

The study investigates an earth slide type rain-induced landslide that occurred at Madithorai, Nilgiris District, India in 2009. It consists of detailed field investigations such as topographical survey and borehole investigation, laboratory and numerical investigations. Based on the topographical survey the slope was modelled in SEEP/W programme to perform the uncoupled transient seepage analysis. The cumulative rainfall of 586 mm was given as an input for a duration of 5 days that caused landslide at Madithorai. The results of transient seepage analysis were used to determine the stability of the slope based on the infinite slope method for unsaturated soil. Based on the analysis the failure mechanism of Madithorai landslide was observed. The influence of antecedent rainfall using three different idealized 5-day antecedent rainfall was studied. A threshold intensity was established from the different 5 days antecedent rainfall and the corresponding factor of safety which can be used as for landslide early warning. The factor of safety from deterministic analysis was greater than one which does not imply that the slope is stable. Therefore, the performance of the slope was studied based on the reliability index and probability of failure using the Monte Carlo simulation. The results highlight that the slope is in hazardous condition with a reliability index of 1.15 and the probability of failure of 11.7%. Hence, it is recommended to adopt a landslide early warning system and suitable remedial measures at Madithorai location.

The stability of subsea tunnels is governed by the strong coupling among stress redistribution, damage evolution, and seepage flow (Stress–Damage–Seepage, SDS). The dynamic interplay, especially under high water pressure, often leads to catastrophic failures, yet its mechanisms, particularly the role of support timing, remain insufficiently understood due to limitations in conventional numerical methods. This study aims to unravel the SDS coupling mechanisms during tunnel excavation under high hydraulic head, and to quantitatively investigate how support timing influences the stability of the surrounding rock within this coupled system. A coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) framework was employed. In this approach, excavation-induced damage, crack propagation, and fluid–particle interactions are explicitly resolved at the particle scale, whereas the macroscopic permeability evolution is captured through an imposed empirical exponential relationship. Simulations were conducted under both steady-state and transient seepage conditions with varying stress ratios and water heads. High-head transient seepage intensifies SDS coupling, dynamically redistributing seepage forces to damage zone edges and amplifying damage. Support timing critically mediates this interaction: premature support risks tensile failure at the tunnel periphery, while delayed support allows a vicious cycle of shear failure and increased inflow. Optimal “timely” support, applied after initial deformation, diverts high seepage forces inward, minimizing final damage. The spatiotemporal synchronization of transient seepage forces with damage evolution is pivotal for stability. Support timing acts as a key control variable. The CFD-DEM framework effectively elucidates these micro-mechanisms, providing a scientific basis for the dynamic design of support in high-pressure subsea tunnels.

Accurate pore pressure prediction under transient seepage conditions following the removal of external water loads, such as rapid drawdown (RDD), requires coupling the effect of changes in both hydraulic boundary conditions and stresses. The Barcelona basic model (BBM), which has been used as a coupled soil constitutive model to predict pore pressures, is used in this paper to predict pore water pressure for RDD stability analysis. RDD stability is evaluated using predicted pore pressures from the BBM for a hypothetical geometry and for the failed Sparmos Dam. The paper compares the pore pressure assumptions for the BBM to other methods of RDD and highlights the limitations of using the BBM in effective stress RDD stability analysis. Sensitivity analysis revealed that pore pressures following RDD are not affected by the values of the BBM parameters, if varied in reasonable bounds. During drawdown, the BBM simulates the influence of shear stresses on predicted pore pressures only related to an elastic response of the soil, which is not realistic for most soils. Despite these limitations, the pore pressures predicted by BBM calculated the most reasonable factor of safety close to unity using the most likely strength parameters for the failed Sparmos Dam when compared to other common RDD procedures.

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