In the past 30 years, research on rainfall-induced landslides has grown remarkably. The contribution of matric suction to soil strength and the physics of water flow in unsaturated soils are widely accepted phenomena among researchers. However, the adoption of unsaturated soil mechanics in geotechnical engineering practice has been relatively slow, in part due to the practicality of design solutions available to the engineer. This paper conducts a parametric study on unsaturated silty slopes under a vertical steady flow rate to identify the suitable slope and hydrologic conditions to incorporate unsaturated conditions for preliminary stability analysis. Notably, the contribution of suction is most significant for silt/clay slopes with a water table located below the mid-height of the slope. For slopes with slope height ≥20 m and a fairly high water table, the slope height is a primary controlling factor of slope stability. Two case studies based on distinct failure mechanisms are presented to review the application of common geotechnical software in rainfall seepage and stability analyses of unsaturated slopes. Focus is placed on the pre-failure and failure stages of each case study. The slip surface search method, failure mode, and coupling approach integrated into each computer program caused notable differences in output results.

Slope stability analysis is one of the most critical and important topics for geotechnical engineers and thus various stability charts have been developed in order to provide quick first assessments of slope stability. However, general chart solutions are often found to be insufficient for purpose when variables to be considered increase. Motivated by that, this study aims to adopt an artificial neural network (ANN) with the extreme learning machine algorithm to develop a convenient and efficient tool for assessing three-layered soil slope stability. The neural network is trained using the obtained solutions from the finite element upper and lower bound limit analysis methods. The proposed tool in this study is capable of providing a quick first assessment of three-layered soil slope stability. The results showed that the slope stability estimation using the proposed ANN-based tool can still maintain good accuracy and convenience.

Seismic effect is one of the most commonly considered factors in rock slope safety design. This study adopts the finite element lower bound limit analysis method to study the seismic stability of disturbed rock slopes considering inhomogeneity caused by rock mass disturbance. Moreover, this research investigates different earthquake magnitudes by considering various seismic coefficients. Results are presented as seismic rock slope stability charts. In addition, the recommended blasting damage zones are also investigated in this study. Results show the chart solutions can provide a reasonable tool for the preliminary evaluations of the seismic safety factors for rock slope stability. The case studies demonstrate that a safe design can be done if the earthquake effects are considered reasonably. Moreover, consideration of varying rock mass disturbance in the slope is helpful to capture the failure mechanism more realistically when compared to the slope case without varying rock mass disturbance. For comparison purposes, the conventional limit equilibrium analysis and the equivalent Mohr-Coulomb parameters are used to perform analyses of the rock slope stability.

This study presents the case of a landslide triggered by a high groundwater level caused by several days of continuous rainfall in the northeastern region of Taiwan. The slope where this landslide occurred consists of closely jointed and weathered bedrock. By means of finite element limit analysis and the Hoek–Brown failure criterion, this study performed a slope failure simulation similar to the actual landslide and deduced the reasonable value range for the combination of key Hoek–Brown failure criterion parameters through back analyses. The results indicate that the key parameters affecting the bedrock’s slope stability were the geological strength index (GSI) and the disturbance factor (D), whereas the effects of the unconfined compressive strength (σ_ci) were less significant. The results of the back analysis reveal that the suitable D-value range and GSI of closely jointed and weathered sandstone in the northeastern region of Taiwan are 0.8 to 0.9 and 20 to 30, respectively. These back-analyzed value ranges can serve as a reference for broader applications in the preliminary stability analysis of similar rock slopes where it is difficult to perform in situ investigation.