Failure Mechanism of Weak Rock Slopes considering Hydrological Conditions

Landslides frequently occur on cutting slopes with weak rock during the rainy season. An understanding of the failure mechanism is essential for designers to adopt effective measures to mitigate potential impacts caused by landslides. Based on an enormous number of highway slope practices in Guangdong Province, China, this paper presents a whole train of numerical investigations on the hydraulic response and stability of weak rock slopes subject to a subtropical climate where the average annual rainfall is approximately 1,800-2,000 mm. This study explored a different failure mechanism of three different types of weak rock slopes in a modeling approach. A physics-based slope model coupled with a hydrological model is used to simulate the factor of safety and porewater pressure development of unsaturated slopes with various rock masses under diverse rainfall scenarios. The input parameters were determined from the rainfall intensity-duration-frequency (IDF) curves of regional hydrological conditions to comprehensively consider the impact of rainfall. The general suction range of rock masses was acquired using three hydraulic characteristic curves associated with sandstone, siltstone, and mudstone. Furthermore, the failure models of weak rock slopes under different rainfall conditions were assessed. Based on the theory of regional hydrology, a unified rainfall and intensity-duration threshold for different weak rock slopes was established. The results indicate that both siltstone and mudstone slopes are more sensitive to long-term rainfall than short-term rainstorms. However, the sandstone slope appears to be unstable under regional rainfall conditions of 40-year return periods. This also proves that the instability mode of the siltstone slope exhibits a retrogressive type in which the sliding surface of the slope increases with an increase in rainfall duration. Translational shallow landslides occurred in the transient saturation zone of the sandstone slope, and the depth of the failure plane was independent of rainfall patterns and duration. This study revealed the instability mechanism and provided a robust early warning measure for weak rock slopes by fusing engineering geology and regional hydraulics, providing insightful information for mitigating potential landslide disasters. The proposed model can also be used to predict the possible infiltration depth of rainfall and the spatial location of the sliding surface, prior to construction to assess whether slope stabilization and reinforcement measures are needed.