Slope stability and critical thresholds of red clay under rainfall and slope‐cutting effects: A case study of Hunan Province, Central China

Abstract

Assessing the stability and critical thresholds of red clay slopes in Central China under rainfall and excavation is crucial for effective risk mitigation. Through field investigations and laboratory tests, a representative red clay slope was characterized at both macro‐ and micro‐scales. A finite element model was developed, and three scenarios were considered: rainfall (continuous and short‐duration), slope cutting (varying height and angle) and their combined effects. The factor of safety (FoS) for each scenario was calculated using the Morgenstern–Price method to determine critical thresholds. Results show that: (1) Although the slope currently exhibits favourable stability, the high illite content presents a significant risk for softening and shallow instability under prolonged rainfall. (2) Short‐duration rainfall had minimal impact, whereas continuous rainfall reduced the FoS below 1.0, highlighting a critical sensitivity to prolonged rainfall. The FoS decreased with increasing cut height and angle, directly reducing stability, though it did not change the slope's response to rainfall. (3) The failure thresholds for the three investigated scenarios were identified as 40 mm/day for rainfall, 10 m/60° for excavation and 20 mm/day of rainfall under critical excavation conditions, demonstrating their synergistic weakening effect. This research provides practical guidance for safe excavation and rainfall early warning in red clay regions.