Field-based investigation of failure modes and thresholds of granite residual soil slopes under heavy rainfall conditions.

Rainfall is a primary coefficient of slope instability. To study patterns of slope failure mode and key index threshold values of slope instability under varying rainfall intensities, real-time data from in-situ monitoring tests of an engineering slope were utilized. This data facilitated the analysis of the temporal and spatial responses of a rainfall infiltration slope. Four field tests were conducted under four different rainfall intensities, namely 75mm/h, 125mm/h, 150mm/h, and 175mm/h, to study the progressive failure characteristics of the granite residual soil slope. These characteristics were studied based on water-time and displacement-time curves, and the rainfall duration and threshold for slope instability were proposed. The result revealed that the granite residual soil slope undergoes progressive failure under rainfall conditions, which can be categorized into four modes: "shallow local sliding","shallow global sliding and collapse", "deep local sliding", and"deep global sliding". It takes 25 ~ 135min for the shallow sliding failure characteristics from significant deformation to instability, while the deep sliding only lasts for 18 ~ 20min. A significant correlation was observed between soil moisture content and slope instability. Instability symptoms such as cracking and peristaltic deformation begin to appear when the soil moisture content in the shallow layer of the slope increases to 42 ~ 45%. When the soil moisture content escalates to 47 ~ 50%, the slope begins to disintegrate, leading to rapid landslides and collapses.