Response of a granite residual soil slope to rainfall based on stable slope angle: from field experiments in northern Guangdong Province, China

Intense soil erosion and landslide events, often initiated by rainfall, present significant challenges in regions characterized by granite residual soil (GRS) in northern Guangdong Province. The GRS layer in this region is divided into two distinct zones: red soil at the surface and sandy soil beneath it. To determine the stable slope angle of GRS slopes in this area, in-situ direct shear tests were conducted at three different soil elevations. The results revealed that the stable slope angle ranged from 32.29° to 34.86°, with a safe stable slope angle set at 32° for greater safety. To further investigate the response of GRS slopes to rainfall at the stable slope angle, field-based artificial rainfall experiments were systematically performed. The findings showed that most of the rainfall infiltrated the slope, with both volumetric water content, pore water pressure (PWP) and earth pressure responding rapidly to the simulated rainfall events. However, as internal drainage channels formed due to seepage erosion within the soil, rainwater was quickly discharged after infiltration, causing only minor fluctuations in PWP, which remained within a low range. This drainage process is a key factor in preventing landslides on GRS slopes with less than or equal to the stable slope angle. The conclusions drawn from this study provide valuable insights into the soil erosion and landslide hazards in northern Guangdong Province, and may also be applicable to other regions with similar slope characteristics and climatic conditions.