Rainfall infiltration depth and formation mechanism of slow-inclination soil landslides in Nanjiang

Lots of slow-inclination soil landslides were triggered by the rainfall on September 16th in 2011 in Nanjiang. The typical landslides have sliding bodies with thickness concentrated in 1~5 m, and they always slide along the bedrock cover interface which is a smooth surface. By designing rainfall infiltration tests, establishing G-A infiltration model and calculating atmosphere influence depth considering the effect of atmosphere on the slope, the rainfall infiltration depth of landslides is analyzed, and the distribution causes of sliding thickness are investigated. The slope stability is analyzed based on the unsaturated soil strength theory when the infiltration peak is located in the soil layer and the base cover interface under the natural and rainfall conditions. The results show that the permeability of red clay in Nanjiang is extremely low, the rain infiltration depth is very limited in short and heavy rainfall, and it is generally only about 50 cm. The atmosphere influence depth is 5.37 m in the study area. In this range the rain easily infiltrates. However, with the increase of depth, the weathering degree of the soil is gradually weakened, the pore ratio and permeability gradually decrease, and infiltration is gradually difficult until the bedrock. The rainfall infiltration depth is the main cause of the typical landslides with shallow slide. Under rainfalls, when the depth of infiltration peak is less than the thickness of soil layer and the potential sliding surface is in the infiltration peak surface, the slope is stable. When the depth of infiltration peak is equal to the thickness of soil layer and the potential slip surface is located on the base cover interface, stagnant water at the interface is generated, the water level rises, the pore pressure increases, and the interface produces softening and lubrication. The interface effect is the main cause for the slow-inclination soil slide along the bedrock cover interface which is a smooth surface.